11: Strings

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 11.1: Using C-Strings 11.1.1: About C-Strings 11.1.2: Declaring and Initializing C-Strings 11.1.3: Performing I/O on C-Strings 11.1.4: Iterating C-Strings 11.1.5: Using C-String Library Functions 11.1.6: C-Strings as Arguments and Parameters 11.1.7: Converting C-Strings to Numbers 11.1.8: Summary Exercise 11.1 11.2: String Class 11.2.1: Using Strings: A Review 11.2.2: Constructors of Class string 11.2.3: Indexing a String 11.2.4: Iterating Strings 11.2.5: Finding Substrings 11.2.6: Erasing and Inserting Substrings 11.2.7: Summary Exercise 11.2 11.3: Handling String Input (Optional) 11.3.1: Problems With cin 11.3.2: Using getline() 11.3.3: Converting to Numbers 11.3.4: Checking for Errors 11.3.5: No More cin 11.3.6: Summary Exercise 11.3 Wrap Up Due Next: A10: Working with Arrays (11/17/05) Exercise 10 and CodeLab 10 (11/17/05) Exercise 11 (12/1/05) Sampler Project (12/8/05) Continuations Homework Questions? A10: Working with Arrays (11/17/05) Exercise 10 and CodeLab 10 (11/17/05) Questions from last class? What is the code to declare and allocate and array or characters named shortString that is initialized to the values 'a', 'b' and 'c'? What to take next? Schedule of Classes Computer Information Systems Computer Science After completing CS-11 you are qualified to take either CS-19 or cs-20J CS-19: C++ programming and software design methodologies CS-20J: Java programming and software design methodologies Each course has a initial review section such that you could learn the syntax with a little effort Other computer-related courses on which you can use your programming skills CIS 130: UNIX/Linux Shell Programming CIS-160GP: Game Programming in Java CIS-165PH: Introduction to Programming Database-Driven Web Sites with PHP ^ top 11.1: Using C-Strings Objectives At the end of the lesson the student will be able to: Initialize C-strings Use C-string library functions Perform I/O on C-strings Convert C-strings to numbers ^ top 11.1.1: About C-Strings Previously we discussed arrays, including arrays of type char C++ offers a special kind of char array known as a C-string Included from the C programming language Many library functions use C-strings Thus we still need to use C-strings How C-Strings Work C-strings are an array of char values Each char is stored in the array with a special null character '\0' ending the string The null character is a control character like '\n' You cannot actually view the character For example, a char-array of size 10 used to store a C-string would look like: ^ top 11.1.2: Declaring and Initializing C-Strings You can declare and allocate a C-string like a char array but with one important difference: char arrayName[MaximumCStringSize + 1]; The "+ 1" reserves the additional space needed by '\0' You can initialize a C-string using double quotes For example: char s[10] = "Hi Mom!"; cout << s << endl; Note that declaring a C-string as char s[10] creates room for only 9 char's The null character terminator requires one space The size variable is actually optional for a C-string For example: char shortString[] = "abc"; cout << shortString << endl; This will create a C-string of size 4 terminated with a '\0' What is wrong with the following declaration? char shortString[] = {'a', 'b', 'c'}; cout << shortString << endl; ^ top 11.1.3: Performing I/O on C-Strings C-strings can be output with the insertion << operator For example: char news[] = "C-strings"; cout << news << " Wow." << endl; The extraction operator >> can fill a C-string For example: char a[80], b[80]; cout << "Enter input: " << endl; cin >> a >> b; cout << a << b << "End of Output"; Note that whitespace ends reading of the data To read an entire line you can use function getline() of the object cin Syntax: cin.getline(cString, maxSize + 1); cin.getline() has two arguments The first is a C-string variable to store input data The second specifies the maximum number of char's to read For example: char line[80]; cout << "Enter a line of input:\n"; cin.getline(line, 80); cout << line << "END OF OUTPUT\n"; cin.getline() stops reading after the maximum number of char's - 1 is read one character is reserved for the null character cin.getline() stops even if the end of the line has not been reached C-string input and output works the same way with file streams inStream >> cString; inStream.getline(cString, 80); Similarly, you can use an output file stream like you use cout outStream << cString; ^ top 11.1.4: Iterating C-Strings You can iterate through a C-string like you can for a char array For example: const int SIZE = 10; char shortString[SIZE] = "Hi Mom!"; int index = 0; while (shortString[index] != '\0') { shortString[index] = 'X'; index++; } cout << shortString << endl; You must be careful not to replace the null character ('\0') If the null character is lost, the array cannot act like a C-string What could you change to still process the array correctly in case the null character was missing? ^ top 11.1.5: Using C-String Library Functions Note that the following statement produces a compiler error: char aString[11]; aString = "Hello"; The assignment operator does not work with C-strings Instead, programmers often use library function strcpy() to assign characters Defined in the cstring library Included automatically by our compiler For example: char aString[11]; strcpy (aString, "Hi Mom!"); cout << aString << endl; Function strcpy() automatically