6: Getting Functional

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Edward Parrish © 2003-2010

6: Getting Functional
What We Will Cover

Reminders:

Please silence all cell phones and pagers

Sit at a computer station, start your computer and login

When class is over, please shut down your computer.

Remember that you lose any files not saved in the student folder.

Continuations

6.1: More About Strings and Characters

6.1.1: Strings Versus Characters

6.1.2: Indexing a String

6.1.3: Iterating Strings

6.1.4: Converting Characters to Digits

Exercise 6.1

6.1.5: String Input With Spaces

6.1.6: Processing Text Input

6.1.7: Summary

6.2: Predefined Functions

6.2.1: About Functions

6.2.2: Calling a Function

6.2.3: Generating Random Numbers

6.2.4: Seeding the Random Generator

Exercise 6.2

6.2.5: Simulating Dice

6.2.6: Flotaing-Point Random Numbers

6.2.7: Summary

6.3: Coding Functions

6.3.1: Repeated Code

6.3.2: Defining a Function

6.3.3: Example Function Definition

Exercise 6.3

6.3.4: Passing Arguments to a Function

6.3.5: Returning a Value

6.3.6: Some Style Requirements for Functions

6.3.7: Summary

6.4: Midterm Preparation

6.4.1: About the Exam

6.4.2: Recommended Preparation

6.4.3: Exam Taking Tips

6.4.4: Questions and Answers

Wrap Up

Due Next:
A5-Loopy Programs (3/18/10)
A6-Programs With Functions (4/8/10)

Continuations

Questions from last class?

What will be output after the following C++ statements have executed?
int count = 1;
while (count <= 3) {
    cout << count << " ";
    count++;
}
1 2

1 2 3

2 3

1 2 3 4

Homework Questions?

A5-Loopy Programs (3/18/10)

Whitespace and astyle

Homework Discussion Questions (Thursday)

Can you suggest a different algorithm than shown in exercise P4.6?

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6.1: More About Strings and Characters

Learner Outcomes

At the end of the lesson the student will be able to:

Iterate through a string and extract each character

Convert characters to digits

Use string functions

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6.1.1: Strings Versus Characters
Remember that a string is a series of characters enclosed in double quotes such as:
"Hello"  "b"  "3.14159"  "$3.95"  "My name is Ed"
We can store text in a variable of type string, like:
string firstName;             // declaration
firstName = "Edward";         // assignment
string lastName = "Parrish";  // declaration + assignment
cout << firstName << " " << lastName << endl;
On the other hand, a character is a single letter, number or special symbol
We enclose characters in a single quote, rather than a double quote, like:
'a'   'b'   'Z'   '3'   'q'   '$'   '*'
Also, you can store a a single character using a variable of type char, such as:
char letterA = 'A';
char letterB = 'B';
Each character is stored as a number, using its ASCII Table

By declaring a char variable or using single quotes, C++ knows to treat the number as a character
Thus, when you print a character, you see a letter rather than a number:
char letter = 'A';
cout << letter << 'B' << endl;
As you can see, a string is made up of characters and characters are numerical codes

We can use this information to work with characters and strings
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6.1.2: Indexing a String
Strings are stored in a character sequence starting at 0 (zero)

You can access any individual character of a string variable using [ ]
The general syntax is:
stringVariable[index];
Where:

stringVariable: the name of your string variable

index: the number of the character position
For example:
string str = "abcdef";
char firstLetter = str[0];
cout << firstLetter << str[1] << endl;
The above code displays:
ab
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6.1.3: Iterating Strings
Recall that member function length() returns the number of characters in a string variable:
string s = "abcdef";
unsigned n = s.length();
Since a string's length is always 0 or a positive number, the length() function returns an unsigned int type
After we know the length, it is easy to iterate through the individual characters of a string using a counting loop:
cout << "Enter a message: ";
string msg;
cin >> msg;
for (unsigned i = 0; i < msg.length(); i++) {
    cout << "Char[" << i << "]: " << msg[i] << endl;
}
Using unsigned

Note the use of unsigned i in the for loop

Recall from lesson 4.1.7 that unsigned ranges from 0 to 4294967295 rather than -2147483647 to 2147483647 for int

The length() function returns an unsigned number because the length of a string is never less than zero

If you compare a signed number with an unsigned number, the compiler may issue a warning:
warning: comparison between signed and unsigned integer expressions

By using unsigned as the counting variable type in the for loop you avoid the warning

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6.1.4: Converting Characters to Digits
Recall that a character is stored by the computer as a number using its ASCII code

In the ASCII table, notice the ASCII codes for the digit characters:

Decimal char

48 '0'

49 '1'

50 '2'

51 '3'

52 '4'

53 '5'

54 '6'

55 '7'

56 '8'

57 '9'

Since each character is a numerical code, we can convert a char to a number
For example:
char digit = '9';
int num = digit - 48;
cout << num << endl;
num = num * 4 + 6;
cout << num << endl;
Since a string is a series of characters, and each character is number, we can convert a string to anumber as well
For example:
string str = "123";
char ch0 = str[0];
char ch1 = str[1];
char ch2 = str[2];
int digit1 = ch0 - 48;
int digit2 = ch1 - 48;
int digit3 = ch2 - 48;
int num = digit1 * 100 + digit2 * 10 + digit3;
cout << num << endl;
Since the algorithm for converting a string to a number is repetitious, we can use a loop

We will explore converting strings to numbers in the next exercise
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Exercise 6.1

In this exercise we convert strings to character and characters to numbers.

