Question

I have two structs pointers to pointers

typedef struct Square {
...
...
}Square;

Square **s1; //Representing 2D array of say, 100*100
Square **s2; //Representing 2D array of say, 200*200

Both are allocated on the heap using malloc().
I have s1 initialized with certain values and s2 initialized completely with the default values.
Basically I need to resize s1 to the size of s2 while maintaining its (s1) values, and the ‘added’ values would be just as they were in s2 – the default value.

I wrote this question memcpy() from smaller array to larger one but apparently I’m confusing between arrays and pointers/

My question is, how to implement this resizing of s1 to the size of s2. I don’t have to keep the original pointer. I can copy s1 to s2 and return s2 if that’s a better way
I hope I explained what I’m after properly.
Thanks!

Answer 1

Two dimensional arrays are laid out in memory sequentially: row1 row2 row3 etc.

memcpy does a linear copy from one memory location to another.

So to achieve what you need:

a) Create a new array

Square **s3 = malloc(sizeof(s2));

b) Copy s2 into it

c) Copy stuff from s1, row by row into new

for(r = 0; r < NROWS_S1; r++)
    memcpy(s3[r], s1[r], sizeof(Square) * NCOLS_S1);

http://www.fredosaurus.com/notes-cpp/arrayptr/23two-dim-array-memory-layout.html