appends the '\0' character to the C-string Note that strcpy() does not know the length of the first char array Thus, strcpy() can write characters beyond the declared size of the array Many versions of C++ have a safer version of strcpy() named strncpy() strncpy() uses a third argument for the maximum number of char's to copy For example: char anotherString[10]; strncpy(anotherString, aString, 9); cout << anotherString << endl; Copies up to 9 characters into another_string, leaving one space for '\0' However, it does not put the '\0' in the array if it stops copying before the end of the string The == operator does not work as you might expect with C-strings You only compare the base address of the two arrays Instead, programmers use strcmp() to compare the content of C-strings For example: #include <cstring> if (strcmp(cString1, cString2)) { cout << "Strings are not the same."; } else { cout << "String are the same."; } strcmp() compares the char codes in the two C-strings one char at a time As soon as the characters do not match: Returns a negative value if the code in the first parameter is less Returns a positive value if the code in the second parameter is less If the two C-strings are the same, strcmp() returns 0 strlen() is another cstring library function that returns the number of characters in a string int x = strlen(aString); strcat() concatenates two C-strings The second argument is added to the end of the first The result is placed in the first argument For example: char stringVar[20] = "The rain"; strcat(stringVar, "in Spain"); Now string_var contains "The rainin Spain" strncat() is a safer version of strcat() A third parameter limits the number of characters to concatenate For example: char string_var[20] = "The rain"; strncat(string_var, "in Spain", 11); Further Information Further information: Standard C String & Character ^ top 11.1.6: C-Strings as Arguments and Parameters C-string variables are arrays C-string arguments and parameters are used just like arrays Like other arrays, be careful with the array size If a function changes the value of a C-string parameter Best to include a parameter for the declared size of the C-string If a function does not change the value of a C-string parameter You can use the null character to detect the end of the string No size argument is needed ^ top 11.1.7: Converting C-Strings to Numbers "1234" is a string of characters 1234 is a number Sometimes you need to convert C-strings to numbers For instance, you may need to read string data because: Reading money may involve a dollar sign Reading percentages may involve a percent sign To read numbers as characters: Read input as characters into a C-string Remove unwanted characters Use a conversion function to convert the C-string to a numeric value The conversion functions are found in the library cstdlib To use the functions use the include directive #include <cstdlib> The most used functions: atoi(), atol() and atof() Function atoi() to converts a C-string to an int value int x = atoi("1234"); // returns 1234 int x = atoi("#1234"); // returns 0 because # is not a digit Function atol() to converts a C-string to a long value long y = atol("1234"); // returns 1234 as a long Function atof() to converts a C-string to a double value double z = atof("9.99"); // returns 9.99 double w = atof("$9.99"); // returns 0.0 because $ // is not a digit ^ top 11.1.8: Summary C++ offers a special kind of char array known as a C-string Has the same shortcoming as arrays You need to know about C-Strings because you often have to work with them Each char is stored in the array with a special null character '\0' ending the string Syntax: char ArrayName[MaximumCStringSize + 1]; C-strings can be output with the insertion << operator The extraction operator >> can fill a C-string To read an entire line you can use function cin.getline() Syntax: cin.getline(cString, maxSize + 1); You can iterate a C-string like an array int index = 0; while (ourString[index] != '\0' && index < SIZE) { ourString[index] = 'X'; index++; } To assign C-strings, use the cstring library function strcpy() A safer version of strcpy() is named strncpy() Programmers use strcmp() to compare the contents of C-strings strlen() returns the number of characters in a string strcat() concatenates two C-strings strncat() is a safer version of strcat() C-string variables are arrays C-string arguments and parameters are used just like arrays To convert C-strings to numbers, use a conversion function Function atoi() to converts a C-string to an int value Function atol() to converts a C-string to a long value Function atof() to converts a C-string to a double value ^ top Exercise 11.1 Specifications Write a program named readcstr.cpp that reads a line of text as a C-string and prints the line vertically (one character at a time) down the page As a comment in your code, answer the following question: Q1: How large should you make the C-string? Why? Submit your source code as the answer to this exercise. ^ top 11.2: String Class Objectives At the end of the lesson the student will be able to: Use string constructors and member functions ^ top 11.2.1: Using Strings: A Review The string class allows a programmer to use strings as a basic data type Defined in the string library and standard namespace #include <string> using namespace std; Included automatically with our version of g++ Strings are stored in a character sequence starting at 0 (zero) String variables and expressions are treated much like simple types You can assign C-strings to string objects using the assignment operator string s1 = "Hello Mom!"; Also, you can assign string objects to a string object string s1, s2, s3; s1 = "Hello "; // assignment s2 = "Mom!"; s3 = s1 + s2; // concatenation cout << s3 << endl; Will the following code compile and run? Why or why not? string s = "abc" + "def"; Comparison operators work with string objects == returns true if two string objects contain the same characters in the same order For example: string s1, s2; s1 = "Hello Mom!"; s2 = s1; cout << (s1 == s2); <, >, <=, >= can be used to compare string objects as well ^ top 11.2.2: Constructors of Class string Since string is a class type, it has constructors The default string constructor initializes the string to the empty string string phrase; // empty string Another string constructor takes a C-string argument string noun("ants"); cout << noun << endl; ^ top 11.2.3: Indexing a String You can access any individual character of a string object using [ ] For example: string line; cout << "Enter your name: "; getline(cin, line); cout << "The first letter is: " << line[0] << endl; What happens if we try to display line[100]? Buffer overflow is a major source of errors and security flaws Member function at accesses an individual character as well cout << "The first letter is: " << line.at(0) << endl; However, at checks for valid index values When an invalid index is found, the program provides an error message and aborts For example: cout << "The first letter is: " << line.at(100) << endl; Produces a message like: 5 [sig] fiddle 2272 open_stackdumpfile: Dumping stack trace to fiddle.exe. stackdump Which is better: An error message or a security flaw? Further Information cppreference.com: C++Strings ^ top 11.2.4: Iterating Strings Member function length() returns the number of characters in the string object string s = "abcdef"; unsigned n = s.length(); After we know the length, it is easy to iterate through the individual characters of a string string s = "abcdef"; for (unsigned i = 0; i < s.length(); i++) { cout << "Char[" << i << "]: " << s.at(i) << endl; } ^ top 11.2.5: Finding Substrings Sometimes you want to know if a string contains a certain word For this you can use the find() member function Syntax: unsigned int haystack.find(needle) Searches for the string or char needle in the string haystack Returns the index of the first occurrence of needle or string::npos if not found For example string str = "Do you want to play again?"; unsigned n = str.find("you"); if (n == string::npos) { cout << "Could not find \"you\" in the str\n"; } else { cout << "We found \"you\" in the str\n"; } ^ top 11.2.6: Erasing and Inserting Substrings Sometimes you want to erase or insert parts of a string To erase parts of a string, you can use the erase() member function Syntax: str.erase(start, length) str.erase(start) The first line removes all the characters between start and start + length The second line removes all the characters from start through the end of the string To insert characters into a string, use the insert() member function Syntax: str.insert(start, newStr) Inserts the string newStr starting at position start For Example string str("Game over"); cout << str << endl; str.erase(4, 5); cout << str << endl; str.insert(0, "I'm "); cout << str << endl; ^ top 11.2.7: Summary The string class allows the programmer to use strings like a primitive type Strings are stored in a character sequence starting at 0 (zero) You can construct string objects like other class types string noun("ants"); You can also assign C-strings to strings using the assignment operator string s1 = "Hello Mom!"; You can concatenate (join) strings with the '+' operator string s3 = s1 + " " + s2; Comparison operators work with strings: ==, !=, <, >, <=, >= You can access individual characters of a string using [] or member function at You should use member function at because it checks for valid index values line.at(0); You 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; } You can find characters in a string using the find() member function string str = "Do you want to play again?"; unsigned n = str.find("you"); You can erase and insert characters in a string as well string str("Game over"); cout << str << endl; str.erase(4, 5); cout << str << endl; str.insert(0, "I'm "); cout << str << endl; ^ top Exercise 11.2 Specifications Write a program named triplex.cpp that replaces every third letter of a string with an 'X'. Submit your source code as the answer to this exercise. ^ top 11.3: Handling String Input Objectives At the end of the lesson the student will be able to: Use getline() as a substitute for cin ^ top 11.3.1: Problems With cin To get input from a user, we have been using cin However, cin has a number of shortcomings When entering input into a string, a space causes the input to stop reading If a user enters a character when a number is expected, unpredictable results occur For Example #include <iostream> using namespace std; const int MAX = 20; int main() { int numItems; string items[MAX]; cout << "Enter the number of items: "; cin >> numItems; for (int i = 0; i < numItems && i < MAX; i++) { cout << "Enter item #" << i << ": "; cin >> items[i]; } cout << "You entered:\n"; for (int i = 0; i < numItems; i++) { cout << i << ": " << items[i] << endl; } return 0; } If you enter a letter, like 