Specifications
Copy the following program into a text editor, save it as stringdigits.cpp, and then compile and run the starter program to make sure you copied it correctly.
#include <iostream>
using namespace std;

int main() {
    // Enter your code here

    return 0;
}
Add code to get user input into an string variable named str. When you run the program after adding this code, the output should look like:
Enter an integer number: 12345
Note that the underlined numbers above shows what the user enters. Also, we do NOT want to allow spaces in user input. For more information see section 3.2.2: String Variables and Simple I/O.
After the input statement, add another line of code that displays the length of the string variable str. When you run the program after adding this code, the output should look like:
Enter an integer number: 12345
The number of characters is 5.
Note that the number 5 in the above example may change depending on the number entered. For more information see section 6.1.3: String Functions.
After the other statements, add the following code to convert the first character of the string to a number:
char ch = str[0];
int digit = ch - 48;
cout << "Digit: " << digit << endl;
Enclose the above code that converts a character to a string inside a loop that iterates through each character, as explained in section 6.1.3: Iterating Strings. IMPORTANT: change the number 0 in square brackets to the index variable of the counting loop. When finished, your code should look like:
In this step, we add the code to display the sum of the digits as follows:
Before the loop, declare a variable named sum and initialize the variable to 0.
int sum = 0;
At the end of the loop, before the closing curly brace (}), add the statement:
sum = sum + digit;
After the closing curly brace of the loop, add a statement to display the sum of the digits, like:
cout << "The sum of the digits is: "
     << sum << endl;
Compile and run your modified program to make sure you made the changes correctly. When you run the program, the output should look like:
Enter an integer number: 12345
The number of characters is 5.
Digit: 1
Digit: 2
Digit: 3
Digit: 4
Digit: 5
The sum of the digits is: 15
Debug any problems you see and ask your neighbor or the instructor for help as needed.
In this step, we add the code to convert the digits into a single number as follows:
At the top of the file, include the cmath library:
#include <cmath>
Before the loop, declare a variable named num and initialize the variable to 0:
int num = 0;
At the end of the loop, before the closing curly brace (}), add the statements:
int exp = str.length() - (int) i - 1;
num = num + (int) (digit * pow(10.0, exp));
After the closing curly brace of the loop, add a statement to display the entire number :
cout << "The number is: " << num << endl;
Compile and run your modified program to make sure you made the changes correctly. When you run the program, the output should look like:
Enter an integer number: 12345
The number of characters is 5.
Digit: 1
Digit: 2
Digit: 3
Digit: 4
Digit: 5
The sum of the digits is: 15
The number is: 12345
Debug any problems you see and ask your neighbor or the instructor for help as needed.
Submit your program source code to Blackboard as part of assignment 6.
Program that Processes Strings with Loops

As time permits, read the following sections and be prepared to answer the Check Yourself questions in the section: 6.1.7: Summary.

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6.1.5: String Input With Spaces
We have been using the >> operator to enter data into a string variable:
string something;
cout << "Enter something: ";
cin >> something;
cout << "You entered: " << something << endl;
However, there are some complications

>> skips whitespace and stops on encountering more whitespace

Thus, we only get a single word for each input variable

If a user types in "Hello Mom!", we would only read "Hello" and not " Mom!"

This is because cin >> s1 works as follows:

Skips whitespace

Reads non-whitespace characters into the variable

Stops reading when whitespace is found
Input Using getline()
To read an entire line we use function getline()
Syntax:
getline(cin, stringVariable);
Where:

stringVariable: the name of the string variable
For example:
string line;
cout << "Enter a line of input:\n";
getline(cin, line);
cout << line << "END OF OUTPUT\n";
Note that getline() stops reading when it encounters a '\n'
The Problem with Newlines
When you press the Enter key, a newline character ('\n') is inserted as part of the input

The newline character can cause problems when you mix cin >> with getline()

Recall that cin >> s1:

Skips whitespace

Reads non-whitespace characters into the variable

Stops reading when whitespace is found

Since whitespace includes newline characters, using cin >> will leave a newline character in the input stream

However, getline() just stops reading when it first finds a newline character
This can lead to mysterious results in code like the following:
cout << "Enter your age: ";
int age;
cin >> age;
cout << "Enter your full name: ";
string name;
getline(cin, name);
cout << "Your age: " << age << endl
     << "Your full name: " << name << endl;
To get around this problem you can use cin >> ws just before getline()
cin >> ws; // clear whitespace from buffer
You can see how to use this fix in the following example
Example Using cin >> ws
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#include <iostream>
using namespace std;

int main() {
    cout << "Enter your age: ";
    int age;
    cin >> age;
    cout << "Enter your full name: ";
    string name;
    cin >> ws; // clear whitespace from buffer
    getline(cin, name);
    cout << "Your age: " << age << endl
         << "Your full name: " << name << endl;
}
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6.1.6: Processing Text Input
Sometimes we need to read input as words and sometimes as lines
To input a sequence of words, use the loop:
string word;
while (cin >> word) {
   // process word
}
cin >> word is the same test as cin.good() (see lesson 5.3.6)
To process input one line at a time, use the getline() function
string line;
while (getline(cin, line)) {
   // process line
}
getline(cin, line) returns true as long as there is input

The following example processes text input by counting words

When reading input in the while test, you need to close the stream using:

