C++ .compare()

In C++, the .compare() method compares two strings lexicographically (dictionary order) and returns an integer that indicates their relative order. The comparison is done character by character based on ASCII values.

Syntax

string1.compare(string2);                              // Compares string1 with string2.
string1.compare(pos, len, string2);                    // Compares a substring of string1 starting at pos with length len to the entirety of string2.
string1.compare(pos, len, string2, subpos, sublen);    // Compares a substring of string1 (starting at pos, length len) with a substring of string2 (starting at subpos, length sublen)

Parameters:

• string1: The string object calling the .compare() method.
• string2: The string to compare with.
• pos (optional: Starting position in string1 for comparison
• len (optional): The number of characters to compare from string1 starting at pos.
• subpos (optional): The starting position in string2 from which to begin comparison.
• sublen (optional): The number of characters to compare from string2 starting at subpos.

Return values:

• Returns 0 if the strings are equal
• Returns a negative value (< 0) if string1 is lexicographically less than string2
• Returns a positive value (> 0) if string1 is lexicographically greater than string2

Example

This example demonstrates the usage of the .compare() method:

#include <iostream>
#include <string>

int main() {
  std::string str1 = "apple";
  std::string str2 = "banana";
  std::string str3 = "apple";

  int result1 = str1.compare(str2);
  int result2 = str1.compare(str3);
  int result3 = str2.compare(str1);

  std::cout << "apple vs banana: " << result1 << std::endl;
  std::cout << "apple vs apple: " << result2 << std::endl;
  std::cout << "banana vs apple: " << result3 << std::endl;

  return 0;
}

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The output of this code is:

apple vs banana: -1
apple vs apple: 0
banana vs apple: 1

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Codebyte Example

Run the following codebyte to understand how the .compare() method works:

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Frequently Asked Questions

1. What is the difference between .compare() and == operator?

The == operator returns a boolean (true or false) indicating whether two strings are equal. The .compare() method returns an integer indicating the relative order of the strings: less than, equal to, or greater than.

2. Can .compare() be used for case-insensitive comparison?

No, .compare() is case-sensitive by default. To perform a case-insensitive comparison, try converting both strings to the same case using methods like std::transform() with ::tolower, or use a custom comparator.

3. What happens when comparing strings of different lengths?

If one string is a prefix of the other, the shorter string is considered lexicographically smaller:

std::string a = "app";
std::string b = "apple";
bool result = a.compare(b) < 0; // true

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