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Strings
Read: Chapter 8.
Q: How can we create and manipulate variables holding character string data?
Two Ways:
- Using “ cstrings ”, supported by both C and C++.
- Using C++ string class.
cstrings in C++:
- A cstring is simply a partially-filled array having base type char and with a null character ‘\0’ to mark the end of the character string stored in the array.
- They are arrays first and “strings” second!
- A cstring variable is declared in exactly the same way as an array of characters but will be used differently.
Syntax in Declaring a cstring Variable: char array_name[max_cstringSize + 1];
This declaration will create a cstring variable (an array of char) that is capable of storing a string with at most max_cstringSize characters.
Initializing cstring Variable during Declaration: Consider char s[10] = “Hi Mom!”;
The array elements are:
s[0] s[1] s[2] s[3] s[4] s[5] s[6] s[7] s[8] s[9] H i M o M! \0??
Remarks:
- The character ‘\0’ is a null character used as a sentinel value to mark the end of a cstring stored in an array.
- The cstring variable s has length 8.
- Warning: The following are illegal in C++. char s[10]; s = “Hi Mom!”;
Q: What if the max_cstringSize is not specified?
Example: char q[] = “jrm” // q has 4 characters
The array elements are: q[0] q[1] q[2] q[3] *j r m *
Example: The following loop will change a cstring, our_string, into a cstring having the same length but with characters all ‘X’:
int index = 0; while (our_string[index] != '\0') { our_string[index] = 'X'; index++; }
Remark: In the loop above, if our_string has no null terminator, the loop will run off into memory, happily writing on every byte in memory beyond the end of our_string until a byte is found with zero value.
Better using: int index = 0; while (our_string[index] != '\0' && index < size) { our_string[index] = 'X'; index++; }
Assignment Operator in cstring: Warning: The assignment operator does not work for cstrings. Hence, the following are illegal in C++. char s[10]; s = “Hi Mom!”;
However, during declaration, it is legal to use: char s[10] = “Hi Mom!”;
To assign a value to a csting variable, use strcopy function defined in the cstring library.
Example: #include … char s[10]; strcpy(s, “Hi Mom!”);
Warning: In C++, strcpy function does not check the declared length of the first argument. Hence, it is possible for strcpy to write characters beyond the declared size of the array!
Remedy: Many versions of C++ have a safer version of strcpy named strncpy, which includes a third argument representing the maximum number of characters to copy.
Comparing cstrings: The “==” comparison operator does not work as expected with cstrings. To compare two cstring variables, use the predefined function strcmp defined in the cstring library.
Example: #include … if (strcmp(c_string1, c_string2)) cout << “Strings are not the same.”; else cout << “String are the same.”;
Remark: The strcmp function compares the numeric codes of the elements in the C-strings one character at a time.
- If the two cstrings are the same, strcmp returns 0, which will be interpreted as false.
- As soon as the characters do not match, strcmp returns a negative (positive) value if the numeric code in the first parameter is less (greater) than the numeric code in the second parameter, which will be interpreted as true.
Example: The following are either disallowed or they don’t do what you would expect: void doThings( ) { char myString[5]; myString = “abc”; char stringA[] = “Hello”; char stringB[] = stringA; if (stringB == “Hello”) cout << “Hello\n”; }
Correct Way: #include void doThings( ) { char myString[5]; strcpy(myString, “abc”); char stringA[] = “Hello”; char stringB[6]; strcpy(stringB, stringA); if (!strcmp(stringA, stringB)) cout << “Hello\n”; }
Passing cstrings as Arguments and Parameters: Since cstring variables are arrays, cstring arguments and parameters are used just like array variables.
