[ILUG] gcc optimisation weirdness?

[John N S Gill]

unless i am missing something when input is 10 then you are writing 
to frequency[9] which is beyond the end of the array and so all bets 
are off.

> gcc seems to do weird things to the following:
> 
> int input, frequency[9];
> 
> //frequency[] initialised to 0.
> 
> do {
>                 scanf("%d", &input);
>                 if (input > 0 && input < 11) {
> 			//point 1
>                         (frequency[input-1])++;
> 			//point 2
>                         }
>         } while (input);
> 
> if i compile with -O2 or -O3 it works as i expect it to, but if i compile
> with "gcc -o test test.c" then:
> 
> it works correctly for digits 0 thru 9, however if i enter 10 it goes
> wrong: at point 1 input == 10, at point 2 input == 11! 
> 
> seems like gcc is applying the increment to variable input,
> despite my bracketing of frequency[input-1]! which i don't
> want. why???
> 
> (even without the brackets it should not increment input)
> 
> --paulj.
> 
> PS: is there anyway to initialise an array at declaration time, rather
> than iterate thru the array and set each element manually? eg something
> like: int array[9]=0; 
> 
> 
> 
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