Don't understand simple C Program if/else result

Question

int main()
{
    unsigned long a= 100000;
    long b = -1;
    if (b > a)
        printf("yes\n");
    else
        printf("No\n");

    return 0;
 }

Why the output comes "Yes" when we know 100000 > -1 . So accordingly it should print "NO" but being a naive, i can really get it

First problem `void main()` is a very old way of defining `main()`, that means that you are using very outdated resources. — Iharob Al Asimi, Jun 21 '15 at 12:43
Second problem: You haven't enabled warning flags or have ignored them. — Spikatrix, Jun 21 '15 at 12:44
This should clear things : http://stackoverflow.com/questions/5416414/signed-unsigned-comparisons — adrian008, Jun 21 '15 at 12:56

score 5 · Accepted Answer · answered Jun 21 '15 at 12:46

5

When you compare long and unsigned long then both get converted to unsigned long first so (depending on the platform) the -1 value becomes 0xFFFFFFFF. The result is clear then.

answered Jun 21 '15 at 12:46

dlask

8,776
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how the value -1 becomes in 0xFFFFFFFF ??? sorry @dlask will give a gist or any kind of link ? – Shijith Thomas Jun 21 '15 at 12:52
it's called two's complement – Pynchia Jun 21 '15 at 20:10

score 2 · Answer 2 · answered Jun 21 '15 at 12:47

2

In this line:

if (b > a)

you are comparing an unsigned long with a long, which leads to the errors.

Simply change

 unsigned long a= 100000;

to

  long a= 100000;

answered Jun 21 '15 at 12:47

Ayushi Jha

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Okay i get what u meant , but i was wondering how **unsigned** made the value of **long a** less than **long b** ? – Shijith Thomas Jun 21 '15 at 12:59
@SJith: See, unsigned means its values will be from 0, and no negatives can be included. So when you try to fit a negative number into an unsigned, implicitly it is converted into a non-negative. So, -1 will be converted to, say 2^32-1, which is larger than 10000, so you get your output. – Ayushi Jha Jun 21 '15 at 13:08
Thanks @karma_geek :) – Shijith Thomas Jun 26 '15 at 14:40

Don't understand simple C Program if/else result

2 Answers2