C++ String Comparison

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C++ string comparison refers to the process of evaluating two strings to determine their equality or their order based on lexicographical rules.

String comparison can be done by using built-in operators such as ==, !=, <, and > or by the compare() method. But by default these comparisons are case-sensitive, which means "tutorial point" and "Tutorial point" are considered different. String comparison plays an important role in performing tasks like sorting, searching, and input validation.

Types of String in C++

Basically there are two primary types of string in C++ −

• C-style strings − This string in C++ is an array of characters with a null character ('\0').
• std::string − This string is a part of the C++ Standard Library which provides a more robust and user-friendly way to handle strings because it manages memory automatically, allows dynamic resizing, and provides a vast set of member functions for manipulation like concatenation, substring extraction, and searching.

Comparing C-Style Strings

Here's how you can compare C-style strings −

1. strcmp()

You can use the strcmp() function from the <cstring> library to compare two strings.

Example

Heres a given example showing a comparison between two C-style strings −

#include <iostream>
#include <cstring>

int main() {
   const char* str1 = "hello";
   const char* str2 = "Tutorialspoint Learner";

   int result = strcmp(str1, str2);

   if (result < 0) {
      std::cout << "str1 is less than str2\n";
   } else if (result > 0) {
      std::cout << "str1 is greater than str2\n";
   } else {
      std::cout << "str1 is equal to str2\n";
   }

   return 0;
}

Output

str1 is greater than str2

Explanation

• The strcmp function compares the two strings and returns negative (str1 < str2), zero(str1 = str2) and positive(str1 > str2).
• In this case, "hello" is lexicographically greater than "Tutorialspoint Learner", so the output is "str1 is greater than str2".
• In lexicographical order, strings are compared character by character based on their ASCII values, where 'h' (ASCII 104) and 'T' (ASCII 84)
• So 104 > 84 then this comparison resulted in "hello" being greater than "Tutorialspoint Learner".

2. strcoll()

The strcoll() function compares two C-strings according to the current locale, which is useful for internationalization. It behaves similarly to strcmp() but takes into account locale-specific rules.

Example

#include <iostream>
#include <cstring>
#include <locale>

int main() {
   const char* str1 = "hello";
   const char* str2 = "Tutorialspoint Learner";

   // Set the locale (optional, depends on your environment)
   std::setlocale(LC_COLLATE, "en_US.UTF-8");

   int result = strcoll(str1, str2);

   if (result < 0) {
      std::cout << "str1 is less than str2\n";
   } else if (result > 0) {
      std::cout << "str1 is greater than str2\n";
   } else {
      std::cout << "str1 is equal to str2\n";
   }

   return 0;
}

Output

str1 is greater than str2

Explanation

• The std::setlocale() function sets the locale for string collation (comparison) to "en_US.UTF-8", which is the U.S. English locale.
• The strcoll() function compares str1 and str2 according to the current locale.
• It returns a "-ve" value if str1 < str2, 0 if they are equal, and a "+ve" value if str1 > str2.
• Since 'h' (ASCII 104) > 'T' (ASCII 84) therefor output is 'h' is greater than 'T'.

Comparing std::string

In C++ for comparing std::string objects have various methods that give different ways of accessing their equality or relative order.

1. Comparison Operators

Comparison operators give access to compare two std::string objects directly.

This comparison is also done lexicographically which means characters are compared based on their ASCII values. These are the following −

• == (Equality)
• != (Inequality)
• < (Less than)
• > (Greater than)
• <= (Less than or equal to)
• >= (Greater than or equal to)

Example

#include <iostream>
#include <string>

int main() {
   std::string str1 = "Allen";
   std::string str2 = "allen";

   std::cout << (str1 == str2) << std::endl; // false (0)
   std::cout << (str1 != str2) << std::endl; // true (1)
   std::cout << (str1 < str2) << std::endl;  // true (1)
   std::cout << (str1 > str2) << std::endl;  // false (0)

   return 0;
}

Output

0
1
1
0

Explanation

Since the ASCII value of A (65) is less than a (97), So receiving output accordingly.

2. std::string::compare() Method

The std::string::compare() method is also used to compare the value of two strings. It returns an integer value based on the lexicographical comparison.

• 0: if both strings are equal
• > 0 (+ve) : if the first string is greater
• < 0 (-ve) : if the first string is lesser

Example

#include <iostream>
#include <string>

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

   std::cout << str1.compare(str2) << std::endl; 
   std::cout << str1.compare("apple") << std::endl; 

   return 0;
}

Output

-1
0

Explanation

• At first, since 'a' (ASCII 97) is less than 'b' (ASCII 98) so output is a negative number (-1).
• In the second, the output is 0 as values stored in str1 and "apple" are the same.