Creating a 2D array with malloc and calloc

Question

I am wondering how exactly this works and if this is even the correct way to do this. It works like I want it to. Basically what I'm doing is creating a float array where all elements are 0 with the height of h and width of w.

float** arr = NULL;
arr = (float**) malloc(sizeof(float*) * h);
for (int i = 0; i < h; ++i) {
    arr[i] = (float*) calloc(h, sizeof(float) * w);
}

@Someprogrammerdude That's not really what I was going for here though — randomboiguyhere
– randomboiguyhere, Commented Dec 7, 2019 at 11:34
A jagged array doesn't have to be "jagged". You can still use it as a "2d" array or a matrix. — Some programmer dude
– Some programmer dude, Commented Dec 7, 2019 at 11:36
@Someprogrammerdude Okay but that doesn't really answer my question.. Is this the correct way to create a 2d array of floats where every element is equal to zero? — randomboiguyhere
– randomboiguyhere, Commented Dec 7, 2019 at 11:38

tdao · Accepted Answer · 2019-12-07 11:50:26Z

3

Assume you want a 2D array - jagged array - with size h x w

Then you need something like:

float** arr = NULL;

size_t h = 2;
size_t w = 3;

arr = (float**) malloc(sizeof(float*) * h);  // 1)
for (size_t i = 0; i < h; ++i) {
    arr[i] = (float*) calloc(w, sizeof(float));  // 2)
}

Note 1) - allocate arr that points to an array size h of pointers to float.

Note 2) - allocate each item arr[i] that points to an array size w of float.

As usual, you should use size_t type instead of int for array size.

answered Dec 7, 2019 at 11:50

tdao

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2 Comments

randomboiguyhere Over a year ago

This is what I was looking for, thanks. Quick question, would it be fine to use arr = (float**) calloc(h, sizeof(float*));?

tdao Over a year ago

Yes, the effect is the same. But use malloc in the first call would be more efficient because you don't really need to zero out the content of arr[i] pointer, you only need to clear the content of what arr[i] points to. In other words, the 2nd call to calloc is necessary, but the 1st is not.

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