2: Programming Essentials

What We Will Cover Announcements 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 any data or program files are lost. Continuations 2.1: Starting Out 2.1.1: Comments 2.1.2: Statements 2.1.3: Identifiers 2.1.4: Libraries and Namespaces 2.1.5: Declaring a main function 2.1.6: Summary Exercise 2.1 2.2: Data and Data Types 2.2.1: Bits and Bytes 2.2.2: Primitive Data Types 2.2.3: Integers 2.2.4: Floating-Point Numbers 2.2.5: Characters 2.2.6: Boolean Values 2.2.7: Summary Exercise 2.2 2.3: Arithmetic 2.3.1: Arithmetic Operators 2.3.2: Mixed-Mode Expressions 2.3.3: Integer Division and Modulus 2.3.4: Arithmetic Precision and Range 2.3.5: Operator Precedence and Associativity 2.3.6: Mathematical Functions 2.3.7: Summary Exercise 2.3 2.4: Variables and Declarations 2.4.1: Memory Addresses and Data Storage 2.4.2: Declaring Variables 2.4.3: Assigning Values to Variables 2.4.4: Assignment Variations 2.4.5: Type Casting 2.4.6: Summary Exercise 2.4 Wrap Up Due Next: A1: Information Please! (9/9/04) Exercise 1 and CodeLab Lesson 1 (9/9/04) A2: Compiling Expressions (9/16/04) Exercise 2 and CodeLab Lesson 2 (9/16/04) Continuations Homework Questions? Nothing Questions from last class? What to do when WebCT has problems Why should you use the compiler options -W, -Wall and --pedantic? 1. To display error messages. 2. To compile the code. 3. To display extra warning messages. 4. To name the output file. ^ top 2.1: Basic Coding Skills Objectives At the end of the lesson the student will be able to: Write appropriate comments in programs List the rules for creating an identifier Identify statements in programs Include libraries and use namespaces in programs Code main functions ^ top 2.1.1: Comments Comments are ... comments -- notes to people reading the code Comments ignored by compiler Use comments to document and describe code Comments can start with // and last to end of the line // this is a comment Comments can span multiple lines: /* ... */ /* This is a multi-line comment which can be split over many lines or a portion of one line. */ Programming Style: Block Comments Use block comments like the following at the start of a program file /** * CS-11 Asn 0 * hello.cpp * Purpose: Prints a message to the screen. * * @author Jane User * @version 1.0 8/20/03 */ ^ top 2.1.2: Statements Statements direct the operation of the program Most statements end in a semicolon (;) cout << "Hello, world!"; However, statements requiring a set of braces {} end with the right brace int main() { cout << "Hello, world!"; } Note that we can have multiple statements within a pair of curly braces Programming Style: Line Length Limit your line length to 80 characters Longer lines can cause problems with many text editors and other tools ^ top 2.1.3: Identifiers Identifier - the name of a variable, procedure, class or other programming construct. Purpose of an identifier is to provide a name for something Programmers create identifiers for the names of many things in a program: variables functions files classes Identifiers are made up of a sequence of characters Each character can be a: Letter Digit Underscore: _ Dollar sign: $ Can not start with a digit Can not contain any periods or spaces Can not be a keyword (e.g. "if, "for", etc.: see C/C++ Keywords) Are cAsE sEnSiTiVe! id, ID, iD and Id are all valid but different identifiers Programming style: identifiers have naming conventions You must follow these naming conventions on your homework Keywords Keywords are reserved words with predefined meanings We used keywords in our hello.cpp program int using namespace #include <iostream> using namespace std; int main() { cout << "Hello, World!\n"; return 0; } All keywords have special meaning in C++ Can only be used for their specified purpose Attempting to use for any other purpose will generate a compiler error Thus cannot use keywords as identifiers For a current list, see C/C++ Keywords Note that the entire C++ language has only about 62 keywords Part of learning a language is learning what the words mean ^ top 2.1.4: Libraries and Namespaces C++ has a number of standard libraries These libraries place their definitions in a namespace Libraries and include Directives Use the include directive to add the contents of a library to your program #include <libraryName> Called a ‘preprocessor directive’ Executes before compiling, and ‘copies’ a library into your program file Most of our programs begin with a declaration like: #include <iostream> Which is a library for console I/O Other libraries exist for math, strings and more Note that some compilers are picky about spaces in include directives