Character strings & String functions

Character Strings :

 

A string is a sequence of characters. Any sequence or set of characters defined within double quotation symbols is a constant string.

 

 Various operations on strings:

Reading string displaying strings

Combining or concatenating strings

Copying one string to another.

Comparing string & checking whether they are equal

Extraction of a portion of a string

 

Strings are stored in memory as ASCII codes of characters that make up the string appended with ‘\0’(ASCII value of null). Normally each character is stored in one byte, successive characters are stored in successive bytes.

                 The last character is the null character having ASCII value zero.

 

Initializing Strings:

Following the discussion on characters arrays, the initialization of a string must the following form which is simpler to one dimension array.

 

char month1[ ]={‘j’,’a’,’n’,’u’,’a’,’r’,’y’};

 

J

A

N

U

A

R

Y

\0

 

/*String.c string variable*/

#include < stdio.h >

main()

{

char month[15];

printf (“Enter the string”);

gets (month);

printf (“The string entered is %s”, month);

} 

 

Character string terminated by a null character ‘\0’.

A string variable is any valid C variable name & is always declared as an array. The general form of declaration of a string variable is

 

Char string_name[size];

 

The size determines the number of characters in the string name.

 

Example:

 

char month[10];

char address[100];

 

The size of the array should be one byte more than the actual space occupied by the string since the complier appends a null character at the end of the string.

 

Reading Strings from the terminal:

The function scanf with %s format specification is needed to read the character string from the terminal.

 

Example:

 

char address[15];

scanf(“%s”,address);

 

Scanf statement terminates the statement as soon as it finds a blank space, suppose if we type the string new york then only the string new will be read and since there is a blank space after word “new” it will terminate the string.

 

Note that we can use the scanf without the ampersand symbol before the variable name.

In many applications it is required to process text by reading an entire line of text from the terminal.

 

The function getchar can be used repeatedly to read a sequence of successive single characters and store it in the array.

 

We cannot manipulate strings since C does not provide any operators for string. For instance we cannot assign one string to another directly.

 

For example:

 

String=”xyz”;   not valid.

String1=string2;   not valid.

 

To copy the chars in one string to another string we may do so on a character to character basis.

 

Writing strings to screen:

The printf statement along with format specifier %s to print strings on to the screen. The format %s can be used to display an array of characters that is terminated by the null character

For example: printf(“%s”,name); can be used to display the entire contents of the array name.

 

String operations (string.h)

C language recognizes that string is a different class of array by letting us input and output the array as a unit and are terminated by null character. C library supports a large number of string handling functions that can be used to array out many o f the string manipulations such as:

·         Length (number of characters in the string).

·         Concatenation (adding two are more strings)

·         Comparing two strings.

·         Substring (Extract substring from a given string)

·         Copy(copies one string over another)

To do all the operations described here it is essential to include string.h library header file in the program.

strlen() function:

This function counts and returns the number of characters in a string. The length does not include a null character.

Syntax: n=strlen(string);

          Where n is integer variable. This receives the value of length of the string.

Example:

length=strlen(“Hollywood”);

        The function will assign number of characters 9 in the string to a integer variable length.

strcat() function:

when you combine two strings, you add the characters of one string to the end of other string. This process is called concatenation. The strcat() function joins 2 strings together. It takes the following form

strcat(string1,string2)

string1 & string2 are character arrays. When the function strcat is executed string2 is appended to string1. the string at string2 remains unchanged.

Example:

strcpy(string1,”sri”);
strcpy(string2,”Bhagavan”);
Printf(“%s”,strcat(string1,string2);

From the above program segment the value of string1 becomes sribhagavan. The string at str2 remains unchanged as bhagawan.

Strcmp() function:

In c you cannot directly compare the value of 2 strings in a condition like if(string1==string2)
Most libraries however contain the strcmp() function, which returns a zero if 2 strings are equal, or a non zero number if the strings are not the same. The syntax of strcmp() is given below:

Strcmp(string1,string2);

String1 & string2 may be string variables or string constants. String1, & string2 may be string variables or string constants some computers return a negative if the string1 is alphabetically less than the second and a positive number if the string is greater than the second.

Example:

strcmp(“Newyork”,”Newyork”) will return zero because 2 strings are equal. strcmp(“their”,”there”) will return a 9 which is the numeric difference between ASCII ‘i’ and ASCII ’r’. strcmp(“The”, “the”) will return 32 which is the numeric difference between ASCII “T” & ASCII “t”.

strcpy() function:

C does not allow you to assign the characters to a string directly as in the statement name=”Robert”;
Instead use the strcpy(0 function found in most compilers the syntax of the function is illustrated below.

strcpy(string1,string2);

Strcpy function assigns the contents of string2 to string1. string2 may be a character array variable or a string constant.

strcpy(Name,”Robert”);

In the above example Robert is assigned to the string called name.