Ctrl + Z in Windows

Ctrl + D in UNIX or Max OS X

Closing the stream acts as a sentinel value for the loop

When the stream fails the loop exits
Example Program that Reverses a Sentence
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#include <iostream>
#include <string>

using namespace std;

int main() {
    cout << "Enter a phrase followed by the Enter"
         << " key and Ctrl-Z/D.\n";
    string reversed;
    string word;
    while (cin >> word) {
        reversed = word + " " + reversed;
    }
    cout << reversed << endl;

    return 0;
}
Redirection of Input and Output
We could use the above program by typing words at the command line

However, that quickly gets tedious

A better way is to use redirection of input

The command line interfaces of most operating systems have a way to link a file to the input of a program

The content of the file gets fed into the program as if all the characters had been typed by a user
For example, after compiling the above program we type something like the following at the command line:
./words < input.txt
Where input.txt is the text file on which we want to count words
You can redirect program output to a file as well using something like:
./words > output.txt
You can combine input and output redirection in one command:
./words < input.txt > output.txt
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6.1.7: Summary
A string is a series of characters enclosed in double quotes
We can store text in a variable of type string, like:
string s1 = "Hello Mom!";
A character is a single letter, number or special symbol
We can store a a single character using a variable of type char, such as:
char letterA = 'A';
char letterB = 'B';
Each character is stored as a number, using its ASCII code

Strings are stored in a character sequence starting at 0 (zero)

You can access individual characters of a string using []

Strings are a special type of variable called objects, just like a Turtle

Because a string is an object, it has member functions
We can iterate through a string using a loop and the length() member function:
string s = "abcdef";
for (unsigned i = 0; i < s.length(); i++) {
    cout << "Char[" << i << "]: " << s.at(i) << endl;
}
Also, we looked at how to convert a char to a number

Since each character has a numerical code, we can convert a char to a number
For example:
char digit = '9';
int num = digit - 48;
cout << num << endl;
Since a string is a series of characters, and each character is number, we can convert a string to anumber as well

We looked at an example of converting strings to numbers as well
To read an entire line, you need to use the getline() function:
getline(cin, line);
Sometimes cin >> can leave a '\n' character in the input stream
To get around this problem you can use cin >> ws before getline()
cin >> ws; // clear whitespace from buffer
Check Yourself

Answer these questions to check your understanding. You can find more information by following the links after the question.
String are enclosed in double quotes. What type of quote marks enclose characters? (6.1.1)

The characters of a string variable can be accessed using what brackets? (6.1.2)

The leftmost character of a string is accessed using which index number? (6.1.2)
To print the following string vertically down the page, what code do you write? (6.1.3)
string str = "Hi mom!";
To convert the following char variable to a number, what code do you write? (6.1.4)
char ch = '7';
What is the value of the expression: 'd' - 'a' + 'A'? (6.1.4)
To convert the following string variable to a number, what code do you write? (6.1.4)
string str = "7";
How many words can you enter with the following code? (6.1.5)
string something;
cout << "Enter something: ";
cin >> something;
cout << "You entered: " << something << endl;
How can you change the previous code to read a string that includes spaces? (6.1.5)

What code can you use to clear newlines and other whitespace from the input stream? (6.1.5)
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6.2: Predefined Functions

Objectives

At the end of the lesson the student will be able to:

Create code that calls predefined functions

Describe the flow of control of a function call

Write code that generates random numbers

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6.2.1: About Functions

Function -- a subprogram (method, procedure or subroutine) that executes a block of code when called.
Most popular programs are large, containing thousands to millions of lines of code

When programs are this large, the best way to develop and maintain them is to construct them from smaller pieces or modules

In C++, these smaller pieces are called functions

Later on we will look at another way to create larger modules called Classes

You may have noticed that we have been using functions already
For instance, main() is a function that we use in every program
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#include <iostream>
using namespace std;

int main() {
    cout << "Hello, World!\n";
    return 0;
}
Function are often collected into libraries so they can be reused by many different programs

C++ comes with many common libraries such as <cmath> which contains functions like sqrt()

We have used many of these library functions and will use more as the course progresses

Functions such as these are known as predefined functions

A predefined functions is simply a function that someone else wrote

Our use of predefined functions shows one of the benefits of functions: reusable code

Later on we will learn to define our own functions so that we can create reusable code as well

For now, we are going to focus on how to use functions
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6.2.2: Calling a Function

When a function is defined or included in a program, C++ ignores the statements at first

In order to run the statements, you must call the function
Function call: an instruction that passes control to a function; after completing the function, control returns to the next instruction in the calling function.

Calling a function is like a boss asking a worker to do something

The "boss" (calling function) asks the "worker" (called function) to complete a task

Flow of Control for a Function Call

To understand how a function works, you need to trace its flow of control

The programs we write in this course have a single flow of control

All the statements in our programs execute in sequence, which can be modified by conditional statements or loops

A function call is another way to change the flow of control

When we call the function, the caller function stops and waits for the called function to finish

You can see this flow in the following diagram:

Arguments and Returned Values
When we call a function we can send the function data called arguments
For example, to calculate the square root of a number, we call the sqrt() function with a particular value:
sqrt(9.0);
In this case the argument is the numerical value 9.0

When the called function finishes executing, it may return a value

In this case our function returns the value 3.0
Example Program with a Function Call
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#include <cmath>
#include <iostream>
using namespace std;

int main() {
    double result = 1 + sqrt(9.0);
    cout << result << endl;

    return 0;
}
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6.2.3: Generating Random Numbers