Do not put spaces before or after the # sign Do not put spaces inside the angle brackets Namespaces Namespace: a collection of name definitions You can only use a name once within a namespace All standard libraries put their definition in the namespace: std To use these standard names, you need the directive: using namespace std; Thus, most of your programs will begin with two declarations: #include <iostream> using namespace std; ^ top 2.1.5: Declaring a main Function Function - a block of code that performs a task Every C++ program has one or more functions Every C++ application has a main function that is declared like this: int main() { // function definition goes here } Programs begin executing at the main function int means main returns a value of type int -- cover later For now, mimic the first line of main Left brace { begins body of every function Right brace } ends body of every function Last statement in main should be a return statement return 0; ^ top 2.1.6: Summary C++ has two styles of comments // /* ... */ Comments help document what a program does Use block comments at beginning of file and before functions Otherwise, use them sparingly Statements direct what a program does Identifiers are any name you create in a C++ program Keyword is a special word reserved by C++ Cannot use a reserved word as an identifier C++ has standard libraries of prewritten code Definitions for these libraries are collected in a namespace called: std Thus, most of your programs will begin with two declarations: #include <iostream> using namespace std; Every C++ application starts with a function named main ^ top Exercise 2.1 With a single partner, if needed, take 5 minutes to complete the following: Start a text file named exercise2.txt. Prepare the exercise header as described in the HowTo on submitting exercises Label this exercise: Exercise 2.1 Submit all exercises for today's lesson in one file unless instructed otherwise Complete the following and record the answers to any questions in exercise2.txt. Exercises and Questions Start your text editor and enter the following code: #include <iostream> using namespace std; int main() { cout << "Hello, World!\n"; return 0; } Save the file as "hello.cpp". Q1: Which lines of code contain an identifier? Compile and execute the code. g++ -W -Wall --pedantic -o hello hello.cpp Q2: Which lines of code end a statement? How can you tell? Add a block comment as shown in section 2.1.1 Re-compile and execute the code. Q3: How does the added block comment change the execution of the code? Remove the directive: using namespace std; and then re-compile the code. Q4: What message does the compiler report? ^ top 2.2: Data and Data Types Objectives At the end of the lesson the student will be able to: Describe the C++ primitive data types Distinguish between an integer, a floating-point number, and a boolean value Decide which data type is appropriate for representing data ^ top 2.2.1: Bits and Bytes "There are only 10 kinds of people in the world, those who understand binary and those who don't." -- Unknown Bit - (Binary Digit) smallest unit of information within a computer Bit has two values: 0 and 1 How do we represent the value "2" in binary? Byte - a group of 8 bits Usually the smallest addressable amount of memory A byte has 256 possible combinations of values -- Why 256? What is the binary number represented by the picture above? What is the decimal equivalent? How do you think computers store the following data? 12 512 1.23 3.14159... The letter 'A' The boolean values true or false More Information Binary Number System: More information on binary numbers Binary numeral system: Wikipedia entry on binary numbers Floating point: Wikipedia entry on floating-point numbers ASCII: Wikipedia entry on ASCII The ASCII Character Set: Appendix 3 of the textbook ^ top 2.2.2: Primitive Data Types C++ has built-in data types for numbers, characters and booleans These built-in types are called primitive types A data type tells a computer how to interpret the data C++ can recognize literal values for each primitive type Literal values are a sequence of characters in a certain form General Type Explanation Examples Integers Numbers without decimal points 123 -987 Floating-Point Numbers with decimal points 1.23 -0.01 Characters Single letters, digits and special symbols 'A' '9' Booleans Logical values true or false true false Since these literal values are compiled into our programs, they cannot be changed ^ top 2.2.3: Integers An integer is zero or any positive or negative number without a decimal point For example: 0 1 -1 +5 -27 1000 -128 Integer literals may