initializing array - quick question

[fcommisso]

Hey, will this guarantee all the elements in my array will be declared zero? or should I got for a for loop instead?

  int array[size] = {0};

[MK27]

Hey, will this guarantee all the elements in my array will be declared zero?

Yes.

[Epy]

Don't forget to compile as C99 if you're going to use variable-length arrays. Alternatively, you could use calloc to allocate the memory for array and initialize it to 0s.

[jeffcobb]

Really? That would seem to only set one if you use size as an index. The most common way I have seen to init an array (to zero for instance) is this:

int myArray[size];
memset(myArray, 0, sizeof(int)*size);

[MK27]

Really? That would seem to only set one if you use size as an index.

Wrong.

int ray[ANY VALUE] = {0};

Initializes the entire array to zero, and it is the most common technique. You do not have to use memset.

[jeffcobb]

Wrong.

int ray[ANY VALUE] = {0};

Initializes the entire array to zero, and it is the most common technique. You do not have to use memset.

Interesting; changing your initializer to 1:

    int myArray[10] = {1};
    for(int x = 0; x < 10; x++)
        printf("myArray[%d] = %d\n", x, myArray[x] );

Produces:

myArray[0] = 1
myArray[1] = 0
myArray[2] = 0
myArray[3] = 0
myArray[4] = 0
myArray[5] = 0
myArray[6] = 0
myArray[7] = 0
myArray[8] = 0
myArray[9] = 0

On GCC in debug mode (which explains the other 0's). I would expect it to be garbage in release mode. The {} are only specifying one element. Is it possible that you are also running in debug mode (hence all vars are initialized, at least to zero) and being fooled by the output?

J.

[MK27]

Is it possible that you are also running in debug mode (hence all vars are initialized, at least to zero) and being fooled by the output?

Nope.

[slingerland3g]

Note: more portable than using memset as earlier mentioned.

   static int myArray[10];

If you specify anything other than 0, when you initialize then the first index is only initialized all others will be set to 0.

[jeffcobb]

After playing with this a bit more it seems that {0} does work, with the caveats being you can only use 0 and only at array instantiation. Any other value at any other time will require stronger measures such if you need to *re*set an array or defer initialization or use a default value other than zero.

But if you need to init the array once only and to zero only, this would seem to be an acceptable solution ^__^

[MK27]

I was just flipping thru the new standard to find out if this is guaranteed. It is:

If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.

Which means:

1653 If an object that has static storage duration is not initialized explicitly, then:

1654 — if it has pointer type, it is initialized to a null pointer;

1655 — if it has arithmetic type, it is initialized to (positive or unsigned) zero;

1656 — if it is an aggregate, every member is initialized (recursively) according to these rules;

1657 — if it is a union, the first named member is initialized (recursively) according to these rules.

That would explain why you can initialize the first few (or one) item to anything, and then everything else is zero, as jeffcobb points out.

[jeffcobb]

This would also be useful in resource-constrained embedded environments too....will keep it in mind.