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carrayspointers
Karampistis Dimitrios
Karampistis Dimitrios

Reputation: 99

Pure C: Copy rows and columns of a 2D array using pointers

Assuming that I have initialized an int array using double pointer

int **array;

I have used malloc to give the wanted size of the array like this

array = malloc( sizeof *array * ROWS );
if ( array ){
    for ( size_t i = 0; i < ROWS; i++ )
        *(array  + i) = malloc( sizeof *(*(array  + i)) * COLS );
}

Every index of the array is declared with a random int number. I am trying to find a faster way to copy a row or a column rather than using for loops. My choice is to use memcpy, it looked like a good idea on the beginning since I had no problem copying a row using 

memcpy((array + 1), (array + 3), (int)sizeof(int **));

However when it comes to columns using

memcpy(*(array + 1), *(array + 3), (int)sizeof(int *));

does not actually worked as I expected and only one element was copied. Is there something that I am missing?

Upvotes: 0

Views: 772

Answers (1)

Yakov Galka
Yakov Galka

Reputation: 72479

For a 2D array, if your rows are laid out continuously in memory (aka row-major array) then your columns will not be continuous.

memcpy, on the other hand, copies only continuous chunks of memory. Therefore you can't copy the entire column with just one call to memcpy. You'll rather need to copy each element one-by-one, for which you don't need memcpy.

Upvotes: 4

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