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std::strncmp() in C++ - GeeksforGeeks

Last Updated : 02 Jul, 2024

std::strncmp() function in C++ lexicographically compares not more than count characters from the two null-terminated strings and returns an integer based on the outcome. This function is a Standard Library function that is defined in <cstring> header file in C++.

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

• str1 and str2: Pointer to the strings to be compared.
• count: Maximum number of characters to be compared.
• size_t: It is an unsigned integral type.

• It returns 0 if str1 is equal to str2.
• It returns an integer value less than zero if str1 is less than str2.
• It returns an integer value greater than zero if str1 is greater than str2.

If there are fewer than count characters in either string, the comparison ends when the first null is encountered.

// C++ program to illustrate the strncmp function
#include <cstring>
#include <iostream>
using namespace std;

int main()
{
    // defining two similar strings
    char* str1 = "GeeksforGeeks";
    char* str2 = "Geeks For Geeks";

    // comparing first 5 characters of each
    int value = strncmp(str1, str2, 5);

    if (!value) {
        cout << "The first 5 characters are same.";
    }
    else {
        cout << "The first 5 characters are not equal.";
    }
  
    return 0;
}

The first 5 characters are same.

When the strings are not equal, the value returned by the strncmp() function is the difference between the ASCII values of the first unmatched character in str1 and str2.

The strncmp() function returns three different types of integer values on the basis of comparison.

A positive value is returned if a character of str1 and str2 doesn't match before the num characters and the ASCII value of the str1 character is greater than the ASCII value of the str2 character. As a result, we can say that str1 is lexicographically greater than str2.

Example

// C, C++ program to demonstrate
// functionality of strncmp()
#include <cstring>
#include <iostream>
using namespace std;

int main()
{
    // Take any two strings
    char str1[10] = "aksh";
    char str2[10] = "akash";

    // Compare strings using strncmp()
    int result = strncmp(str1, str2, 4);

    if (result == 0) {
        // num is the 3rd parameter of strncmp() function
        cout << "str1 is equal to str2 up to num characters"
             << endl;
    }
    else if (result > 0)
        cout << "str1 is greater than str2" << endl;
    else
        cout << "str2 is greater than str1" << endl;

    cout << "Value returned by strncmp() is: " << result
         << endl;

    return 0;
}

str1 is greater than str2
Value returned by strncmp() is: 18

A negative value is returned, if a character of str1 and str2 doesn't match before the num characters and the ASCII value of the str1 character is lesser than the ASCII value of the str2 character. As a result, we can say that str2 is lexicographically greater than str1.

Example

// C, C++ program to demonstrate
// functionality of strncmp()

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

int main()
{
    // Take any two strings
    char str1[10] = "akash";
    char str2[10] = "aksh";

    // Compare strings using strncmp()
    int result = strncmp(str1, str2, 4);

    if (result == 0) {
        // num is the 3rd parameter of strncmp() function
        cout << "str1 is equal to str2 up to num characters"
             << endl;
    }
    else if (result > 0)
        cout << "str1 is greater than str2" << endl;
    else
        cout << "str2 is greater than str1" << endl;

    cout << "Value returned by strncmp() is: " << result
         << endl;

    return 0;
}

str2 is greater than str1
Value returned by strncmp() is: -18

This function returns zero if the characters of str1 match with the characters of the str2 up to num characters. As a result, we cannot say that str1 is equal to str2 until num is equal to the length of either string.

Example

// C, C++ program to demonstrate
// functionality of strncmp()

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

int main()
{
    // Take any two strings
    char str1[10] = "akash";
    char str2[10] = "akas";

    // Compare strings using strncmp()
    int result = strncmp(str1, str2, 4);

    if (result == 0) {
        // num is the 3rd parameter of strncmp() function
        cout << "str1 is equal to str2 up to num characters"
             << endl;
    }
    else if (result > 0)
        cout << "str1 is greater than str2" << endl;
    else
        cout << "str2 is greater than str1" << endl;

    cout << "Value returned by strncmp() is: " << result
         << endl;

    return 0;
}

str1 is equal to str2 up to num characters
Value returned by strncmp() is: 0

• This function takes two strings and a number num as arguments and compares at most the first num bytes of both strings.
• num should be at most equal to the length of the longest string. If num is defined as greater than the string length then the comparison is done till the null character ('\0') of either string.
• This function compares the two strings lexicographically. It starts comparison from the first character of each string. If they are equal to each other, it continues and compares the next character of each string, and so on.
• This process of comparison stops until a terminating null character of either string is reached or the num characters of both strings match.

• Difference between strncmp() and strcmp in C/C++

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