Random numbers are a series of numbers whose order cannot be predicted

The C++ library has a random number generator, which produces numbers that appear to be random

Many computational problems need to use random numbers

rand() is a library function that produces a series of psuedorandom numbers:

Range is 0 up to and including RAND_MAX

Returns an "random" int value

Random numbers actual come from a a very long sequence of numbers computed from fairly simple formulas; they just behave like random numbers

For that reason they are often called psuedorandom numbers

The following program uses rand to produce the same output every time it runs

The reason that the numbers are the same is because the numbers are generated with formulas

Example Program Using rand()
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#include <iostream>
#include <cstdlib>
using namespace std;

int main() {
    int i;
    for (i = 0; i < 10; i++) {
        int num = rand();
        cout << num << "\n";
    }

    return 0;
}
Another Random Number Generator

Random Number: a less useful random number generator from xkcd

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6.2.4: Seeding the Random Generator
When running programs we do not always want the same sequence of numbers every time

For instance, if we run a dice simulation for a game we want different numbers every time the program runs

To get a different number, we must seed the random number generator

We set the seed with the srand() function

If we change the seed value, we will get different numbers every time

One common strategy for changing the seed value is to use time

Since the time changes every second, we get a new random number sequence every time we run our program
To generate a number from time, we can use the time() function from the C time library:
srand(time(0));
The expression time(0) returns the number of seconds since January 1, 1970
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Exercise 6.2

In this exercise we look at how to call functions and learn how to generate random numbers.

Specifications
Copy the following program into a text editor, save it as dice.cpp, and then compile the starter code to make sure you copied it correctly.
#include <iostream>
using namespace std;

int main() {
    // Enter your code here

    return 0;
}
Random numbers are generated by using the rand() function. Declare two integer variables, die1 and die2, and assign them values returned by the rand() function using code like the following:
int die1 = rand();
int die2 = rand();
cout << "You rolled a " << die1
     << " and a " << die2 << endl;
For more information, see section: 6.2.3: Generating Random Numbers.
Compile and run your program and make sure your output looks like the following. Note that the actual numbers may be different.
You rolled a 0 and a 1481765933
Notice the size of the numbers displayed. We want to limit the size of the random numbers generated by rand() to only six numbers. We limit the range by using the modulus (%) operator.
Change the two assignment statements with the following code:
int die1 = rand() % 6;
int die2 = rand() % 6;
The number 6 in the above code is known as the scaling factor since it limits the scale of the numbers produced by the rand() function.
Compile and run your program and make sure your output looks like the following. Note that the actual numbers may be different.
You rolled a 0 and a 5
Seeing just one roll of the dice is not very useful. Add a counting loop to roll and display the dice 10 times. For more information, see section: 5.1.1: Using Loops to Count. You may want to use a for loop for counting as described in section 5.1.2: for Statements.
Compile and run your program and make sure your output looks like the following. Note that the actual numbers may be different.
You rolled a 0 and a 5
You rolled a 3 and a 2
You rolled a 5 and a 0
You rolled a 0 and a 4
You rolled a 0 and a 2
You rolled a 1 and a 3
You rolled a 5 and a 3
You rolled a 5 and a 0
You rolled a 5 and a 0
You rolled a 0 and a 5
Notice that we get numbers in the range of 0 to 5 when we want 1 to 6. To correct this problem, we must add one to each of the statements generating the random numbers. Go ahead and make this change now. For more information, see section: 6.2.5: Simulating Dice.
Rerun your code two or more times and check the numbers rolled. Do you see any patterns? To correct the problem we must "seed" the random number generator. Add the following code after the start of main and before your counting coop:
srand(time(0));
For more information, see section: 6.2.4: Seeding the Random Generator.
Compile and run your program and make sure your output looks like the following. Note that the actual numbers may be different.
You rolled a 1 and a 1
You rolled a 4 and a 5
You rolled a 5 and a 6
You rolled a 3 and a 6
You rolled a 2 and a 4
You rolled a 4 and a 6
You rolled a 3 and a 4
You rolled a 1 and a 5
You rolled a 4 and a 5
You rolled a 4 and a 6
Submit your final program source code to Blackboard as part of assignment 6.
As time permits, read the following sections and be prepared to answer the Check Yourself questions in the section: 6.2.7: Summary.

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6.2.5: Simulating Dice
A simulation is an imitation of some real thing or process

We create a simulation in a computer program by generating random events and evaluating their outcome

One simple simulation we can do is a pair of dice

We need a number between 1 and 6, but rand() returns a number between 0 and RAND_MAX

We need only random numbers for a die and so we scale the range of RAND_MAX using the % operator

Now we can generate random integer numbers between 0 and 5

To get a number between 1 and 6, we shift the numbers by adding 1
Our code to simulate a single die with random numbers is now:
int die = 1 + rand() % 6;
cout << die << endl;
We can generalize our formula for producing integer random numbers to:
rand() % SCALING_FACTOR + SHIFTING_VALUE;
The following program implements our dice rolling simulation

Note how the program simulates rolling 2 dice using 1 die at a time

You would get a different result if you just generated a random number between 2 and 12

