Summing up integers in an array and storing the result in an array

Question

I am attempting to do this for a smaller example so that I can do it for a larger sample.

Here is my attempted code:

int counts[0];
int numbers[] = {1,2,3,4,5,6,7,8,9,10};
for(int i = 0; i < 10; i++)
    counts[0] += numbers[i];

printf("%d ", counts[0]);

This should yield 55, but my output is only 14. There is an issue with how I am setting up the numbers array, but I am not quite sure how to fix this. Any help is appreciated, thank you.

The array `int counts[0]` has no size. You just need `int counts = 0;` and then use `counts` instead of `counts[0]` in the rest of it. — Weather Vane, Jul 03 '19 at 19:02
Try `int counts[1] = {0};` instead of `int counts[0];` An array of size 0 is not useful. — user3386109, Jul 03 '19 at 19:07
[ISO/IEC 9899:TC2:6.7.5.2](http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1124.pdf) — Agnius Vasiliauskas, Jul 03 '19 at 19:08
@AgniusVasiliauskas the comments under that answer indicate that a compiler is allowed to build a zero-length array, as long as a diagnostic message is issued. But of course, a zero length array cannot be indexed by `[0]` as it has no elements. — Weather Vane, Jul 03 '19 at 19:13
@WeatherVane So what ? Many things are allowed for a compiler by a standard, because **they are not dis-allowed by it**. But this makes these "allowed" things undocumented by a standard, thus implying IDB (implementation-defined behavior) or UB (undefined-behavior). None of these behaviors are really useful for a developer. — Agnius Vasiliauskas, Jul 03 '19 at 19:46
https://math.stackexchange.com/questions/2260/proof-for-formula-for-sum-of-sequence-123-ldotsn — Neil, Jul 03 '19 at 19:49
@AgniusVasiliauskas you since changed the [answer link](https://stackoverflow.com/questions/9722632/what-happens-if-i-define-a-0-size-array-in-c-c/9723093#9723093) but I supposed you had posted it for a reason, and the comments under the answer said it was incorrect. It wasn't a useful discussion for a beginner, but I didn't notice when you edited the comment to a different link. — Weather Vane, Jul 03 '19 at 19:57
FWIW: Zero-length arrays do have some great uses in C. I use them a lot in binary packing and network communications (https://stackoverflow.com/questions/14643406/whats-the-need-of-array-with-zero-elements) — abelenky, Jul 03 '19 at 20:41
@abelenky no they don't. All that garbage has undefined behaviour. That's why c99+ has the flexible array member — Antti Haapala -- Слава Україні, Jul 04 '19 at 02:51

score 2 · Answer 1 · answered Jul 03 '19 at 19:03

int counts[0];   // This declares an array that holds ZERO elements!  No place to store a value.
int numbers[] = {1,2,3,4,5,6,7,8,9,10};
for(int i = 0; i < 10; i++)
    counts[0] += numbers[i];

printf("%d ", counts[0]);

To fix it:

int counts = 0;   // Declare a variable, and start it at 0
int numbers[] = {1,2,3,4,5,6,7,8,9,10};
for(int i = 0; i < 10; i++)
    counts += numbers[i];

printf("%d ", counts);

Summing up integers in an array and storing the result in an array

1 Answers1