Comparing two strings in C++

Comparing strings in C++ determines whether two strings are equal or which one comes first in lexicographical order.

String comparison is case-sensitive and compares characters from left to right.
Relational operators and compare() can be used to compare std::string objects.

Examples: Given two strings, how to check if the two strings are equal or not.

Input : ABCD, XYZ
Output : ABCD is not equal to XYZ
XYZ is greater than ABCD

Input : Geeks, forGeeks
Output : Geeks is not equal to forGeeks
forGeeks is greater than Geeks

Note: String comparison in C++ is case-sensitive and follows lexicographical order, so uppercase and lowercase characters can produce different comparison results.

Approaches to Compare Two Strings

The following two approaches can be used to compare std::string objects in C++:

1. Comparing Strings Using Relational Operators

Relational operators such as ==, !=, <, >, <=, and >= can directly compare two std::string objects.

For example, s1 < s2 checks whether s1 comes before s2 lexicographically, while s1 == s2 checks whether both strings are equal.

CPP

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

void relationalOperation(const string& s1, const string& s2)
{
    if (s1 != s2)
    {
        cout << s1 << " is not equal to " << s2 << endl;

        if (s1 > s2)
            cout << s1 << " is greater than " << s2 << endl;
        else
            cout << s2 << " is greater than " << s1 << endl;
    }
    else
        cout << s1 << " is equal to " << s2 << endl;
}

int main()
{
    string s1("Geeks");
    string s2("forGeeks");

    relationalOperation(s1, s2);

    string s3("Geeks");
    string s4("Geeks");

    relationalOperation(s3, s4);

    return 0;
}

Output

Geeks is not equal to forGeeks
forGeeks is greater than Geeks
Geeks is equal to Geeks

Explanation: The comparison is case-sensitive. For example, "geeks" is greater than "forGeeks" because the first characters g and f are compared first, and g comes after f.

2. Comparing Strings Using compare()

The compare() function compares two strings lexicographically and returns:

A value less than 0 if the first string is smaller.
0 if both strings are equal.
A value greater than 0 if the first string is greater.

CPP

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

void compareFunction(const string& s1, const string& s2)
{
    int result = s1.compare(s2);

    if (result != 0)
    {
        cout << s1 << " is not equal to " << s2 << endl;

        if (result > 0)
            cout << s1 << " is greater than " << s2 << endl;
        else
            cout << s2 << " is greater than " << s1 << endl;
    }
    else
        cout << s1 << " is equal to " << s2 << endl;
}

int main()
{
    string s1("Geeks");
    string s2("forGeeks");

    compareFunction(s1, s2);

    string s3("Geeks");
    string s4("Geeks");

    compareFunction(s3, s4);

    return 0;
}

Output

Geeks is not equal to forGeeks
forGeeks is greater than Geeks
Geeks is equal to Geeks

Relational Operators Vs compare()

Relational operators return a Boolean value, while compare() returns a negative, zero, or positive integer.
Relational operators are convenient for direct comparisons, while compare() also supports substring comparisons.
compare() can compare specific portions of two strings without creating separate substrings.

Comparing Substrings Using compare()

The compare() function can compare specific parts of strings directly.

C++

string str1 = "GeeksforGeeks"; 
string str2 = "Geeks"; 

if (str1.compare(5, 3, str2, 0, 3) == 0) 
    cout << "Equal"; 
else 
    cout << "Not equal";

Here, 3 characters starting from index 5 of str1 are compared with 3 characters starting from index 0 of str2. The compare() function is useful when specific portions of strings need to be compared without manually extracting them.