Difference Between initializing an array

Question

I have been getting a weird output if I initialize an array using a variable instead of a constant the two code below produces a different output for me.

int x =7;
int arr[x];

and

int arr[7];

the first one generates this output

78  9  73  32  6422216  50  42

and the 2nd one

78  9  73  54  29  50  42

I need to use the size of an vector for the array size. I have tried making the variable constant but it doesn't makes a difference.

edit using the array here

int arr[size];
for(int j=i;j<nums.size();j++)
    arr[j+1]=nums[j];
arr[i]=nums[signs.size()];
for(int j=0;j<nums.size();j++)
    nums[j]=arr[j];

As local (automatic) variables defined in a function: neither is initialised. — Weather Vane, Feb 20 '18 at 21:00
C or C++? The latter doesn't even have variable-length arrays. — Angew is no longer proud of SO, Feb 20 '18 at 21:00
You are not initialising array, what you see is leftover garbage — Killzone Kid, Feb 20 '18 at 21:01
Also note that variable length arrays can't be initialized. You can assign values to the array elements using loop of `memset` function. And VLAs are not supported in C++. — haccks, Feb 20 '18 at 21:01
How does the first one even compile on c++? It would have to be a const int — Zebrafish, Feb 20 '18 at 21:02
The first loop (in the "edit" code) starts with `i` and the second loop starts at `0`. That will use uninitialized entries in the array unless `i` is 0. — user3386109, Feb 20 '18 at 21:08
http://en.cppreference.com/w/c/language/array_initialization — yano, Feb 20 '18 at 21:09
Possible duplicate of [Is uninitialized local variable the fastest random number generator?](https://stackoverflow.com/questions/31739792/is-uninitialized-local-variable-the-fastest-random-number-generator) — , Feb 21 '18 at 00:36
Also: https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard — , Feb 21 '18 at 00:37

score 1 · Answer 1 · answered Feb 20 '18 at 21:04

1

In neither of your two cases is the array initialized.

Without an explicit initializer, the contents of an array (or any variable) are indeterminate. You can't predict what those values will be. And if one of those values happens to be a trap representation, you invoke undefined behavior if you attempt to read that value.

If you want your array to have a particular set of values to start, you need to set those values explicitly, either with an initializer or by assignment.

answered Feb 20 '18 at 21:04

dbush

205,898
23
218
273

Note that you can't initialize VLAs. – haccks Feb 20 '18 at 21:04
i edited my post im initializing the array in the loop later on – irrelevent Feb 20 '18 at 21:05
@irrelevent; That's called assignment, not initialization. – haccks Feb 20 '18 at 21:05
Are arrays defined in the global namespace zero initalised like all other variables? – Zebrafish Feb 20 '18 at 21:11
@Zebrafish If defined at file scope or `static`, then yes all array elements are zero initialized. – dbush Feb 20 '18 at 21:12

Difference Between initializing an array

1 Answers1