To repeat the dice rolling, we use a loop
Example Program Simulating the Rolling of a Pair of Dice
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#include <iostream>
using namespace std;

int main() {
    srand(time(0));

    for (int i = 0; i < 10; i++) {
        int die1 = 1 + rand() % 6;
        int die2 = 1 + rand() % 6;
        cout << "You rolled a " << die1
             << " and a " << die2 << endl;
    }

    return 0;
}
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6.2.6: Flotaing-Point Random Numbers
Sometimes we want random numbers that are of data type double

For example, we may want a random number for simulations

However, the rand() function returns an int and we need to convert the int to a double
For this we can use the formula:
double r = rand() / (RAND_MAX + 1.0);
This produces a number between 0 (inclusive) and 1 (exclusive), often written as [0, 1)

One we have this random double, we multiply it by the scaling factor
Also, we can shift the range using addition or subtraction
double num = r * SCALING_FACTOR + SHIFTING_VALUE;
For example, we can get a random floating-point number between -10 and +10 (not including +10) using:
double r = rand() / (RAND_MAX + 1.0);
double y = r * 20 - 10;
If we want to include +10 then we use the simpler:
double r = rand() / RAND_MAX;
double y = r * 20 - 10;
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6.2.7: Summary
A function is a block of code get executed when called

During a function call, control is passed to the called function

When the called function finishes executing, control returns to the calling function and statement

C++ has many predefined functions in several libraries
Libraries must be "included" in a program:
#include <cmath>
Newer standard libraries, such as cmath, also require the directive:
using namespace std;
One of the library functions generate "random" numbers: rand()

To seed the random number generator, you use the srand() function
We can use these functions to simulate the rolling of a die:
srand(time(0));

int die = 1 + rand() % 6;
cout << die << endl;
You can use random numbers for other computations as well, such as x and y coordinates:
double r = (double) rand() / ((double) RAND_MAX + 1.0);
double y = r * 20 - 10;
Check Yourself

Answer these questions to check your understanding. You can find more information by following the links after the question.

What is a function? (6.2.1)

Why do we use functions? (6.2.2)

Where does the control flow during a function call? (6.2.2)

What is an argument? (6.2.2)

What is a returned value? (6.2.2)

What is a random number? (6.2.3)

What is the value of RAND_MAX on our classroom computers? (6.2.3)

How do you make the rand() function return a different random sequence when you run your program again? (6.2.4)

How do you code a random number that simulates rolling a die? (6.2.5)

What statement do we use to limit the range of the values returned by the rand() function to the values 1 through 10? (6.2.5)

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6.3: Coding Functions

Learner Outcomes

At the end of the lesson the student will be able to:

Define functions with and without parameters

Pass arguments to functions

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6.3.1: Repeated Code

Recall our test code to validate user input:
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#include <iostream>
using namespace std;

int main() {
    double input = 0.0;
    do {
        cout << "Enter a positive number: ";
        cin >> input;
        if (cin.fail()) {
            cout << "You must enter digits, not words\n";
            cin.clear();
            cin.ignore(INT_MAX, '\n');
            input = -1; // set loop test to fail
        } else if (input <= 0.0) {
            cout << "You must enter a positive number\n";
        }
    } while (input <= 0.0);
    cout << "You entered: " << input << endl;

    return 0;
}
What if we need to enter two validated numbers into a program?

We want to process the first number after input and then input the second number

Doing so, we would end up with code like this:
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#include <iostream>
using namespace std;

int main() {
    double input = 0.0;
    do {
        cout << "Enter a positive number: ";
        cin >> input;
        if (cin.fail()) {
            cout << "You must enter digits, not words\n";
            cin.clear();
            cin.ignore(INT_MAX, '\n');
            input = -1; // set loop test to fail
        } else if (input <= 0.0) {
            cout << "You must enter a positive number\n";
        }
    } while (input <= 0.0);

    // Process the input
    cout << "You entered: " << input << endl;

    double input2 = 0.0;
    do {
        cout << "Enter a positive number: ";
        cin >> input2;
        if (cin.fail()) {
            cout << "You must enter digits, not words\n";
            cin.clear();
            cin.ignore(INT_MAX, '\n');
            input2 = -1; // set loop test to fail
        } else if (input <= 0.0) {
            cout << "You must enter a positive number\n";
        }
    } while (input <= 0.0);

    // Process the second input
    cout << "You entered: " << input2 << endl;

    return 0;
}
Our program would be easier to write if we could get the second input without repeating the code

Ideally, we would give the list of commands a name and then run the list using the name

We can do this in C++ by writing a function

Programming Style: Avoid Duplicating Code

Duplicate code is the mark of a poor or lazy programming style

It can lead to problems such as:

Long repeated sections that are more difficult to understand than shorter sequences

Repetition of largely identical code within which it is difficult to see the different purposes of each section

Update anomalies where you make changes in some sections but overlook making changes in other sections

If you find yourself writing similar code of three or more lines multiple times, then consider writing a function

For more information see the textbook page 192

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6.3.2: Defining a Function

Function -- a named block of code.

In other languages, a function is known as a method or subroutine.

