C Programming | Character Strings

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What we would learn about strings

• Representation in C
• String Literals
• String Variables
• String Input/Output ( printf, scanf, gets, fgets, puts, fputs )

Introduction

A string is an array of characters.

• Individual characters are stored in memory in ASCII code.
• A string is represented as a sequence of characters terminated by the null (‘\0’) character whose ASCII value is zero

String Literals

String literal values are represented by sequences of characters between double quotes (“)

Examples

• “” represents empty string
• “hello”

“a” versus ‘a’

• ‘a’ is a single character value (stored in 1 byte) as the ASCII value for the letter, a
• “a”is an array with two characters, the first is a, the second is the character value \0

Referring to String Literals

String literal is an array, can refer to a single character from the literal as a character

Example:

printf(”%c”, ”hello”[1]); 
//outputs the character ‘e’

During compilation, C creates space for each string literal (# of characters in the literal + 1)

Declaring String Variables

A string is declared like any other array:

char string-name [size];

• size determines the number of characters in string_name.

When a character string is assigned to a character array, it automatically appends the null character (‘\0’) at the end of the string.

• size should be equal to the number of characters in the string plus one.

Examples

char name[30]; 
char city[15]; 
char dob[11];

A string may be initialized at the time of declaration.

char   city[15] = “Calcutta”; 
char   city[15] = {‘C’, ‘a’, ‘l’, ‘c’, ‘u’, ‘t’, ‘t’, ‘a’}; 
char   dob[] = “12-10-1975”;

Duplicate String Literals

Each string literal in a C program is stored at a different location

So even if the string literals contain the same string, they are not equal (in the == sense)

Example:

char string1[6] = “hello”; 
char string2[6] = “hello”; 
/*but string1 does not equal string2
(they are stored at different locations)*/

Changing String Variables

Cannot change string variables connected to string constants, but can change pointer variables that are not tied to space.

Example:

char *str1 = “hello”;          
/* str1 unchangeable */ 
char *str2 = “goodbye”;        
/* str2 unchangeable */
char *str3;                   
/* Not tied to space */ 
str3 = str1;  
/* str3 points to same space s1 connected to */ 
str3 = str2;

Can change parts of a string variable

char str1[6] = “hello”; 
str1[0] = ‘y’; /* str1 is now “yello” */ 
str1[4] = ‘\0’; /* str1 is now “yell” */

Important to retain end-of-string marker (replacing str1[5] in the original string with something other than ‘\0’ makes a string that does not end)

Have to stay within limits of array –responsibility of programmer

Reading Strings from the Keyboard

Two different cases will be considered:

• Reading words
• Reading an entire line

Reading words

scanf can be used with the “%s” format specification.

char   name[30]; 
scanf (“%s”, name);

The ampersand (&) is not required before the variable name with “%s”.

• “name” represents an address

The problem here is that the string is taken to be upto the first white space (blank, tab, carriage return, etc.)

• If we type “Rupak Biswas”
• name will be assigned the string “Rupak”

Reading a line of text

In many applications, we need to read in an entire line of text (including blank spaces).

We can use the getchar() function for the purpose.

char   line[81], ch; 
int  c=0; 

do { 
  ch = getchar(); 
  line[c] = ch; 
  c++; 
} while (ch != ‘\n’); 
c = c – 1; 
line[c] = ‘\0’;

Reading a line :: Alternate Approach

Reads a string containing uppercase characters and blank spaces

char line[81]; 
scanf (“%[ ABCDEFGHIJKLMNOPQRSTUVWXYZ]”, line);

Reads a string containing any characters until a newline character is received

char line[81]; 
scanf (“%[^\n]”, line);

More on String Input

Edit set input %[ListofChars]

• ListofChars specifies set of characters (called scan set)
• Characters read as long as character falls in scan set
• Stops when first non scan set character encountered
• Note, does not ignored leading white space
• Any character may be specified except ]
• Putting ^ at the start to negate the set (any character BUT list is allowed)

Examples:

scanf (“%[−+0123456789]”, Number); 
scanf (“%[^\n]”,Line);   
/* read until newline char */

Writing Strings to the Screen

We can use printf with the “%s”format specification.

char name[50];
printf (“\n %s”, name);

Input / Output Example

#include <stdio.h>
void main( ) { 
  char LastName[11]; 
  char FirstName[11];
  printf("Enter your name (last , first): "); 
  scanf("%10s%*[^,],%10s", LastName, FirstName);
  printf("Nice to meet you %s %s\n", FirstName, LastName);
}