String comparison giving different output when used with variable

Question

Why is the output of the following codes different?

I'm comparing two strings. I don't understand why they give different outputs?

Code 1:

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

int main() {
    if("35" <= "255")
    {
        cout << 1;    
    }
    cout << 0;
}

Code 2:

#include <bits/stdc++.h>
using namespace std;
int main() {
    string num = "35";
    if(num <= "255")
    {
        cout << 1;    
    }
    cout << 0;
}

The output of code 1 is 10. The output of code 2 is 0.

Oh and `num<="255"` doesn't compare the integer values inside the strings, it compares the strings as strings of *characters*. — Some programmer dude, Aug 12 '19 at 13:33
Please don't use `` this is an implementation specific header and non-portable use `` instead. — Mgetz, Aug 12 '19 at 13:42

eerorika · Answer 1 · 2019-08-12T13:56:54.127

3

You made the second program different by using std::string.

std::string has an overload for the comparison operator, which compares the content of the operands lexicographically. Lexicographical ordering, which is different from numerical ordering, is same that would be used in a dictionary: 255 comes before (i.e. "is less than") 35, just like aardvark comes before zoo.

The string literals on the other hand are arrays, which will decay to a pointer to first element, and pointer comparison compares the relative location in memory, which has nothing to do with the text content, and which in this case is at best unspecified and you could either see 1 output or not.

edited Aug 12 '19 at 13:56

answered Aug 12 '19 at 13:38

eerorika

232,697
12
197
326

@TobySpeight Exact wording of the latest draft is "neither pointer is required to compare greater than the other.". Which is unspecified, as I stated in the answer; either result is possible. – eerorika Aug 12 '19 at 13:46
1

@formerlyknownas_463035818 added explanation. – eerorika Aug 12 '19 at 13:51
you already had my vote ;) – 463035818_is_not_an_ai Aug 12 '19 at 13:58

score 0 · Answer 2 · answered Aug 12 '19 at 13:40

A string is not a magic object that understands what it contains and acts differently on it.

In your case, you are comparing the address that hold a buffer of chars (containing '3', '5', 0) with the address containing another buffer of chars (containing '2', '3', '5', 0).

The output is random (in fact, it isn't, but for now, let's assume it is).

If you want to compare strings, you can use the second example (or strcmp) but that will compare the buffer content based on some logical rules, that are not those you except (you expect semantic logic, but it's not).

The rules are:

Compare each buffer char based on their ASCII/Unicode order, and return -1 if the first one is lower than the second one, 1 if it's higher. (If using < operator, it returns true if -1, false otherwise and so on)
If they are equal, continue to next char.

In the previous example, '3' is higher than '2' (even if 35 is smaller than 235).

You'll need to either convert your string to integer before comparing (and deal with potential conversion errors) or use integers from the beginning.

score 0 · Answer 3 · answered Aug 12 '19 at 13:42

0

First convert the string to Int and then compare.

Example:

#include <iostream>
using std::cout;

string value1 = "22";
string value2 = "222";
int main()
{
if(std::stoi(value1)<=std::stoi(value2)) 
{

cout<<"1";

}
cout<<"0";


}

answered Aug 12 '19 at 13:42

Pradeep Kumar

109
1
10

1

this is probably what OP was aiming for in the first place, but you do not answer the question – 463035818_is_not_an_ai Aug 12 '19 at 13:43

String comparison giving different output when used with variable

3 Answers3