Once you define a function, you can execute the block of code using the function's name

We have used functions already, such as:

sqrt(x) -- computes the square root of a floating point number

pow(x, y) -- computes the power of x^y

Many functions have input values (a.k.a. arguments or parameters) that are transferred or passed into the function

For example:

The x in: y = sqrt(x);

Both the x and the y in: z = pow(x, y);

An expression as in: sqrt(b * b - 4 * a * c);

Many functions have output or return values

For example the y in: y = sqrt(x);

Both parameters and return values are a particular type

For instance:

You cannot compute: sqrt("Ed Parrish");

You cannot assign: string thing = sqrt(5.5);

Function Syntax
The general syntax for defining a function is:
returnType functionName(parameter1, ..., parametern) {
    statements
}
Where:

returnType: the data type of the value returned

functionName: the name you make up for the function

parameterx: the input values, if any

statements: the statements to execute when the function is called
As an example, we can define our function to draw a square like this:
double readPosNum(String prompt) {
    // Statements to read and validate the input
}
Lets look more details of the syntax in order from left to right
Return Type

Sometimes you will want a function to return a value

If you do, then you need to specify the type of data it will return

If you do not want a function to return a value, then you use the keyword void

In our example the return type is a double

Function Name

Technically, you can use any valid identifier for a function name

However, you should use a name that suggests the action the function performs

For instance, readPosNum suggests that the function will get a positive number from the user

Parameter List

You must have parenthesis after the function name

Inside the parenthesis, you define a list of zero or more parameters

Parameters are like variables except they are declared inside the function parenthesis

Each parameter must have both a type and a name, just like a variable

If you have more than one parameter, you separate each one with commas

A parameter type can be any primitive type or class type, like a variable

A class type is usually followed by an ampersand "&" (we will discuss why later)

Any parameter that you declare must be given a value when you call the function

Code Block

After the parenthesis, you define the block of code that you want to execute

The block starts with an opening curly brace: {

The block ends with a closing curly brace: }

Between the curly braces, you place all the statements you want the function to execute

For readability, you indent the statements within the curly braces

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6.3.3: Example Function Definition

Let us define a function for reading positive numbers

We are taking our previous repeated code and packaging it inside a function

You can see the entire function in the following program

Example Program with a Function for Reading Input
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#include <iostream>
using namespace std;

double readPosNum(string prompt) {
    double input = 0.0;
    do {
        cout << prompt;
        cin >> input;
        if (cin.fail()) {
            cout << "You must enter digits, not words\n";
            cin.clear();
            cin.ignore(INT_MAX, '\n');
            input = -1; // set loop test to fail
        } else if (input <= 0.0) {
            cout << "You must enter a positive number\n";
        }
    } while (input <= 0.0);

    return input;
}

int main() {
    double input = readPosNum("Enter a positive number: ");
    // Process the input
    cout << "You entered: " << input << endl;

    double input2 = readPosNum("Enter another number: ");
    // Process the second input
    cout << "You entered: " << input2 << endl;

    return 0;
}
Note that the function readPosNum() comes before main()

In C++, the compiler requires us to declare a function before we call the function

Since we call function readPosNum() from main(), we must put the function before main()

Also notice that the function readPosNum() is outside of main()

You cannot nest functions

Instead, each function is separate from every other function

Variable Scope, Parameters and Return Statements
A variable declared inside a function can only be used within that function
Local variable: a variable that can only be accessed within a function or block.
For example, we declared input inside the readPosNum() function:
double input = 0.0;
In addition, we declared another variable named input inside main():
double input = readPosNum("Enter a positive number: ");
These are not the same variable even though the have the same name

The area of code that a variable can operate within is known as it's scope
Scope: the enclosing area within which a variable exists
To send information to a function we must use a parameter:
double readPosNum(string prompt)
Within main() we send the information to the parameter as part of the function call:
readPosNum("Enter a positive number: ")
The string, "Enter a positive number: ", is passed to the parameter variable: prompt
To return information from a function, we use a return statement:
return input;
We must save the returned information if we want to it for processing:
double input = readPosNum("Enter a positive number: ");
This is the same process we used for calling sqrt():
y = sqrt(x);
Flow of Control for a Function Call
To use functions well, you must understand their flow of control

In our example, the program starts executing in the main() function
When our program gets to the following statement, it stops executing in main() and jumps to our function:
double input = readPosNum("Enter a positive number: ");
The program executes the statements in the function and then returns to the statement from which it jumped

After returning to the calling statement, the program completes processing the statement and then moves on to the next statement

In our example, the statement saves the returned value in the variable: input

Every time the flow of control reaches a function call, the program:

Temporarily stops executing in the current function

Jumps to the called function and executes the statements of that function

Returns to the point in the code from which it jumped
^ top

Exercise 6.3

In this exercise we define our own function.

Specifications
Copy the following program into a text editor, save it as absvalue.cpp, and then compile the starter code to make sure you copied it correctly.
#include <iostream>
using namespace std;

int main() {
    // Enter your code here

    return 0;
}
Inside the main() function, enter these statements:
cout << "This program calculates absolute values\n";
cout << "Number to calculate: ";
double num = 0.0;
cin >> num;
double absnum = calcAbs(num);
cout << "Absolute value is: " << absnum << endl;
The fifth line contains the function call. For more information on function calls, see section: 6.2.2: Calling a Function.
Try to compile your code. You should see an error message like:
absvalue.cpp:9: error: `calcAbs' undeclared
The reason is that the we have not yet written the calcAbs() function. We must define a function before we can call the function.
Before the main() function, define a function named calcAbs() as shown below:
double calcAbs(double value) {
    // Insert statements here

    return value;
}
Your code should compile at this time. Even though the function is not complete, we have supplied enough information to the compiler to define the calcAbs() function.
Inside the curly braces of the calcAbs() function, write the statements to calculate the absolute value using the following psuedocode:
if value is less than zero return -1 * value
return value
Compile and run your modified program and verify the output looks like:
This program calculates absolute values
Number to calculate: -42
Absolute value is: 42
Also, verify that a positive number works correctly:
This program calculates absolute values
Number to calculate: 42
Absolute value is: 42
In addition to completing the programming code, prepare a second file named trace.txt.
In the trace.txt file, list the line numbers of each statement of your program in the order the lines are executed. For example, if main() starts on line 11, statements are executed as follows:
11, 12, 13, 14, ...
Do not bother to list blank lines or lines containing only the closing curly brace (}) of a function definition.
Review the hand trace with another student in the class. Then add a comment to the top of the file that contains the name of the person with whom you reviewed the code, like:
Reviewed trace with Fred George.
Submit both files absvalue.cpp and trace.txt to Blackboard as part of assignment 6.
Completed Program

When finished, your application should look like the following.

