C++ cstring strncmp() Function

The C++ cstring strncmp() function is used to compare two given strings.

This function is similar to the strcmp() function but the difference is that strcmp() function compares two strings completely whereas strncmp() function compares only n number of characters of two strings.

Assume that you have a string s1 and s2, and you want to compare only the first 5 characters of both strings. In this case, you can use the strncmp() function.

Syntax

Following is the syntax of the C++ cstring strncmp() function −

int strncmp(const char *str1, const char *str2, size_t n);

Parameters

Parameters of the strncmp() function are below −

• str1: This is the first string to be compared.
• str2: This is the second string to be compared.
• n: This is the number of characters to be compared.

Return Values

• 0: if both strings are equal.
• 1: if the first character that does not match has a lower value in str1 than in str2.
• -1: if the first character that does not match has a greater value in str1 than in str2.

Example 1

Following is the basic example of the strncmp() function.

In here, we will take two strings str1 and str2 and then compare only the first 5 characters of both strings using the strncmp() function.

#include <iostream>
#include <cstring>
using namespace std;

int main() {
   char str1[15];
   char str2[15];
   int ret;

   strcpy(str1, "abcdef");
   strcpy(str2, "ABCDEF");

   ret = strncmp(str1, str2, 5);

   if(ret > 0) {
      cout << "str1 is greater than str2"<<endl;
   } else if(ret < 0) {
      cout << "str1 is less than str2"<<endl;
   } else {
      cout << "str1 is equal to str2"<<endl;
   }
   return 0;
}

Output

Following is the output of the above C++ program −

str1 is less than str2

Example 2

Now, Let's take another example where we will compare output of the strncmp() function with the strcmp() function.

#include <iostream>
#include <cstring>
using namespace std;

int main() {
   char str1[15];
   char str2[15];
   int ret;

   strcpy(str1, "abcdef");
   strcpy(str2, "abcdefgh");

   ret = strncmp(str1, str2, 5);

   if(ret > 0) {
      cout << "strncmp: str1 is greater than str2"<<endl;
   } else if(ret < 0) {
      cout << "strncmp: str1 is less than str2"<<endl;
   } else {
      cout << "strncmp: str1 is equal to str2"<<endl;
   }

   ret = strcmp(str1, str2);

   if(ret > 0) {
      cout << "strcmp: str1 is greater than str2"<<endl;
   } else if(ret < 0) {
      cout << "strcmp: str1 is less than str2"<<endl;
   } else {
      cout << "strcmp: str1 is equal to str2"<<endl;
   }
   return 0;
}

Output

Following is the output of the above C++ program −

strncmp: str1 is equal to str2
strcmp: str1 is less than str2

The difference in the output of the above program is because the strncmp() function compares only the first 5 characters of both strings whereas the strcmp() function compares both strings completely.