2.5 Strings

Basic of strings

A string is a continuous sequence of characters terminated by '\0', the null character. The length of a string is considered to be the number of characters excluding the terminating null character. There is no string type in C, and consequently there are no operators that accept strings as operands.

Instead, strings are stored in arrays whose elements have the type char or wchar_t. Strings of wide characters that is, characters of the type wchar_tare also called wide strings . The C standard library provides numerous functions to perform basic operations on strings, such as comparing, copying, and concatenating them.

Declarations and uses of strings

You can initialize arrays of char or wchar_t using string literals. For example, the following two array definitions are equivalent:

An array holding a string must always be at least one element longer than the string length to accommodate the terminating null character. Thus the array str1 can store strings up to a maximum length of 29. It would be a mistake to define the array with length 8 rather than 30, because then it wouldn't contain the terminating null character.

If you define a character array without an explicit length and initialize it with a string literal, the array created is one element longer than the string length. An Example

The following statement uses the standard function strcat() to append the string in str2 to the string in str1. The array str1 must be large enough to hold all the characters in the concatenated string.

The output printed by the puts() call is the new content of the array str1:

Let's go to London!

The names str1 and str2 are pointers to the first character of the string stored in each array. Such a pointer is called a pointer to a string , or a string pointer for short. String manipulation functions such as strcat() and puts() receive the beginning addresses of strings as their arguments. Such functions generally process a string character by character until they reach the terminator, '\0'. The function in is one possible implementation of the standard function strcat() . It uses pointers to step through the strings referenced by its arguments.

Built-in functions for character and string processing

Character processing functions

The standard library provides a number of functions to classify characters and to perform conversions on them. The header ctype.h declares such functions for byte characters, with character codes from 0 to 255.

The results of these functions, except for isdigit() and isxdigit() , depends on the current locale setting for the locale category LC_CTYPE. You can query or change the locale using the setlocale() function.

Character classification functions

The functions listed in [link] test whether a character belongs to a certain category. Their return value is nonzero, or true, if the argument is a character code in the given category.