3 Ways to Compare Strings in C++

Introduction

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

Strings in C++ can be compared using one of the following techniques:

1. String strcmp() function
2. built-in compare() function
3. C++ Relational Operators (==, !=)

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

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

strcmp() Syntax

• The input string has to be a char array of C-style String.
• The strcmp() compares the strings in a case-sensitive form as well.

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

This function returns the following values according to the matching cases:

• Returns 0 if both the strings are the same.
• Returns < 0 (less than zero) if the value of the character of the first string is smaller as compared to the second string input.
• Results out to be > 0 (greater than zero) when the second string is greater in comparison.

strcmp() Example 1

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 will generate the following output:

Output
String 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.

strcmp() Example 2

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 will generate the following output:

Output
String 1: String Match
String 2: String Match

Both the input strings are equal.

strcmp(str_inp1, str_inp2) results in 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.

compare() Syntax

The compare() function compares two strings:

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

This function returns the following values according to the matching cases:

• Returns 0 if both the strings are the same.
• Returns < 0 (less than zero) if the value of the character of the first string is smaller as compared to the second string input.
• Results out to be > 0 (greater than zero) when the second string is greater in comparison.

Example 1: Using compare()

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 the strings are the same lexicographically, so the function returns 0.

Example 2: Using compare()

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 compared a string with another input string to the compare() function.

3. Relational Operators in C++

C++ Relational operators such as == (double equals) and != (not equals) can be helpful in the comparison of strings.

Relational Operators Syntax

Check if two values are equal:

string1 == string2

Check if two values are not equal:

string1 != string2

Example 1: Using 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 values for “String 1” and “String 2”:

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

The code will compare the two strings with ==.

Example 2: Using 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 values for “String 1” and “String 2”:

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

The code will compare the two strings with !=.

Conclusion

In this article, you learned methods to compare strings in C++. This included String’s strcmp() function, the built-in compare() function, and relational operators (==, !=).

Continue your learning with more C++ tutorials.