have a sign (+ or -) before the number Cannot have any commas, decimal points or special symbols (like $) Integer Data Types Some C++ integer data types are: Type Name Memory Size Range of Values short 2 bytes -32768 to 32767 int 4 bytes -2,147,483,647 to 2,147,483,647 long 4 bytes -2,147,483,647 to 2,147,483,647 In C++, you may add these modifiers to the type: signed unsigned short long ^ top 2.2.4: Floating-Point Numbers A floating-point literal is any signed or unsigned number with a decimal point For example: 0.0 1.0 -1.1 +5. -6.3 3234.56 0.33 Note that 0.0, 1.0 and +5. are floating-point numbers, but could be rewritten as integers Like integers, cannot have any commas or special symbols Floating-Point Data Types Some C++ floating-point data types are: Type Name Memory Size Range of Values float 4 bytes about ±1.4 x 10-45 to ±3.4 x 1038 double 8 bytes about ±3.4 x 10-308 to ±1.7 x 10308 Exponential Notation Floating-point literals can be written in exponential notation Similar to scientific notation Used to express both very large and very small values in compact form For example: Decimal Notation Exponential Notation Scientific Notation 1234 1.234E3 1.234 x 103 98765 9.87654E4 9.8765 x 104 0.0123 1.23E-2 1.23 x 10-2 0.000625 6.25E-4 6.25 x 10-4 Letter E stands for exponent Number following E represents a power of 10 Indicates the number of decimal places to move for standard decimal value How Large is 1.7E308? Largest possible double is 17 followed by 307 zeros How large is that? Current estimate of the number of atoms in the universe: about 1.0E78 Mathematicians use the term "google" for a very large number: 1.0E100 Data type double easily encompasses these numbers What value cannot be represented by type double? ^ top 2.2.5: Characters A character is a letter, number or special symbol For example: 'a' 'b' 'Z' '3' 'q' '$' '*' C++ provides the char data type to represent characters Stores characters as an 8-bit unsigned value using ASCII Escape Sequences First 32 character codes are not visible on our monitors Represent control codes C++ can access some of the control codes using escape sequences Backslash (\) directly in front of a certain character tells the compiler to escape from the normal interpretation Following table has some nonprinting and hard-to-print characters: Sequence Meaning \a Alert (sound alert noise) \b Backspace \f Form feed \n New line \r Carriage return \t Horizontal tab \\ Backslash \" Double quote \' Single quote Some examples: cout << '\a' << endl; // alert cout << '\n' << endl; cout << "Left \t Right" << endl; cout << "one\ntwo\nthree" << endl; Programming Style No tab characters in your source code ^ top 2.2.6: Boolean Values Boolean types have just one of two values: either true or false Mostly used in conditional statements -- will cover later in course cout << true << endl; cout << false << endl; cout << boolalpha; // show output as true or false cout << true << endl; cout << false << endl; ^ top 2.2.7: Summary Computers store data in units of 8 bits called a byte Some primitive C++ data types are: Type Bytes Use char 1 All ASCII characters. short 2 Short integers from -32,768 to 32,767. int 4 Integers from -2,147,483,647 to 2,147,483,647. long 4 Integers from -2,147,483,647 to 2,147,483,647. float 4 Single-precision, floating-point numbers from about E-45 to E38 with 6 or 7 significant digits. double 8 Double-precision, floating-point numbers from E-308 to E308 with from 14 to 15 significant digits. bool 1 A true or false value. Note that number of bytes used for storage depends on the system and compiler ^ top Exercise 2.2 Determine the data types that are appropriate for the following problems and record your answers in exercise2.txt: The average number of four grades The number of days in a month The length of the Golden Gate Bridge The numbers in the state lottery ^ top 2.3: Arithmetic Objectives At the end of the lesson the student will be able to: Use arithmetic operators Perform integer division and use the modulus operator Translate algebraic expressions into C++ code ^ top 2.3.1: Arithmetic Operators Can perform arithmetic on integers and floating-point numbers Use special symbols to perform the operations: + for addition - for subtraction * for multiplication / for division % for modulus (remainder) For example: 3 + 4 12 - 7 12.3 + 4.56 .065 * 1200 18 / 2 17.4 / 2.3 These combination of numbers and operators are called arithmetic expressions Expressions are parts of a program that return a value We can use these expressions to perform arithmetic and display the results cout << 3 + 4; // prints 7 Negation Operators +, -, *, /, % are