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On initializing a Variable length array compiler gives an error message: 

[Error] variable-sized object may not be initialized

Code snippet:

int n; 
printf("Enter size of magic square: ");
scanf("%d",&n);

int board[n][n] = {0};

How should Variable Length arrays be initialized? And why it's all elements are not initialized to 0 in the way give below;

   int board[n][n];
   board[n][n] = {0};

?

haccks
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  • If you are using cpp then use fill(&board[0][0], &board[n-1][n-1]+1, 0) to initialize all elements with 0 or any value you choose – router Dec 15 '22 at 05:39

3 Answers3

27

You'll have to use memset:

memset(board, 0, sizeof board);
Carl Norum
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22

VLAs cannot be initialized by any form of initialization syntax. You have to assign the initial values to your array elements after the declaration in whichever way you prefer.

C11: 6.7.9 Initialization (p2 and p3):

No initializer shall attempt to provide a value for an object not contained within the entity being initialized.

The type of the entity to be initialized shall be an array of unknown size or a complete object type that is not a variable length array type.

Community
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AnT stands with Russia
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    you mean to say `int board[n][n];` `board[n][n] = {0};`. right? – haccks Jun 27 '13 at 00:05
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    @haccks: No. Arrays are not assignable. This is why I said "assign the initial values to your array *elements*". Barring such raw-memory operations like `memset` or `memcpy`, it has to be done in element by element fashion. So, I meant to say `for (i = 0; i < n; ++i) ...` and so on. – AnT stands with Russia Jun 27 '13 at 00:37
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    I read that array can be initialized as `array[m][n] = {0}`, this will initialize its first element to `0` and remaining elements get initialized to `0` itself. – haccks Jun 27 '13 at 00:46
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    @haccks: "Initialized" means that `= { 0 }` can be specified as an initializer in a declaration. What you have in your first comment is completely incorrect. And even in a declaration it works with non-VLA arrays only. For example, you can do `int array[10][10] = { 0 }`. Yet this question is specifically about VLA arrays. VLA arrays do not accept initializers. You cannot do `int array[m][n] = { 0 }` when `m` and `n` are not constants. – AnT stands with Russia Jun 27 '13 at 01:59
0

1.You can simply initialize the array as follows- 

int n; 
printf("Enter size of magic square: ");
scanf("%d",&n);

int board[n][n];
for(int i=0; i<n; i++)
   for(int j=0; j<n; j++)
   {
      board[i][j] = 0;
   }
}

2. memset() should only be used when you want to set the array to "0".

The Matt
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PG1
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  • "*... memset() should be usesd only when you want to set the array to "0".*" Hu! Why this? – alk Dec 24 '13 at 16:42
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    @alk : The `memset()` function fills the first n bytes of the memory area pointed to by s with the constant byte c. Whereas, size of integer is generally 4 bytes so we cant set the integer to our desired number but "0" is a special case since with "0", `memset()` sets all bytes to "0". – PG1 Dec 26 '13 at 07:49
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    @ParagGangil; *memset() should be usesd only when you want to set the array to "0"*: [Not true](http://stackoverflow.com/q/24207698/2455888). – haccks Mar 10 '15 at 19:02