Initialised array to value 0 inside a block of code

Question

Here's the snippet

main(){
//
  while(){
  int b;
  scanf("%d",&b);
  int arr[b];
  }
}

Ok now, Since I don't know the exact bound of array, I have to take in as a variable*using scanf*. And the array created contains garbage value. And for that purpose I have to initialize each element to 0, because that is what I want.

Is there a way I can get this thing to work, without having to initialize every time inside the loop?

Thanks

EDIT I guess I can use calloc to allocate the memory. Will there be a different in memory used?

Compile with -Wall... like `gcc -Wall prog.c -pedantic -o prog` . — 0decimal0, Jul 14 '13 at 08:05
@meaning-matters Then instead of asking he should have tried it first ? what say? — 0decimal0, Jul 14 '13 at 08:07
@meaning-matters Yeah, I did try it before editing it, but will there be any difference in the memory usage in two cases? — Kraken, Jul 14 '13 at 08:10
@haccks Can't do that, I will have to run the same process multiple times, within the while loop, and each time I'll have different boundary index. and also the index is user input. Is there a way of incorporating this using macro? — Kraken, Jul 14 '13 at 08:11
If you use `calloc`, it will exist on the heap, while what you have will exist on the stack. The stack has limited memory, so if `b` is several million, you are going to have problems. — Mark Lakata, Jul 14 '13 at 08:11
@MarkLakata how does the memory allocation differ in two ways, when the value of `b` is not yet taken from the user? Is the memory allocated at run time? — Kraken, Jul 14 '13 at 08:12
Yes, the memory is allocated (from the stack) at run-time, but this is true even if `b` was a constant. — Mark Lakata, Jul 14 '13 at 08:15
Then it supports c99. You can use VLA(variable length array). Hope [this](http://stackoverflow.com/questions/17332360/initializing-variable-length-array) will help you. — haccks, Jul 14 '13 at 08:16
@Kraken You can do it like this `scanf("%d",b); arr[b];`.As if you just want to decide the size of array at run time. Look you got your answer below :) — 0decimal0, Jul 14 '13 at 08:19
@MarkLakata If b was constant and I used `int arr[5]` then the memory would have been run time or compile time? Then why do we call malloc allocation dynamic if both are the same and occur at runtime? — Kraken, Jul 14 '13 at 08:19
It depends on the compiler. Most compilers running on a normal operating system (Linux, Windows, etc) will take memory from the stack at run-time for local variables, also known as `auto` variables. The amount that they take is either known (`int arr[5]`) or unknown (`int arr[b]`). But the memory left in the stack is generally unknown to the application, so there is no guarantee that there will be enough memory on the stack at run-time. Some embedded compilers do NOT use a stack. In that case, all allocation happens at compile time, and `int arr[b]` is not allowed. — Mark Lakata, Jul 15 '13 at 16:44

score 4 · Accepted Answer · answered Jul 14 '13 at 08:20

If you need an array of run-time size, then the proper approach depends on how large your array is going to be.

If the value of b can be large, then declaring it as a local array can lead to stack overflow. In that case the better idea would be to dynamically allocate your array using calloc. calloc will properly zero-initialize an int array.

If the array is relatively small and safe to allocate on the stack, the you can simply declare it as a local array (which is what you did in your original code) and then set it to all-zeros by using memset(arr, 0, sizeof arr).

Of course, in both cases you might end up with a hidden memory-zeroing cycle implemented inside the library function. In general case it is not possible to set a large region of memory to zero without using a cycle.

haccks · Answer 2 · 2015-03-10T19:14:54.163

1

If your compiler supports C99 then you can easily use VLA's;

int a, b; 
printf("Enter size of array: ");
scanf("%d",&b);

int arr[b];

and to initialize its all elements to zero in your case use an inner loop or standard library function memset.

memset(b, 0, sizeof(b));

edited Mar 10 '15 at 19:14

answered Jul 14 '13 at 08:23

haccks

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@PHIfounder Yeah, but your comment is just after mine :) – haccks Jul 14 '13 at 08:25
@AndreyT; I think no! – haccks Jul 14 '13 at 08:26
OOOOps I didn't see it , +1 nevertheless, AndreyT answer is recommendable :) – 0decimal0 Jul 14 '13 at 08:27
Actually, That is what I am already doing. Sorry If it was not clear. – Kraken Jul 14 '13 at 08:28
1

@Kraken But that ain't visible in your code , So what is the problem . It should work though. – 0decimal0 Jul 14 '13 at 08:30
1

@Kraken; But you have written *Since I don't know the exact bound of array* in your question........I am surprised! – haccks Jul 14 '13 at 08:30
@Kraken; Still it is not clear. If you have used VLA, then it must not take garbage value after initialization. – haccks Jul 14 '13 at 08:32
@Kraken; still my technique will work if you will use an inner loop for initialization. – haccks Jul 14 '13 at 08:35

Initialised array to value 0 inside a block of code

2 Answers2