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Strings in C++

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• The char type
  • The char (built-in) type represents the characters

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  • The encoding scheme for characters is ASCII (American Standard Code for Information Interchange)

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  • A char type variable takes up 1 byte of computer memory

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• String
  • A string is a sequence of characters

    The content (i.e., the characters of a string is always stored in an array of char

    There is one more piece of information to determine the string:

    Where is the end of the string ?

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  • There are two common ways to represent a string....

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• String representations
  • There are two common ways to represent a string:

    Content and length Content and sentinel

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  • Content with length:
    • In the content with length representation, a string is represented by the starting address of the character array and the length of the string

      Example:

      The string in the above figure is:

      ASCII codes: 72 101 108 108 Characters: H e l l

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  • Content with sentinel
    • In the content with sentinel representation, a string is represented by the starting address of the character array and a special terminating character

      In C++, the ASCII code 0 is used This special character is denoted as '\0'

      Example:

      The string in the above figure is:

      ASCII codes: 72 101 108 108 Characters: H e l l

      NOTE: the sentinel '\0' is not part of the string

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• String in C++
  • Defining strings in C++:

    "content of the string"

    Example:

    "Hello World !"

    This expression will return the address of the first character of the string

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  • Defining string variables in C++:

    char s[] = "content of string";

    By omitting the SIZE of the array from the definition, C++ will create an array of the size that is given in the data initialization

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    Example:

    char s[] = "Hello World !";

    This will create an array of 14 bytes (13 letters and one more byte for the sentinel '\0')

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• Strings in C++ is mutable
  • An important difference between strings in C++ and Java is that strings in C++ is mutable

    Example:

    char s[] = "Hello World !"; s[4] = ','; // s is now "Hell, World!"

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• Character pointer as strings
  • You can also define an string and make a char * variable point to that string

    Example:

    char *s = "Hello World !"; // CONSTANT string

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  • C++ interprets this as a constant (immutable) string (a string literal)

    When you try to change the string, the program will crash

    Example:

    char *s = "Hello World !"; // String literal s[4] = ','; // CRASH !!! Cannot change a literal

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• Copying strings
  • Pointer-arithmetics used on a char * pointer variable is commonly used to copy strings

    Example:

    char s[10]; // s stores a string char *p; // Help variable used to copy string char *q = "Hello World !"; // Source string // Copy string q to string s p = s; // p points to receiving array while ( *q != '\0' ) *p++ = *q++; *p = '\0'; // Mark end of string

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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  • NOTE: a C/C++ guru may show you this version of the string copy code:

    char s[10]; // s stores a string char *p; // Help variable used to copy string char *q = "Hello World !"; // Source string // Copy string q to string s p = s; // p points to receiving array while ( (*p++ = *q++) != '\0' );

    It takes advantage of the fact that the assignment expression "*p++ = *q++" in C/C++ returns the value assigned

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• Built-in string manipulation function in C/C++
  • The C/C++ programming language comes with a standard C/C++ library of functions.

  • To view the documentation on the C library, use the command:

    man libc

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  • There are many string manipulation functions in the C/C++ library.

    Some example functions:

    char * strcpy(char *s1, char *s2): copy string s2 to string s1 int strcmp(char *s1, char *s2): compare strings s1 and string s2. Returns 0 is strings are equal, int strcat(char *s1, char *s2): append string s2 at the end of string s1. NOTE: make sure the character array s1 is sufficiently large