How do I allocate a 2 D array with contigious memory ? How Do I use it to access rows and columns? Give me an example

Question

I have created a 2 d array which reads as follows

     int i,j,lx,ly;// lx,ly are the row and column respectively
     double** a;

     a=(double**) malloc(         


        

Answer 1

This code allocates a 10 by 5 contiguous block of memory, initializes it with incrementing doubles, and then prints the values indexed by x and y:

#include "2d.h"

int main(void){

    unsigned int x,y;
    const unsigned int width = 10;
    const unsigned int height = 5;

    //we need an index into the x of the array
    double * index[width];

    //need the memory to store the doubles
    unsigned int memorySizeInDoubles = width * height;
    double * memory = malloc(memorySizeInDoubles * sizeof(double));

    //initialize the memory with incrementing values
    for(x = 0; x < memorySizeInDoubles; ++x){
        memory[x] = (double) x;
    }

    //initialize the index into the memory
    for(x = 0; x < width; ++x){
        index[x] = memory + height * x;
    }

    //print out how we did
    for(x = 0; x < width; ++x){
        for(y = 0; y < height; ++y){
           printf("[%u, %u]: Value = %f\n", x, y, index[x][y]);
        }
    }

    free(memory);

    return 0;
}

The 2d.h file should contain these lines:

#include 
#include 

int main(void);

Note: The memory created is only contiguous for some definitions. The memory is logically contiguous, but not necessarily physically contiguous. If this memory is for a device driver for instance, malloc won't work.