called binary operators Always need two operands (numbers) to work with them C++ also provides a unary operator: - (minus sign) Reverses the sign of a number For example: -1 -5.3 - -1 C++ also provides a plus sign (+) unary operator Generally not used ^ top 2.3.2: Mixed-Mode Expressions Recall the different data types are stored in different forms Computer needs both operands in the same form before it can perform an operation If one operand is different than the other, the compiler converts it to a wider type For example: 2 + 2.2 First number (2) is an int Second number (2.2) is a double First number is narrowest, thus gets promoted to a double (i.e. 2.0) Then the arithmetic operation can take place to produce a result of 4.2 ^ top 2.3.3: Integer Division and Modulus Dividing two integers can produce unexpected results for the unwary For example, what is the result of the following expression? 7 / 2 Since both numbers are of type int, one does not get a floating-point result Remainder portion is truncated (discarded, thrown away) Modulus Operator Sometimes you want to know the remainder of an integer division C++ provides the modulus operator: % (percent sign) Returns the remainder of the division of first number by the second For example: 7 % 2 // returns 1 7 % 2 returns 1 because 1 is the remainder when 7 is divided by 2 Check Ourselves What is the result of the following arithmetic operations? 9 / 4 17 / 3 14 / 2 9 % 4 17 % 3 14 % 2 ^ top 2.3.4: Arithmetic Precision and Range There are only a finite set of numbers in numerical data types Can lead to problems for the unwary Integer Overflow What happens when an integer is too big for its type? cout << "Big number: "; cout << 2147483647 + 1 << endl; cout << "Small number: "; cout << -2147483647 - 2 << endl; Wraps around from the highest number to the lowest Must be careful that your program will not go beyond the range of its data types Floating-Point Precision and Range Floating-point numbers are not exact representations of real numbers Rounding errors occur in repeated calculations Type double has about twice the precision of type float However, even type double can have rounding errors cout.precision(17); cout << .8F + .1F << endl; cout << .8 + .1 << endl; When floating point numbers get too large, they are set to inf cout << 2E38F + 2E38F << endl; Similarly, when too small they are set to 0.0 ^ top 2.3.5: Operator Precedence and Associativity Often need to process expressions of greater complexity than two numbers May need more than one operator C++ has certain rules that must be followed C++ Rules Two binary operators cannot be placed side by side Invalid expression: 5 * % 6 Operators * and % cannot be next to each other Parenthesis can be used to group expressions Anything within parenthesis is evaluated first Can have parenthesis within parenthesis Innermost parenthesis evaluated first Parenthesis cannot be used to indicate multiplication Must use the * operator Programming style: add spaces around operators 2 + 3, not 2+3 Programming style: no spacing after opening or before closing parenthesis (2 / 3), not ( 2/3 ) Precedence and Associativity Rules Arithmetic operators processed in algebraic order: Parenthesis: ( ) Unary operators: +, - Multiplication, division, modulus: *, /, % Addition, subtraction: +, - Binary operators of same precedence evaluated from left to right Examples of Expressions Algebra Expression C++ Expression Fully Parenthesized 2(10 + 5) 2 * (10 + 5) (2 * (10 + 5)) 1 12.2 + 3 · 7.3 1 / (12.2 + 3 * 7.3) (1 / (12.2 + (3 * 7.3))) 10 - 7 3.3 + 9 · 1.6 (10 - 7) / (3.3 + 9 * 1.6) ((10 - 7) / (3.3 +(9 * 1.6))) 2 · 42 2 * 4 * 4 ((2 * 4) * 4) ^ top 2.3.6: Mathematical functions Only the simplest mathematical operations are provided by operators For more complex operations, you use mathematical functions cout << sqrt(9.0) << endl; C++ provides a standard cmath library that contains many such functions Name Description Example Result abs absolute value abs(-3.9) abs(3.9) 3.9 3.9 exp exponent exp(1.0) 2.718... log natural log log(10.0) 2.40... pow powers pow(2.0, 3.0) 8.0 sqrt square root sqrt(4.0) 2.0 In addition, it includes two similar functions: ceil, and floor Name Description Example Result ceil ceiling: round up ceil(3.3) ceil(3.7) 4.0 4.0 floor floor: round down floor(3.3) floor(3.7) 3.0 3.0 Both return whole numbers, although they are of type double To use the cmath library, you must include it in your source code file #include <cmath> using namespace std; ^ top 2.3.7: Summary C++ uses the following operators for arithmetic + for addition - for subtraction * for multiplication / for division % for