Example: Replace all vowels in a string with an underscore character. void replaceVowels(char str[ ]) // we don’t pass a length { int i = 0; while (str[i] != ‘\0’) { switch (str[i]) { case ‘a’: case ‘e’: case ‘i’: case ‘o’: case ‘u’: case ‘A’: case ‘E’: case ‘I’: case ‘O’: case ‘U’: str[i] = ‘_’; break; }
i++; } }
void tryIt( ) { char command[ ] = “Let’s eat apples”; replaceVowels(command); cout << command << endl; }
Remarks:
- If a function changes the value of a cstring parameter, it is best to include a parameter for the declared size of the cstring.
- If a function does not change the value of a cstring parameter, the null character can detect the end of the string and no size argument is needed.
Consider the code: char a[80], b[80]; cin >> a >> b ; cout << a << b << “END OF OUTPUT\n”;
Dialogue: Do be do // input DobeEND OF OUTPUT // output
Q: How can we read in an entire line?
Remarks:
- To get an entire line, you can write a loop to extract the line one character at a time, but this won’t read the blanks.
- To read in blanks, use the member function get.
- To get an entire line, you can also use the predefined member function getline (of every input stream).
- getline has two arguments: The first argument is a cstring variable for receiving the input, and a second argument is an integer specifying the number of characters to be extracted to the cstring, allowing for the null terminator.
The getline Function: Syntax: input_stream.getline(cString, max_char + 1);
One line of input is read and stored in the cstring cString. The getline member function will stop reading when max_char characters have been read.
Example: char a[80]; cin.getline(a, 80); cout << a << “END OF OUTPUT.\n”;
Dialogue: Do be do // input Do be doEND OF OUTPUT // output
Remark: These cstring I/O techniques work the same for file I/O streams.
- Replace cin (cout) with the name of an input (ouput) file stream, we may use: in_stream >> c_string; in_stream.getline(c_string, 80); out_stream << c_string;
- The function atoi takes a cstring of digits and returns the int value represented by the digit characters in cstring. It returns 0 if the cstring contains a non-digit character. Example: atoi(“168”) returns 168. atoi(“#168”) returns 0.
- The function atol is similar to atoi except it converts its cstring argument to the long value the cstring represents.
- The function atof converts its cstring argument to the double value the cstring represents. Like atoi, the function atof returns 0.0 if the cstring argument does not represent to a double. Example: double x = atof(“1.68”); // x = 1. double x = atof(“$1.68”); // x = 0.
Q: How do we read the input?
Consider the following function read_and_clean ,
- It reads a line of input.
- Discards all characters except the digits '0' through '9'.
- Uses atoi to convert the "cleaned-up" cstring to int.
Array of cstrings:
- A cstring is an array of base type char.
- An array of cstrings can be considered as a two- dimensional array with base type char.
- Like any array, an array of cstrings can be manipulated by using its index values.
Example: char name[5][20]; // define 5 cstrings of size 19 cout << “Enter 5 names, one per line:\n “; for (int index = 0; index < 5; index++) cin.getline(name[index], 20); for (int index = 0; index < 5; index++) cout << name[index] << endl;
Example: Passing Array of Cstring as Parameter. // Define 10 cstrings, each can hold up to 49 characters char stringArray[10][50]; for (int i=0 ; i<10 ; i++) cin >> stringArray[i]; // read in the string … void lookForBoo(char words[][50], int nWords) { for (int i=0 ; i<nWords ; i++) if (strcmp(words[i], “Boo”) == 0) // Finding “Boo”. cout << “You scared me!\n”; }
Manipulating Character Strings:
- Using cstring.
- Using string class
Problems with Cstrings:
- Does not behave like a simple data type; standard operators do not apply. Example: Assignment and compare operators do not work on cstrings.
- Using cstrings with predefined cstring functions is not as safe as we would like. Example : Using strcpy to copy a longer cstring to another (shorter) cstring will overwrite memory that may be important to your program.
C++ string Class:
- Developed as a safer and easier way to manipulate character strings.
- Class strings behave very much like built-in data types.
- Have extensive utility in the Standard Library.
Using Standard String Class in C++: Include Directive: #include using namespace std;
