[ILUG] gcc optimisation weirdness?

[Paul Jakma]

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;