modulus (remainder) Arithmetic expressions are combinations of numbers and operators Results of integer division are truncated Must use modulus operator (%) to get the remainder value The dash (minus sign) is used for negation Operators have the same precedence as in algebra Parenthesis: ( ) Unary operators: +, - Multiplication, division, modulus: *, /, % Addition, subtraction: +, - Multiplication operator always required, unlike algebra More complex mathematical operations are available using the cmath library ^ top Exercise 2.3 Through the miracles of modern computer technology, we will now convert your $1000 computer into a $10 calculator! Copy the following program into a text editor; compile and run the program. #include <iostream> using namespace std; int main() { cout << "Hello, World!\n"; return 0; } Replace the "Hello, World!" portion of the program with the arithmetic expression 7 + 2 and then recompile and run the program. Q1: What value was displayed by the computer? Use the expression 7 / 2 and then recompile and run the program. Q2: What value was displayed by the computer? Why? Change the expression to use 7 % 2 and then recompile and run the program. Q3: What value was displayed by the computer? Why? The following are algebraic expressions and an incorrect translation to code. Find the errors and write corrected code. Save the corrected expressions in exercise2.txt. Algebraic Expression Incorrect Conversion (2)(3) + (4)(5) (2)(3) + (4)(5) 6 + 18 2 6 + 18 / 2 5 9 (100 - 32) 5 / 9 * (100 - 32) 2 · 42 2 * 4 ^ 2 ^ top 2.4: Variables and Declarations Objectives At the end of the lesson the student will be able to: Use declaration statements to declare variables Assign values to variables ^ top 2.4.1: Memory Addresses and Data Storage Computer data is stored in and retrieved from memory Before high-level languages, memory was referred to by its address Storing integers 45 and 12 would allocate memory like the following Quickly becomes cumbersome to write such programs Have to manually keep track of each location Have to make sure locations do not overlap More intuitive to assign each variable a name Let the computer remember the actual memory location Then each memory location can be referred to by name Now our programs can look like the following: num1 = 45; num2 = 12; total = num1 + num2; Each of these statements is called an assignment statement Tells the computer to assign (store) a value into a variable Assignment statements always have a variable name followed by an equal (=) sign To the right of the equals sign is an expression that yields a value ^ top 2.4.2: Declaring Variables Declaration statement both names a variable and specifies the data type it can store All C++ program variables must be declared before using them General syntax: dataType VariableName1, VariableName2, ...; For example: int num1, num2, total; long dateNum; float firstNum; double secNum; char letter; When variables are declared, computer allocates storage space Amount of storage space depends on the data type Contents of the storage space is undefined until a value is assigned Type Bytes Use char 1 All ASCII characters. short 2 Short integers from -32,768 to 32,767. int 4 Integers from -2,147,483,647 to 2,147,483,647. long 4 Integers from -2,147,483,647 to 2,147,483,647. float 4 Single-precision, floating-point numbers from about E-45 to E38 with 6 or 7 significant digits. double 8 Double-precision, floating-point numbers from E-308 to E308 with from 14 to 15 significant digits. bool 1 A true or false value. Programming Style: Variable Naming Conventions Use meaningful names that are easy to remember as you code. Two commonly-used styles you may use Start with a lower-case letter and use uppercase letters as separators. Do not use underbars ('_'). int myVar Use all lower case letters and use underbars ('_') as separators. int my_var You must be consistent and only use one of them in a program. The instructor's preference is the first style. ^ top 2.4.3: Assigning Values to Variables Use the assignment operator "equals sign" (=) variable = expression; Assigns value of expression (right side) to the variable (left side) The simplest expression is a literal value: length = 25; width = 17.5; In each statement, value on right is assigned to the variable on the left Can also assign results of more complex expressions to a variable total = num1 + num2; slope = (y2 - y1) / (x2 - x1); Assigning Initial Values to Variables Initial values may or may not be assigned when variables are declared: // These are not initialized when declared // and have unknown values int sum, number1, number2; // These