FAQ > Comparing strings (C)


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FAQ > How do I... (Level 1) > Comparing strings (C)

This item was added on: 2003/07/06

In C, you can compare single characters (chars) by using the comparion operator ==, however, this method is not valid for comparing arrays of chars, or strings. Instead, you must use a function that compares each of the chars within the arrays in turn. This may sound complex to the beginner, but fortunately there is a standard C function that does this for you.

#include <string.h>
int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);

The first, strcmp(), does exactly as described above, the second, strncmp(), checks only the first n characters of the array for equality.

Here is an example of how not to do things.


#include <stdio.h>  
#include <string.h>  

int main(void)
{
  char *foo = "hello";
  char *bar = "hello";
  if (foo == bar)  /* Wrong ! */
    puts("Strings equal");
  else
    puts("Strings do not equal");
  
  return 0;
}


Here are some examples of these functions in use.


#include <stdio.h> 
#include <string.h> 

int main(void)
{
  char *foo = "hello";
  char *bar = "world";
  if (strcmp(foo, bar) == 0) 
    puts("Strings equal");
  else 
    puts("Strings do not equal");
  
  return 0;
}

/* 
 * Output
 Strings do not equal
 *
 */
 

This next example shows the consideration needed when reading a string from the user. It deals with removing the newline character from the input array. This issue is also covered here.

#include <stdio.h> 
#include <string.h> 

int main(void)
{
  char Owner[] = "Hammer";
  char buf[BUFSIZ];
  
  printf ("You are at the house door.\nEnter your name:");
  fflush(stdout);
  if (fgets(buf, sizeof(buf), stdin))
  {
    char *p = strchr(buf, '\n');
    if (p) *p = '\0';
    if (strcmp(Owner, buf) == 0)
      printf ("Welcome home %s\n", Owner);
    else
      printf ("Agghh, who let %s in here?!\n", buf);
  }
  
  return 0;
}

/*
 * Output:
 You are at the house door.
 Enter your name:Hammer
 Welcome home Hammer
 
 ---
 
 You are at the house door.
 Enter your name:Trouble
 Agghh, who let Trouble in here?!
 *
 */


This next example shows how to use strncpy() to compare the first few characters of the arrays, rather than all of them.

#include <stdio.h>  
#include <string.h>  

int main(void)
{
  char *foo = "hello";
  char *bar = "help me";
  if (strncmp(foo, bar, 3) == 0)
    puts("First 3 chars are equal");
  else
    puts("First 3 chars are NOT equal");
  
  return 0;
}

/*
 * Output
 First 3 chars are equal
 *
 */

Written by Hammer

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