From: Brady, Padraig (Padraig.Brady at domain compaq.com)
Date: Wed 10 May 2000 - 18:13:08 IST
Few points.
1. Shouldn't it be: int frequency[10];
2. Try to get assembler output for the various optimisation options and diff
them.
3. C compilers can init arrays at domain 3 different times.
1. Compile time. Use static arrays of appropriate. All values are
set to 0
2. Program startup code, sets module level initialised arrays. I.E.
int arr[]={1,2};
3. Runtime code generated by compiler inits local initialised arrays
(using
the same construct as in 2).
p.s. a handy macro when working with arrays is
#define ARRAY_SIZE(array) (sizeof(array)/sizeof(array[0]))
This will be worked out by the compiler at compile time.
So this tidys things a lot when you don't have to explicitly
specify (configure) the number of elements (99% of time).
e.g.
int arr[]={1,2,3,4,5};
int arr_idx=0;
for(;arr_idx<ARRAY_SIZE(arr);arr_idx++) printf("%d,",arr[arr_idx]);
> -----Original Message-----
> From: Paul Jakma [mailto:paulj at domain itg.ie]
> Sent: 10 May 2000 17:39
> To: ilug at domain linux.ie
> Subject: [ILUG] gcc optimisation weirdness?
>
>
> 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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