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

compare() is a public member function of string class. It compares the value of the string object (or a substring) to the sequence of characters specified by its arguments. 
The compare() can process more than one argument for each string so that one can specify a substring by its index and by its length.
Return type : compare() returns an integer value rather than a Boolean value.
Different Syntaxes for string::compare() :

• Syntax 1: Compares the string *this with the string str.

int string::compare (const string& str) const
Returns:
0 : if both strings are equal.
A value < 0 : if *this is shorter than str or,
first character that doesn't match is smaller than str.
A value > 0 : if *this is longer than str or,
first character that doesn't match is greater

CPP

// CPP code for demonstrating
// string::compare (const string& str) const

#include<iostream>
using namespace std;

void compareOperation(string s1, string s2)
{
    // returns a value < 0 (s1 is smaller than s2)
    if((s1.compare(s2)) < 0)
        cout << s1 << " is smaller than " << s2 << endl;

    // returns 0(s1, is being compared to itself)
    if((s1.compare(s1)) == 0)
        cout << s1 << " is equal to " << s1 << endl;
    else
        cout << "Strings didn't match ";

}

// Driver Code
int main()
{
    string s1("Geeks");
    string s2("forGeeks");
    compareOperation(s1, s2);

    return 0;
}

Output: 

Geeks is smaller than forGeeks
Geeks is equal to Geeks

• Syntax 2: Compares at most, len characters of string *this, starting with index idx with the string str.

int string::compare (size_type idx, size_type len, const string& str) const
Throws out_of_range if index > size().

CPP

// CPP code to demonstrate
// int string::compare (size_type idx, size_type len,
// const string& str) const

#include<iostream>
using namespace std;

void compareOperation(string s1, string s2)
{
    // Compares 5 characters from index number 3 of s2 with s1
    if((s2.compare(3, 5, s1)) == 0)
        cout << "Here, "<< s1 << " are " << s2;

    else
        cout << "Strings didn't match ";
}
// Driver Code
int main()
{
    string s1("Geeks");
    string s2("forGeeks");
    compareOperation(s1, s2);

  return 0;
}

Output: 

Here, Geeks are forGeeks

• Syntax 3: Compares at most, len characters of string *this starting with index idx with at most, str_len characters of string str starting with index str_idx.

int string::compare (size_type idx, size_type len, const string& 
str, size_type str_idx, size_type str_len) const
Throws out_of_range if idx > size().
Throws out_of_range if str_idx > str.size().

CPP

// CPP code to demonstrate
// int string::compare (size_type idx, size_type len, const string&
// str, size_type str_idx, size_type str_len) const

#include<iostream>
using namespace std;

void compareOperation(string s1, string s2)
{
    // Compares 5 characters from index number 0 of s1 with
    // 5 characters from index 3 of s2
    if((s1.compare(0, 5, s2, 3, 5)) == 0)
        cout << "Welcome to " << s1 << s2 << " World";

    else
        cout << "Strings didn't match ";
}
// Driver Code
int main()
{
    string s1("Geeks");
    string s2("forGeeks");
    compareOperation(s1, s2);

  return 0;
}

Output: 

Welcome, to GeeksforGeeks World

• Syntax 4: Compares the characters of string *this with the characters of the C-string cstr. 

int string::compare (const char* cstr) const

CPP

// CPP code to demonstrate
// int string::compare (const char* cstr) const

#include<iostream>
using namespace std;

void compareOperation(string s1, string s2)
{
    // returns < 0 (s1 < "GeeksforGeeks")
    if((s1.compare("GeeksforGeeks")) < 0)
        cout << s1 << " is smaller than string " << "GeeksforGeeks";

    //returns 0 (s2 is "forgeeks")
    if((s2.compare("forGeeks")) == 0)
        cout << endl << s2 << " is equal to string " << s2;

    else
        cout << "Strings didn't match ";

}
// Driver Code
int main()
{
    string s1("Geeks");
    string s2("forGeeks");
    compareOperation(s1, s2);

  return 0;
}

Output: 

Geeks is smaller than string GeeksforGeeks
forGeeks is equal to string forGeeks

• Syntax 5: Compares at most, len characters of string *this, starting with index idx with all characters of the C-string cstr.

int string::compare (size_type idx, size_type len, const char* cstr) const

Note that cstr may not be a null pointer (NULL).

CPP

// CPP code to demonstrate
// int string::compare (size_type idx, size_type len,
// const char* cstr) const
#include<iostream>
using namespace std;

void compareOperation(string s1)
{
    // Compares 5 characters from 0 index of s1 with "Geeks"
    if((s1.compare(0, 5, "Geeks")) == 0)
        cout << s1 << " are " << "awesome people";

    else
        cout << "Strings didn't match ";

}

// Driver Code
int main()
{
    string s1("Geeks");
    compareOperation(s1);

  return 0;
}

Output: 

Geeks are awesome people

• Syntax 6: Compares, at most, len characters of string *this, starting with index idx with chars_len characters of the character array chars. 

int string::compare (size_type idx, size_type len, const char* chars, 
size_type chars_len)const

Note that chars must have at least chars_len characters. The characters may have arbitrary values. Thus, ‘\0’ has no special meaning.

CPP

// CPP code to demonstrate
// int string::compare (size_type idx, size_type len,
// const char* chars, size_type chars_len)const

#include<iostream>
using namespace std;

void compareOperation(string s1, string s2)
{
    // Compares 5 characters from 0 index of s1 with
    // 5 characters of string "Geeks"
    if((s1.compare(0, 5, "Geeks", 5)) == 0)
        cout << "This is " << s1 <<  s2 ;

    else
        cout << "Strings didn't match ";
}

// Driver Code
int main()
{
    string s1("Geeks");
    string s2("forGeeks");
    compareOperation(s1, s2);

  return 0;
}

Output: 

This is GeeksforGeeks

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