As time permits, read the following sections and be prepared to answer the Check Yourself questions in the section: 6.3.7: Summary.

^ top

6.3.4: Passing Arguments to a Function
Let us look how parameters work in more detail
When a program makes a function call, it sends the data inside the parenthesis to the function:
readPosNum("Enter a positive number: ");
The string, "Enter a positive number: ", is passed to the parameter variable: prompt:
double readPosNum(string prompt)
The function call must include an argument for every parameter

In addition, the data type of each argument must be compatible with the data type of the parameter
Arguments and Parameters
Depending on your programming background, you might use the term arguments or parameters for the values passed into functions

The terminology is not that important
However, the way I will use the terms is:
A called function has parameters
double readPosNum(string prompt) {
    // Commands to execute in the function
}
A caller passes arguments
readPosNum("Enter a positive number: ");
Arguments are values you pass into functions

When the argument drops into a function, it lands in a parameter

The parameter is just like a local variable in the function

Except that a parameter gets initialized by an argument

The important part is:
If a function takes a parameter, you must pass it a value.

Also, the value must be the same type as the parameter
^ top

6.3.5: Returning a Value
Functions that return a value must execute a return statement
return result;
For instance, in our example, function readPosNum() had a return statement
double readPosNum(string prompt) {
    double input = 0.0;
    do {
        cout << prompt;
        cin >> input;
        if (cin.fail()) {
            cout << "You must enter digits, not words\n";
            cin.clear();
            cin.ignore(INT_MAX, '\n');
            input = -1; // set loop test to fail
        } else if (input <= 0.0) {
            cout << "You must enter a positive number\n";
        }
    } while (input <= 0.0);

    return input;
}
We must save the returned information if we want to it for processing:
double input = readPosNum("Enter a positive number: ");
Returning Early

Sometimes you do not want to execute all the code in a function

You can use a return statement to stop execution and return from anywhere

Control returns immediately from a function whenever a return is reached

^ top

6.3.6: Some Style Requirements for Functions

Consider again our example function
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#include <iostream>
using namespace std;

double readPosNum(string prompt) {
    double input = 0.0;
    do {
        cout << prompt;
        cin >> input;
        if (cin.fail()) {
            cout << "You must enter digits, not words\n";
            cin.clear();
            cin.ignore(INT_MAX, '\n');
            input = -1; // set loop test to fail
        } else if (input <= 0.0) {
            cout << "You must enter a positive number\n";
        }
    } while (input <= 0.0);

    return input;
}

int main() {
    double input = readPosNum("Enter a positive number: ");
    // Process the input
    cout << "You entered: " << input << endl;

    double input2 = readPosNum("Enter another number: ");
    // Process the second input
    cout << "You entered: " << input2 << endl;

    return 0;
}
Note the placement of the curly braces
There are two common styles of curly brace placement for functions:
Place the opening brace on the same line as the function heading:
void myFunction() {
    // statements of the function
}
Place the opening brace under and lined up with the first letter of the return type:
void myFunction()
{
    // statements of the function
}
You can use either style as long as you are consistent

Also notice the indentation of the statements inside the function

As before, you always indent 3-4 more spaces after an opening curly brace

After the closing curly brace, you no longer indent the extra 3-4 spaces

Indenting makes it easier to see the block of code
^ top

6.3.7: Summary
When we reuse a block of code in various parts of a program, we should put the code into a function
To define a function, we use the following syntax:
returnType functionName(parameter1, ..., parametern) {
    statements
}
If the function does not return a value, we use the return type: void

Otherwise, we specify the type of data we want to return

When we define a function, we want it to be reusable

To make a function more reusable, we avoid hard-wiring important values

Instead, we pass the key values by defining parameters

When we call the function, we supply an argument for each parameter
Check Yourself

Answer these questions to check your understanding. You can find more information by following the links after the question.

