Computer Programming | Education

C Programming | String Functions

What we would learn

  • String Functions ( strlen, strcpy, strncpy, strcmp, strncmp, strcat, strncat, strchr, strrchr, strstr, strspn, strcspn, strtok )
  • Reading from/Printing to Strings ( sprintf, sscan )

Processing Character Strings

There exists a set of C library functions for character string manipulation.

  • strcpy :: string copy
  • strlen :: string length
  • strcmp :: string comparison
  • strtcat :: string concatenation

It is required to add the line

#include <string.h>

1. strcpy()

Works very much like a string assignment operator.

char *strcpy (char *str1, char *str2);
  • Assigns the contents of str2 to str1.
  • Returns address of the destination string.


strcpy (city, “Calcutta”); 
strcpy (city, mycity);


  • Assignment operator do not work for strings.
city  =  “Calcutta”; // INVALID

2. strlen()

Counts and returns the number of characters in a string.

intstrlen(char *str);


len = strlen (string); /* Returns an integer */
  • The null character (‘\0’) at the end is not counted.
  • Counting ends at the first null character.
char  city[15]; int  n; 
strcpy (city, “Calcutta”); 
n = strlen (city);

3. strcmp()

Compares two character strings.

int strcmp(char *str1, char *str2); 
  • Compares the two strings and returns 0 if they are identical; non-zero otherwise.


if (strcmp(city, “Delhi”) == 0) {  
// Do some task
if (strcmp(city1, city2) != 0) { 
// Do some task

Actually, the function returns the difference in ASCII values of the first letter of mismatch.

Less than 0

  • If the ASCII value of the character they differ at is smaller for str1, or str2 is longer than str1

Greater than 0

  • If the ASCII value of the character they differ at is greater for str1, or str1 is longer than str2

Equal to 0

  • If the two strings are identical

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String Comparison

strcmp examples:

strcmp(“hello”,”hello”);   //returns 0 
strcmp(“yello”,”hello”);   // returns value > 0 
strcmp(“Hello”,”hello”);  //returns value < 0 
strcmp(“hello”,”hello there”);  //returns value < 0 
strcmp(“some diff”,”some dift”);  //returns value < 0

expression for determining if two strings s1, s2 hold the same string value:

! strcmp(s1, s2)

Sometimes we only want to compare first n chars:

int strncmp(char *s1, char *s2, intn);
/*Works the same as strcmp except 
that it stops at the nth character */
  • looks at less than n characters if either string is shorter than n
strcmp(“some diff”,”some DIFF”); //returns value > 0 
strncmp(“some diff”,”some DIFF”,4); //returns 0

String Comparison (ignoring case)

int strcasecmp(char *str1,  char *str2);
/*  similar to strcmp except that
upper and lower case characters 
(e.g., ‘a’ and ‘A’) are considered to be equal */

int strncasecmp(char *str1, char *str2,  int n);
/* version of strncmp that ignores case */

4. strcat()

Joins or concatenates two strings together

char *strcat (char *str1, char *str2); 
  • str2 is appended to the end of str1.
  • The null character at the end of str1 is removed, and str2 is joined at that point.
  • str1 should have enough space — Programmer’s responsibility


strcpy(name1, “Amit ”); 
strcpy(name2, “Roy“); 
strcat(name1, name2);

Example :: count uppercase

/* Read a line of text and count the number of uppercase letters */ 
#include  <stdio.h> 
#include  <string.h> 
main() { 
  char  line[81]; 
  int  i, n, count=0; 
  scanf (“%[^n]”, line); 
  n = strlen (line); 
  for (i=0; i<n; i++) 
    if (isupper(line[i]) 
  printf (“n The number of uppercase letters in the string %s is %d”, line, count); 

Example :: compare two strings

#include <stdio.h> 
int my_strcmp(char s1[ ],char s2[ ]) { 
  int i=0; 
  while(s1[i]!='' && s2[i]!=''){ 
    else i++; 

main() { 
  char string1[100],string2[100]; 
  printf("Give two strings n"); 
  printf("Comparison result: %d n", 

If you want to know briefly about character strings in C language then it is recommended to read this article : C Programming | Character Strings

Searching for a Character/String

char *strchr(char *str, int ch);
  • returns a pointer (char *) to the first occurrence of chin str
  • returns NULL if chdoes not occur in str
  • can subtract original pointer from result pointer to determine which character in array is ch
char *strstr(char *str, char *searchstr)
  • similar to strchr, but looks for the first occurrence of the string searchstrin str
char *strrchr(char *str, int ch)
  • similar to strchrexcept that the search starts from the end of string strand works backward

Printing to a String

  • The sprintf function allows us to print to a string argument using printf formatting rules
  • First argument of sprintf is the string to print to, remaining arguments are as in printf


char buffer[100]; 
sprintf (buffer, ”%s, %s”, LastName, FirstName); 
if (strlen(buffer) > 15) 
  printf(“Long name %s %sn”, FirstName, LastName);

Reading from a String

  • The sscanf function allows us to read from a string argument using scanf rules
  • First argument of sscanf is the string to read from, remaining arguments are as in scanf


char buffer[100] = “A10 50.0”; 
sscanf (buffer,”%c%d%f”, &ch, &inum, &fnum); 
/* puts ‘A’ in ch, 10 in 
inum and 50.0 in fnum*/

Example :: Duplicate Removal

Write a C function that takes a string as an argument and modifies the string so as to remove all consecutive duplicate characters, e.g., mississippi-> misisipi.

void remove_duplicates(char word[ ] ) { 
    int k, j; 
    char prev = ''; 
    for (k = j = 0; word[k]!=''; k++)  { 
      if (prev != word[k]) 
     prev = word[k]; 
    word[j] = ''; 

Thanks for reading this article,

Team BloggerBoy.


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