'A', for the number of items you get unpredictable results Also, if you enter items with whitespace between the words, the second word ends up in the wrong variable The reason for this is the way cin operates Recall that cin >> numItems: Skips whitespace Reads characters Stops reading when the first unusable character is found This leaves the variable numItems uninitialized in the first case In the second case, the string input stops whenever whitespace is encountered ^ top 11.3.2: Using getline() Our solution to the first problem was to use getline() getline(cin, items[i]); This works because getline() reads until it encounters a '\n' It turns out that a solution to the second problem uses getline() as well Using getline() to Input Numbers To use getline() to read numbers, we use the following algorithm: Read the input as a string using getline() Convert the string to a number ^ top 11.3.3: Converting to Numbers To convert strings to numbers, we can use functions from cstdlib These functions take a C-string parameter, so you must convert a string to a C-String using the c_str() function For example: double z = 0.0; string text = "9.99"; z = atof(text.c_str()); cout << (z + 1 ) << endl; Functions to Convert C-Strings to Numbers Function Description atof(str.c_str()) Returns the contents of str as a double. atoi(str.c_str()) Returns the contents of str as an int. atol(str.c_str()) Returns the contents of str as a long int. Examples Converting Strings to Numbers Using function atoi(): string s1 = "1234"; int x; x = atoi(s1.c_str()); // returns 1234 cout << x << endl; s1 = "#1234"; x = atoi(s1.c_str()); // returns 0 because # is not a digit cout << x << endl; Using function atol(): string s2 = "1234"; long y = atol(s2.c_str()); // returns 1234 as a long cout << y << endl; Using function atof(): string s3 = "9.99"; double z = 0.0; z = atof(s3.c_str()); // returns 9.99 cout << z << endl; s3 = "$9.99"; z = atof(s3.c_str()); // returns 0 because $ is not a digit cout << z << endl; ^ top 11.3.4: Checking for Errors Note that some of the example shown above do not return the desire results For example, if the user enters "$9.99", the function atof() returns 0 To prevent this problem, we must check for erroneous characters To check, we iterate through every character of the input string and verify they are valid The valid characters for a positive double are: '0' - '9' and '.' Ignoring exponential notation In addition, we must verify that the user entered some number Otherwise, we get an empty string ("") Example of Verifying User Input When Using getline() #include <iostream> using namespace std; const int MAX = 20; int main() { string input; double value = 0; bool error = true; while (error == true) { cout << "Enter a number: "; getline(cin, input); error = false; for (unsigned i = 0; i < input.length(); i++) { char ch = input.at(i); if (ch != '.' && ch < '0' || ch > '9') { error = true; cout << "You must enter a numeric value\n"; } } } value = atof(input.c_str()); cout << "You entered: " << value << endl; return 0; } Note that the verification loop does not catch all possible errors What other input errors might we want to check for? ^ top 11.3.5: No More cin We can use our getline() + verification code to replace cin To make it easier to reuse the code, we wrap the code in a function cin Replacement Function for Entering Positive doubles #include <iostream> using namespace std; const int MAX = 20; double inputDouble(string prompt); int main() { string input; double value = inputDouble("Enter a positive number: "); cout << "You entered: " << value << endl; return 0; } double inputDouble(string prompt) { string input; double value = 0; bool error = true; while (error == true) { cout << prompt; getline(cin, input); error = false; for (unsigned i = 0; i < input.length(); i++) { char ch = input.at(i); if (ch != '.' && ch < '0' || ch > '9') { error = true; cout << "You must enter a numeric value\n"; break; } } } value = atof(input.c_str()); return value; } ^ top 11.3.6: Summary cin is useful for getting user input, but has a number of shortcomings For example: entering letters when expecting numbers We can design improved input functions using the getline() function The algorithm we use is: Get user input using getline() Check for unwanted characters Loop until the user enters only correct characters Use a conversion function to convert the string to a numeric value ^ top Exercise 11.3 Write a program named inputint.cpp that uses getline() to read integer values. Use the code below to get started. Submit your source code as the answer to this exercise. #include <iostream> using namespace std; const int MAX = 20; double inputDouble(string prompt); int main() { string input; double value = inputDouble("Enter a positive number: "); cout << "You entered: " << value << endl; return 0; } double inputDouble(string prompt) { string input; double value = 0; bool error = true; while (error == true) { cout << prompt; getline(cin, input); error = false; for (unsigned i = 0; i < input.length(); i++) { char ch = input.at(i); if (ch != '.' && ch < '0' || ch > '9') { error = true; cout << "You must enter a numeric value\n"; break; } } } value = atof(input.c_str()); return value; } ^ top Wrap Up Reminders Due Next: A10: Working with Arrays (11/17/05) Exercise 10 and CodeLab 10 (11/17/05) Exercise 11 (12/1/05) Sampler Project (12/8/05) 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: November 17 2005 @16:59:05