std::string::compare() in C++

The std::string::compare() function in C++ is used to perform lexicographical comparison between two strings or substrings. It is a member function of the std::string class defined in the <string> header file.

• It compares strings character by character using ASCII values.
• It can compare entire strings as well as specific portions of strings.

Return Value of compare()

The compare() function returns:

• 0 if both strings (or substrings) are equal.
• A positive value if the first mismatching character in the first string has a greater ASCII value.
• A negative value if the first mismatching character in the first string has a smaller ASCII value.

Overloads of string::compare()

Compare Two Strings

The string::compare() method can be used to compare the one string with another string that is passed to it as argument.

Syntax

str1.compare(str2);

Parameters: str2 - String to compare with str1.

#include <bits/stdc++.h>
using namespace std;

int main()
{
    string str1("Geeks");
    string str2("Geeks");

    // Comparing str1 and str2
    if ((str1.compare(str2)) == 0)
        cout << "String Matched" << endl;
    else
        cout << "String Not Matched" << endl;

    return 0;
}

String Matched

Explanation: The strings "Geeks" and "Geeks" are identical, so compare() returns 0.

Compare Substring with Another String

The string::compare() method also supports the comparison between a part of the string with another string.

Syntax

str1.compare(pos, len, str2);

Parameters

• pos: Starting index in str1.
• len: Number of characters to compare.
• str2: String to compare.

#include <bits/stdc++.h>
using namespace std;

int main()
{
    string str1("forGeeks");
    string str2("Geeks");

    // Compares 5 characters starting from index
    // 3 of str1 with str2
    if ((str1.compare(3, 5, str2)) == 0)
        cout << "Substring Matched";
    else
        cout << "Strings Not Matched";

    return 0;
}

Substring Matched

Explanation: The substring of str1 starting at index 3 with length 5 is "Geeks", which matches str2.

Comparing Two Substrings

The compare() function also allows comparison between two substrings.

Syntax

str1.compare(pos1, len1, str2, pos2, len2);

Parameters

• pos1: Starting index in str1.
• len1: Number of characters from str1.
• str2: Second string.
• pos2: Starting index in str2.
• len2: Number of characters from str2.

#include <bits/stdc++.h>
using namespace std;

int main()
{
    string str1("Geeks");
    string str2("forGeeks");

    // Compares 5 characters of str1 starting from
    // index 0 with 5 characters of str2 starting
    // from index 3
    if ((str1.compare(0, 5, str2, 3, 5)) == 0)
        cout << "Both Substring Matches";

    else
        cout << "Substrings Not Matched";

    return 0;
}

Both Substring Matches

Explanation: The first 5 characters of str1 ("Geeks") are compared with the 5 characters of str2 starting at index 3 ("Geeks"). Since both substrings are equal, compare() returns 0.

Advantages of std::string::compare()

The compare() function provides a convenient way to perform lexicographical comparisons.

• Supports complete string as well as substring comparison.
• Returns ordering information in addition to equality.
• Performs comparisons efficiently without creating temporary strings.
• Provides clearer intent than manual character-by-character comparison.