Nested if-else not working for custom structure c++

Question

I have a custom data structure date in C++11:

struct date {
   int day;
   int month;
   int year;
};

I want to compare two dates and write function for it:

int compare_dates(date a, date b) {
    int result = 0;

    if (a.year < b.year) {
        result = -1;
    } else if (a.year == b.year) {
        if (a.month < b.month) {
            result = -1;
        } else if (a.month == b.month) {
            if (a.day < a.day) {
                result = -1;
            } else if (a.day > a.day) {
                result = 1;
            }
        } else {
          result = 1;
        }
    } else {
        result = 1;
    }

    return result;
}

But this function doesn't work correctly. I spent a lot of time to debug it and found some issue in the following part of code:

} else if (a.month == b.month) {
    if (a.day < a.day) {
        result = -1;
    } else if (a.day > a.day) {
        result = 1;
    }
} else {
  result = 1;
}

There are two screenshots during debugging, first on } else if (a.month == b.month) { and second when I click next line in debugger. Such happens for all inputs. Why debbuger did not enter into if (a.day < a.day) { or result = 1; everytime?

You may have undefined behaviour elsewhere, which can lead to all kinds of strange things. The error cannot be reproduced by the portions you have shown us. Post a complete piece of code, please. See http://stackoverflow.com/help/mcve — Christian Hackl, Nov 19 '16 at 10:52
Thank you @ChristianHackl for the comment, next time I will add code according to *mcve* standard. — Dmytro Rudnitskikh, Nov 19 '16 at 11:06

max66 · Accepted Answer · 2016-11-19T11:03:55.543

2

Try with

if (a.day < b.day) {
    result = -1;
} else if (a.day > b.day) {
    result = 1;
}

instead of

if (a.day < a.day) {
    result = -1;
} else if (a.day > a.day) {
    result = 1;
}

The two tests a.day < a.day and a.day > a.day are ever false, so result = -1 and result = 1 are never executed.

I suppose that the compiler optimize the code as follows

} else if (a.month == b.month) {
} else {
  result = 1;
}

edited Nov 19 '16 at 11:03

answered Nov 19 '16 at 10:52

max66

65,235
10
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How exactly can this explain why the program skips a perfectly valid `==` comparison? – Christian Hackl Nov 19 '16 at 10:53
@ChristianHackl - I suppose it's because the code that should be executed when the condition is true is an `if`/`else if` with condition that are ever false – max66 Nov 19 '16 at 10:57
Is really solve my issue, thank you! But I still confused why debugger does not help me. – Dmytro Rudnitskikh Nov 19 '16 at 11:02
@DmytroRudnitskikh - expanded my answer; hope this respond to your question about the dubbugger. – max66 Nov 19 '16 at 11:04
@max66 Big thank you again! So stupid mistake, on which I have spent a lot of time. – Dmytro Rudnitskikh Nov 19 '16 at 11:10
@max66 maybe you are interesting to improve this algorithm http://stackoverflow.com/questions/40692789/how-can-i-optimize-compare-function-for-custom-date-structure – Dmytro Rudnitskikh Nov 19 '16 at 12:20

score 1 · Answer 2 · answered Nov 19 '16 at 11:13

It may be that compiler optimisations get in your way.

As others have observed, you have two typos inside the block. a.day < a.day and a.day > a.day are by definition always false. Consequently, nothing can ever happen in the block. There is no observable behaviour. That's probably why the compiler just completely eliminates the entire thing to save speed and/or space.

I don't know which compiler you are using, or which flags you pass it, but if you disable all optimisation, then the debugger should work as you expect. You can also try to put a std::cout << "...\n"; into the block to enforce some observable behaviour and see if that changes something.

This is also an excellent reason to actually learn how your compiler really works, without all the IDE fluff covering its functionality.

I use GNU SCC Compiler with `-std=c++11` – Dmytro Rudnitskikh Nov 19 '16 at 11:20 — Dmytro Rudnitskikh, Nov 19 '16 at 11:20
@DmytroRudnitskikh: Then try to add `-O0`. – Christian Hackl Nov 19 '16 at 11:21 — Christian Hackl, Nov 19 '16 at 11:21

score 0 · Answer 3 · answered Nov 19 '16 at 10:58

0

Line no 25 in screenshot first,

if (a.day < a.day)

It should be

if (a.day < b.day)

same mistake is in line 27.

answered Nov 19 '16 at 10:58

Rahul

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Nested if-else not working for custom structure c++

3 Answers3