How to Compare Two Strings in C Programming

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Last Updated: August 7, 2019

Comparing string length is a common function in C programming, as it allows you to see which string contains more characters. This is very useful for sorting data. Comparing strings requires a special function; do not use != or ==.

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1
There are two functions that allow you to compare strings in C. Both of these functions are included in the <string.h> library.
- strcmp() - This function compares two strings and returns the comparative difference in the number of characters.
- strncmp() - This is the same as strcmp(), except that it compares the first n characters. It is considered more secure as it helps prevent crashes from overflow.
2
Start the program with your necessary libraries. You'll want both the <stdio.h> and <string.h> libraries, along with any others you may need for your specific program.
```
#include <stdio.h>
#include <string.h>
```
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3
Start an .int function. This is the easiest way to learn this function, as it will return an integer that compares the length of the two strings.
```
#include <stdio.h>
#include <string.h>

int main ()

{

}
```
4
Define the two strings you want to compare. For this example, we will be comparing two predefined char strings. You will also want to define the return value as an integer.^{[1]
X
Research source}
```
#include <stdio.h>
#include <string.h>

int main ()
{
  char *str1 = "apple";
  char *str2 = "orange";
  int ret;
}
```
5
Add the comparison function. Now that you have your two strings defined, you can add the comparison function. We are going to use strncmp(), so we need to ensure that the number of characters to be measured is set in the function.
```
#include <stdio.h>
#include <string.h>

int main ()
{
  char *str1 = "apple";
  char *str2 = "orange";
  int ret;

  ret = strncmp(str1, str2, 6);

  /*This will compare the two strings 
  up to 6 characters long */
}
```
6
Use an .If...Else statement to perform the comparison. Now that you have the function in place, you can use an If...Else statement to display which string is longer. strncmp() will return 0 if the strings are the same length, a positive number if str1 is larger, and a negative number if str2 is larger.
```
#include <stdio.h>
#include <string.h>

int main ()
{
  char *str1 = "apple";
  char *str2 = "orange";
  int ret;

  ret = strncmp(str1, str2, 6);

  if(ret > 0)
  {
    printf("str1 is longer");
  }
  else if(ret < 0)
  {
    printf("str2 is longer");
  }
  else
  {
    printf("The two strings are equal");
  }

  return(0);
}
```

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What is the different between strcmp and strncmp?

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Strcmp() simply compares two strings; strncmp() compares the first n characters of two string together.

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Can I use = or != when comparing two strings?

WalkerTyme

Community Answer

You can use == or != if you need to check if the strings are exactly the same.

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Question

When i read the line ret = strcomp (str1, str, 6), I don't understand it.

Community Answer

The function 'strncomp' returns three values: 0 if the strings are equal, >0 if the first string is longer.

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Warnings

Remember that the return value is 0 if the strings are the same. This could confuse you because 0 is also the value of FALSE.

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