3 Ways to Compare Strings in C++

Introduction

In this article, you will learn how to compare strings in C++.

1. Using the strcmp() function in C++

C++ String has built-in functions for manipulating String data. The strcmp() function is a C library function used to compare two strings lexicographically.

The input string must be a character array of C-style strings.
strcmp() also compares strings in a case-sensitive form.

int strcmp(const char *str1, const char *str2);

This function returns the following values depending on the matching items:

Returns 0 if both strings are the same.
If the character value of the first string is smaller compared to the second string input, it returns < 0 (less than zero).
When the second string in the comparison is larger, the results are > 0 (greater than zero).

Run the following code:

#include <iostream>
#include <string.h>
int main()
{
const char *str_inp1 = "String Match";
const char *str_inp2 = "String Unmatch";
std::cout << "String 1: " << str_inp1 << std::endl;
std::cout << "String 2: " << str_inp2 << std::endl;
if (strcmp(str_inp1, str_inp2) == 0)
std::cout << "\nBoth the input strings are equal." << std::endl;
else
std::cout << "\nThe input strings are not equal." << std::endl;
}

This produces the following output:

OutputString 1: String Match
String 2: String Unmatch
The input strings are not equal.

strcmp(str_inp1, str_inp2) results in -9. The values of str_inp1 and str_inp2 are different.

Run the following code:

#include <iostream>
#include <string.h>
int main()
{
const char *str_inp1 = "String Match";
const char *str_inp2 = "String Match";
std::cout << "String 1: " << str_inp1 << std::endl;
std::cout << "String 2: " << str_inp2 << std::endl;
if (strcmp(str_inp1, str_inp2) == 0)
std::cout << "\nBoth the input strings are equal." << std::endl;
else
std::cout << "\nThe input strings are not equal." << std::endl;
}

This produces the following output:

Output
String 1: String Match
String 2: String Match
Both the input strings are equal.

strcmp(str_inp1, str_inp2) returns 0. The values of str_inp1 and str_inp2 are the same.

2. Using the compare() function in C++

C++ has a built-in compare() function to compare two strings.

The compare() function compares two strings:

int compare (const string& string-name) const;

This function returns the following values depending on the matching items:

Returns 0 if both strings are the same.
If the character value of the first string is smaller compared to the second string input, it returns < 0 (less than zero).
When the second string in the comparison is larger, the results are > 0 (greater than zero).

Run the following code:

#include <iostream>
int main()
{
std::string str_inp1("String Match");
std::string str_inp2("String Match");
std::cout << "String 1: " << str_inp1 << std::endl;
std::cout << "String 2: " << str_inp2 << std::endl;
int res = str_inp1.compare(str_inp2);
if (res == 0)
std::cout << "\nBoth the input strings are equal." << std::endl;
else if (res < 0)
std::cout << "\nString 1 is smaller as compared to String 2." << std::endl;
else
std::cout << "\nString 1 is greater as compared to String 2." << std::endl;
}

In this example, str_inp1 and str_inp2 are compared with compare():

Output
String 1: String Match
String 2: String Match
Both the input strings are equal.

Both strings are lexically identical, so the function returns 0.

Run the following code:

#include <iostream>
int main()
{
std::string str_inp0("String Match");
std::string str_inp1("String Match");
std::string str_inp2("String Unmatch");
std::cout << "String 1: " << str_inp1 << std::endl;
if (str_inp1.compare(str_inp0) == 0)
std::cout << "\nStrings are equal." << std::endl;
else
std::cout << "\nStrings are not equal." << std::endl;
std::cout << "String 2: " << str_inp2 << std::endl;
if (str_inp2.compare(str_inp0) == 0)
std::cout << "\nStrings are equal." << std::endl;
else
std::cout << "\nStrings are not equal." << std::endl;
}

In this example, str_inp0 is compared to str_inp1:

Output
String 1: String Match
Strings are equal.

Then str_inp0 is compared to str_inp2:

Output
String 2: String Unmatch
Strings are not equal.

This code directly compares a string to another input string with the compare() function.

3. Relational operators in C++

C++ relational operators such as == (equals) and != (not equal) can be useful in comparing strings.

Check if two values are equal:

string1 == string2

Check if two values are not equal:

string1 != string2

Example 1: Using the C++ == operator

Run the following code:

#include <iostream>
int main()
{
std::string str_inp1;
std::string str_inp2;
std::cout << "Enter the String 1:\n";
std::cin >> str_inp1;
std::cout << "Enter the String 2:\n";
std::cin >> str_inp2;
if (str_inp1 == str_inp2)
std::cout << "Strings are equal" << std::endl;
else
std::cout << "Strings are not equal" << std::endl;
}

Provide the values "String 1" and "String 2":

Enter the String 1:
DigitalOcean
Enter the String 2:
digitalocean
Strings are not equal

The code compares two strings with ==.

Example 2: Using the C++ != operator

Run the following code:

#include <iostream>
int main()
{
std::string str_inp1;
std::string str_inp2;
std::cout << "Enter the String 1:\n";
std::cin >> str_inp1;
std::cout << "Enter the String 2:\n";
std::cin >> str_inp2;
if (str_inp1 != str_inp2)
std::cout << "Strings are not equal" << std::endl;
else
std::cout << "Strings are equal" << std::endl;
}

Provide the values "String 1" and "String 2":

Enter the String 1:
DigitalOcean
Enter the String 2:
DigitalOcean
Strings are equal

The code compares two strings with !=.

Result

In this article, you learned about string comparison methods in C++. This included the String strcmp() function, the built-in compare() function, and the relational operators (==, !=).