如何分配内存并取消引用结构中的数组数组？

Question

I was wondering how I would access a double pointer inside of a struct, for example:

typedef struct Example {
   char **set;
   int size;
 }Example;

The struct is called inside the function as:

struct Example exmpl;

If I needed to create a new array within that array of arrays, how would I call it in a function/main? Or is it just the same as regular, exmpl->set ?

Answer 1

It doesn't matter what the type of set is, you access it with exmpl.set . If you need to access a single element it would be exmpl.set[0][0] for example.

If you need to allocate memory for it, you need to allocate memory for the outer array and then for each inner array independently, like this to create an array of 10x20:

struct Example exmpl;
exmpl.set = malloc(10 * sizeof(char*));       // 10 "arrays" in the set
for (int i = 0; i < 10; i++)
    exmpl.set[i] = malloc(20 * sizeof(char)); // 20 chars within each "array"

Then for setting each element's value:

for (int i = 0; i < 10; i++)
    for (int j = 0; j < 20; j++)
        exmpl.set[i][j] = calculate(i, j);

And, an array and a pointer are different, so what you have here is a pointer to a pointer to char (which works similarly to a 2-dimensional array of chars)