Comparing strings using std::string::compare, c++ [duplicate]

Question

I have a question:

Let's say there are two std::strings and I want to compare them, there is the option of using the compare() function of the string class but I also noticed that it is possible using simple < > != operators (both of the cases are possible even if I don't include the <string> library). Can someone explain why the compare() function exists if a comparison can be made using simple operators?

btw I use Code::Blocks 13.12 here is an example of my code:

#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::getline;

int main()
{
    string temp1, temp2;
    cout << "Enter first word: ";
    getline (cin,temp1);
    cout << "Enter second word: ";
    getline (cin,temp2);
    cout << "First word: " << temp1 << endl << "Second word: " << temp2 << endl;
    if (temp1 > temp2)
    {
        cout << "One" << endl;
    }
    if (temp1.compare(temp2) < 0)
    {
        cout << "Two" << endl;
    }
    return 0;
}

Sometimes it's more useful to have a positive, zero, or negative number. — Qaz
– Qaz, Commented Jul 25, 2014 at 14:24

Tom Fenech · Accepted Answer · 2014-07-25 14:34:50Z

47

.compare() returns an integer, which is a measure of the difference between the two strings.

A return value of 0 indicates that the two strings compare as equal.
A positive value means that the compared string is longer, or the first non-matching character is greater.
A negative value means that the compared string is shorter, or the first non-matching character is lower.

operator== simply returns a boolean, indicating whether the strings are equal or not.

If you don't need the extra detail, you may as well just use ==.

edited Jul 25, 2014 at 14:34

answered Jul 25, 2014 at 14:25

Tom Fenech

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Comments

ruohola · Accepted Answer · 2019-05-13 19:47:33Z

string cat = "cat";
string human = "human";

cout << cat.compare(human) << endl;

This code will give -1 as a result. This is due to the first non-matching character of the compared string 'h' is lower or appears after 'c' in alphabetical order, even though the compared string, 'human' is longer than 'cat'.

I find the return value described in cplusplus.com is more accurate which are-:

0 : They compare equal

<0 : Either the value of the first character that does not match is lower in the compared string, or all compared characters match but the compared string is shorter.

more than 0 : Either the value of the first character that does not match is greater in the compared string, or all compared characters match but the compared string is longer.

Moreover, IMO cppreference.com's description is simpler and so far best describe to my own experience.

negative value if *this appears before the character sequence specified by the arguments, in lexicographical order

zero if both character sequences compare equivalent

positive value if *this appears after the character sequence specified by the arguments, in lexicographical order

Cheers and hth. - Alf · Accepted Answer · 2014-07-25 14:47:52Z

2

Regarding the question,

” can someone explain why the compare() function exists if a comparison can be made using simple operands?

Relative to < and ==, the compare function is conceptually simpler and in practice it can be more efficient since it avoids two comparisons per item for ordinary ordering of items.

As an example of simplicity, for small integer values you can write a compare function like this:

auto compare( int a, int b ) -> int { return a - b; }

which is highly efficient.

Now for a structure

struct Foo
{
    int a;
    int b;
    int c;
};

auto compare( Foo const& x, Foo const& y )
    -> int
{
    if( int const r = compare( x.a, y.a ) ) { return r; }
    if( int const r = compare( x.b, y.b ) ) { return r; }
    return compare( x.c, y.c );
}

Trying to express this lexicographic compare directly in terms of < you wind up with horrendous complexity and inefficiency, relatively speaking.

With C++11, for the simplicity alone ordinary less-than comparison based lexicographic compare can be very simply implemented in terms of tuple comparison.

edited Jul 25, 2014 at 14:47

answered Jul 25, 2014 at 14:28

Cheers and hth. - Alf

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2 Comments

rubenvb Over a year ago

auto int f(int, int) -> int; shudders. Why the trailing return type?

Cheers and hth. - Alf Over a year ago

@rubenvb: nobody's yet made a case for using two different function declaration syntaxes. i guess the "shuffers" comment and the invalid declaration means you react emotionally to any change here. but i fail to see the utility in using two different syntaxes, when a single one suffices.

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