Why do programmers write functions? (6.3.1)

What is a function? (6.3.2)

What are the four main parts of a function? (6.3.2)

How can you tell the difference between a function name and a variable name? (6.3.2)

How many parameters can you declare for a function? (6.3.2)

How is a variable declaration different than a parameter? (6.3.2)

If you declare a variable inside main() can you access that variable inside another function? (6.3.3)

Why do functions need parameters? (6.3.3)

If you call a function in main(), do you place the function definition before or after main()? (6.3.3)

What happens to the flow of control when a function call is reached? (6.3.3)

If you declare three parameters, how many arguments must you include in a function call? (6.3.4)

Which of the following is a function definition and which is a function call? (6.3.4)

void drawSquare(Turtle& turtle, double side) { }

drawSquare(myrtle, 7.5);

What is the difference between an argument and a parameter? (6.3.4)

What statement is used to return values from functions? (6.3.5)

True or false? You should indent code within a function. (6.3.6)

^ top

6.4: Midterm Preparation

Learner Outcomes

At the end of the lesson the student will be able to:

Discuss how to prepare for the midterm exam

Describe how to take the midterm exam

^ top

6.4.1: About the Exam

You must attend the exam or you will receive a score of zero (0)

Except by prior arrangement with the instructor

I am using Blackboard to administer the test

Since there are more students than computers, we must take the exams in shifts

Shifts are determined by your last name and section as follows:

Last Name Section Start Time

A-L Day 10:20 AM

M-Z Day 11:20 AM

A-K Evening 6:00 PM

L-Z Evening 8:00 PM

If you want to change shifts then make arrangements to switch with another student who is on the other shift

The exam is closed books and closed notes

However, you may have one 3" x 5" card of handwritten notes for the exam

You may use a computer from the classroom, but only to take the exam in Blackboard

You may have a sheet of blank scratch paper

You may NOT use the computer to compile or run programs

You may NOT use the computer to view documents on the Internet

You may NOT use a calculator or other electronic device

Thus, you may NOT use your own computer to take the exam

If you have a cell phone visible or in use during the exam, you will automatically fail

You may NOT communicate with anyone but the instructor during the exam

3"x5" Card Requirements

Put your name on your card

Maximum card or paper size is 3 inches by 5 inches

You may use both sides of the card

Notes must be handwritten and NOT photocopied

Notes cannot have any complete functions -- only code snippets

Any 3" x 5" cards violating these rules will be confiscated before the test

You must turn in your 3" x 5" card after the exam in any case

^ top

6.4.2: Recommended Preparation

Try the Example Midterm in Blackboard

These questions are intended to help you get a "feel" for taking an exam in Blackboard

The questions are NOT intended to tell you everything that is on the exam

Review the instructor's notes and make sure you can answer the Check Yourself questions, making notes about anything that you have difficulty answering

Review your notes and prepare your 3" x 5" card

Review your homework assignments and solutions

Review your CodeLab exercises

You should be prepared to write short programs using:

Mathematical expressions

User I/O

Conditional statements

Testing multiple conditions

"main" loops

Counting loop algorithms such as accumulating values in a loop

Using library functions (pow(), rand(), sqrt(), etc.)

Declaring and defining functions

More quiz tips: Basic Rules For Taking a Multiple-Choice Test

^ top

6.4.3: Exam Taking Tips

Save after every answer -- you can change your mind and choose another

If you get stuck on a question, make your best guess and return later

If you are equally uncertain between two choices, go with first impression

When writing code, do NOT add more than the problem asks for

You do not need to comment code for tests and exams

Unless specifically instructed to in the exam question

Use the full time available

Check your work if you finish early

^ top

6.4.4: Questions and Answers

Any questions?

^ top

Wrap Up

Due Next:
A5-Loopy Programs (3/18/10)
A6-Programs With Functions (4/8/10)

When class is over, please shut down your computer

You may complete unfinished exercises at the end of the class or at any time before the next class.

^ top
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Syllabus | Help | FAQ's | HowTo's | Links Last Updated: March 20 2010 @20:09:42

What We Will Cover

Continuations

Questions from last class?

Homework Questions?

Homework Discussion Questions (Thursday)

6.1: More About Strings and Characters

Learner Outcomes

6.1.1: Strings Versus Characters

6.1.2: Indexing a String

6.1.3: Iterating Strings

Using unsigned

6.1.4: Converting Characters to Digits

Exercise 6.1

Specifications

Program that Processes Strings with Loops

6.1.5: String Input With Spaces

Input Using getline()

The Problem with Newlines

Example Using cin >> ws

6.1.6: Processing Text Input

Example Program that Reverses a Sentence

Redirection of Input and Output

6.1.7: Summary

Check Yourself

6.2: Predefined Functions

Objectives

6.2.1: About Functions

6.2.2: Calling a Function

Flow of Control for a Function Call

Arguments and Returned Values

Example Program with a Function Call

6.2.3: Generating Random Numbers

Example Program Using rand()

Another Random Number Generator

6.2.4: Seeding the Random Generator

Exercise 6.2

Specifications

6.2.5: Simulating Dice

Example Program Simulating the Rolling of a Pair of Dice

6.2.6: Flotaing-Point Random Numbers

6.2.7: Summary

Check Yourself

6.3: Coding Functions

Learner Outcomes

6.3.1: Repeated Code

Programming Style: Avoid Duplicating Code

6.3.2: Defining a Function

Function Syntax

Return Type

Function Name

Parameter List

Code Block

6.3.3: Example Function Definition

Example Program with a Function for Reading Input

Variable Scope, Parameters and Return Statements

Flow of Control for a Function Call

Exercise 6.3

Specifications

Completed Program

6.3.4: Passing Arguments to a Function

Arguments and Parameters

6.3.5: Returning a Value

Returning Early

6.3.6: Some Style Requirements for Functions

6.3.7: Summary

Check Yourself

6.4: Midterm Preparation

Learner Outcomes

6.4.1: About the Exam

3"x5" Card Requirements

6.4.2: Recommended Preparation

6.4.3: Exam Taking Tips

6.4.4: Questions and Answers

Wrap Up

Input Using `getline()`

Example Using `cin >> ws`

Example Program Using `rand()`