C++ strncmp() function

The strncmp() is a string manipulation function defined in the cstring header file, which works on a string which is stored in a c-style char array and it is used to compare the first n number of characters of a string to the another string.

Signature of strncmp() function

int ( char * str1, const char * str2, size_t num )

This function compares the characters of str1 string to another string str2 and returns the result of Comparison in the form an int value i.e. the ASCII value difference between the first mismatching characters between the two string, such as -

int value	description
0	when the characters compared between two strings are the same.
A positive int value	when the non-matching character in the first string is having an ASCII* value larger than the character at the same index in second string.*
A negative int value	when the non-matching character in the first string is having an ASCII* value smaller than the character at the same index in second string.*

Example of strncmp() function

In the upcoming code, we have called strncmp() function to compares the first n number of characters between the following three char[] arrays, such as -

The first char[] array i.e. first is initialized with a string value North POLE.
The second char[] array i.e. second is also initialized with the same value North pole.
The third char[] array i.e. third is initialized with the value Norway.

// Program to compare n number of characters of first string with another string. 

#include<iostream>
#include<cstring>

using namespace std;

int main()
{
char first[20] = "North POLE";
char second[20] = "North pole";
char third[10] = "Norway";

cout<<"The content of first string : " <<first <<"\n";
cout<<"The content of second string : " << second <<"\n";


// comparing the first 4 characters of first string with the second string
int result = strncmp(first, second, 4);
cout<<"Comparing first 4 characters between " <<first <<" and " <<second <<" : " <<result <<"\n";

// comparing the first 4 characters of second string with the third string 
result = strncmp(second, third,4); // strncmp gives the ASCII value difference between the first mismatching characters between the two strings */

cout<<"Comparing first 4 characters between " <<second <<" and " <<third <<" : " <<result;

return 0;
}

Output is -

The content of first string : North POLE
The content of second string : North pole
Comparing first 4 characters between North POLE and North pole gives : 0
Comparing first 4 characters between North pole and Norway gives : -3

On the first call to the strncmp(), it compares the first 4 characters in the first char[] array with the first 4 characters in the second char[] array and returns 0 i.e. compared characters are the same.
On the second call to the strncmp(), it compares the first 4 characters in the second char[] array with the first 4 characters in the third char[] array and returns -3 i.e. the difference between the ASCII value of non-matching characters.

Another example of strncmp() function

strcmp()

char[]

The first char[] array i.e. first is initialized with a string value Never give up!
The second char[] array i.e. second is also initialized with the same value Don't give up!.

// strncmp example 

#include<iostream>
#include<cstring>

using namespace std;

int main()
{
char first[15] = "Never give up!";
char second[15] = "Don't give up";

cout<< "The content of first string : " << first << "\n";
cout<< "The content of second string : " << second << "\n";


// comparing the characters of first string with the characters in second string
int result = strncmp(first, second,4);

cout<< "Comparing the first 4 characters between " << first << " and " << second << " : " << result;

return 0;
}

Output is -

The content of first string : Never give up!
The content of second string : Don't give up
Comparing the first 4 characters between Never give up! and Don't give up gives : 10

On the call to the strcmp(), it compares the characters in the first char[] array with the characters in the second char[] array and returns 10 i.e. because when comparing Never give up! with Don't give up!, the ASCII value of N(78) in first char [] array is larger than ASCII value of D(68) in the second char[] array, hence strncmp() gives their difference i.e. 10.