Initializing C dynamic arrays

Question

How do I initialize a dynamic array allocated with malloc  Can I do this   int  p  p   malloc 3   sizeof  p    p    0  1  2         free p     Or do I need to do something like this   int  p  x  p   malloc 3   sizeof  p    for  x   0  x  lt  3  x        p x    0        free p     Or is there a better way to do it

User · Answer

You cannot use the syntax you have suggested. If you have a C99 compiler, though, you can do this:

int *p;

p = malloc(3 * sizeof p[0]);
memcpy(p, (int []){ 0, 1, 2 }, 3 * sizeof p[0]);

If your compiler does not support C99 compound literals, you need to use a named template to copy from:

int *p;

p = malloc(3 * sizeof p[0]);
{
    static const int p_init[] = { 0, 1, 2 };
    memcpy(p, p_init, 3 * sizeof p[0]);
}

User · Answer

Instead of using  int   p  p    1 2 3     we can use   int   p  p   int 3   1 2 3

User · Answer

I think the more tedious way is the only way to do it  I tried the first one and it doesn t compile  After commenting the         No many shortcuts in  C  I guess ac

User · Answer

p    1 2 3  is wrong   You can never use this   int   p  p    1 2 3     loop is right  int  p i  p   malloc 3 sizeof int    for i   0  i lt 3    i      p i    i

User · Answer

You need to allocate a block of memory and use it as an array as   int  arr   malloc  sizeof  int    n      n is the length of the array    int i   for  i 0  i lt n  i        arr i    0      If you need to initialize the array with zeros you can also use the memset function from C standard library  declared in string h    memset  arr  0  sizeof  int    n     Here 0 is the constant with which every locatoin of the array will be set  Note that the last argument is the number of bytes to be set the the constant  Because each location of the array stores an integer therefore we need to pass the total number of bytes as this parameter   Also if you want to clear the array to zeros  then you may want to use calloc instead of malloc  calloc will return the memory block after setting the allocated byte locations to zero   After you have finished  free the memory block free  arr    EDIT1  Note that if you want to assign a particular integer in locations of an integer array using memset then it will be a problem  This is because memset will interpret the array as a byte array and assign the byte you have given  to every byte of the array  So if you want to store say 11243 in each location then it will not be possible   EDIT2  Also note why every time setting an int array to 0 with memset may not work  Why does  quot memset arr  -1  sizeof arr  sizeof int   quot  not clear an integer array to -1   as pointed out by  Shafik Yaghmour

[c] Initializing C dynamic arrays

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