are initialized when declared int sum = 0; int number1 = 5, number2 = 10; Good programming practice: initialize variables when declared Programming Style: Constant Variables and Magic Numbers The meaning of literal numbers is hard to remember Sometimes called "magic" numbers" Magic because no one has a clue what it means after 3 months, including the author A better approach is to assign the number to a descriptive constant variable C++ uses the keyword const to indicate a variable that cannot change const int MY_CONST = 1; Note that the name is all uppercase letters with an underscore separator This is a common coding convention that you must follow ^ top 2.4.4: Assignment Variations You can have the same variable on both sides of an equals sign int sum = 25; // initialize sum to 25 sum = sum + 10; // add to sum Note that the value of the variable changed in the second line Reading variables from memory does not change them Values placed into a variable replace (overwrite) previous values Abbreviated Assignment Expressions Any statement of form: variable = variable <op> (expression); May be written in an abbreviated form: variable <op>= expression; <op> is one of: +, -, *, /, or % For example: sum = sum + 10; Can be written as: sum += 10; Incrementing and Decrementing Adding 1 to a number occurs frequently when programming count = count + 1; You can write this in an abbreviated form like: count += 1; Since it is so common, programmers wanted an even shorter form: count++; This is known as incrementing a variable Similarly, you can decrement a variable count--; ^ top 2.4.5: Type Casting Cast: change the data type of the returned value of an expression Recall that different data types are stored in different forms Sometimes need to change from one form to another Example: arithmetic adding a double and an int value C++ will automatically cast one value to another Also known as implicit casting May not know how to convert some values May not cast how the programmer wants Programmers can also explicitly cast data types Explicit casting changes the data type for a single use of the variable Precede the variable name with the new data type in parentheses: (<dataType>) variableName Data type is changed only for the single use of the returned value For example: int n; double x = 2.0; n = (int) x; Value of x is converted from type double to integer before assigning the value to n New Style Casting C++ has many styles of casting Newer style of casting is of the form: static_cast<toType>(expression) For example: int x = 9; int y = 2; double ans = x / static_cast<double>(y) Casting a char to an int Casting a char value to int produces the ASCII value For example, what would the following display? char answer = 'y'; cout << answer; cout << (int) answer; Answer: >y >89 Truncation When Casting Floating-Point to Integer type Converting (casting) a floating-point to integer type does not round -- it truncates Fractional part is lost (discarded, ignored, thrown away) For example: int n; double x = 3.99999; n = (int) x; // x truncated Value of n is 3 ^ top 2.4.6: Summary Variables store a value that can change as a program executes. Constants store a value that cannot change. Variables and constants must be declared before use. To initialize a variable or constant, declare a name and assign a value. Simple assignment statements have a variable, equals sign and an expression. variable = expression; The expression is computed before the assignment C++ has assignment variations of the form: variable <op>= expression; C++ will automatically cast data of one type to another when necessary You can explicitly cast values of an expression to a different type. You must use an explicit cast when C++ does not perform as you want. ^ top Exercise 2.4 Recall the data that we explored in previous exercises: The average number of four grades The number of days in a month The length of the Golden Gate Bridge The numbers in the state lottery Write a program for one of these that: Declares one or more variables Initializes the variable(s) with any value you like Prints the variable(s) to the screen Submit your code along with exercise2.txt as a separate file. ^ top Wrap Up Reminders Due Next: A1: Information Please! (9/9/04) Exercise 1 and CodeLab Lesson 1 (9/9/04) A2: Compiling Expressions (9/16/04) Exercise 2 and CodeLab Lesson 2 (9/16/04) 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. Instructions on submitting exercises are available from the HowTo's page. ^ top Home | WebCT | Announcements | Day Schedule | Eve Schedule Course info | Help | FAQ's | HowTo's | Links Last Updated: October 20 2004 @11:41:46