c_matrices_with_pointers_with_dynamic_memory_allocation.c

/*

    c_matrices_with_pointers_with_dynamic_memory_allocation.c

    Updated: 2021.05.05  ,  by Dragan Milicev, https://www.facebook.com/dmilicev

https://www.geeksforgeeks.org/dynamically-allocate-2d-array-c/

1) Using a single pointer:
A simple way is to allocate memory block of size r*c
and access elements using simple pointer arithmetic.


    You can find all my C programs at Dragan Milicev's pastebin:

    https://pastebin.com/u/dmilicev

*/

#include <stdio.h>
#include <stdlib.h>

// Prints a matrix arr that has r rows and c columns.
// title describes the shown matrix arr.
void printMatrix( char *title, int *arr, int r, int c ){
    int i,j;
    printf("%s", title );
    for (i=0; i<r; i++){
        for (j=0; j<c; j++)
            printf("%4d", *(arr + i*c + j) );
        printf("\n\n");
    }
} // printMatrix()

// Fills in the matrix arr[i][j] which has r rows and c columns with ordinal numbers.
void fillMatrix( int *arr, int r, int c ){
    int i, j, count=0;
    for (i=0; i<r; i++)
        for (j=0; j<c; j++)
            *(arr + i*c + j) = ++count;
} // fillMatrix()

// Reallocate matrix space of the matrix arr[i][j] which has r rows and c columns.
int *reallocateMatrixSpace( int *arr, int r, int c ){
    // allocate sufficient space
    arr = (int *)realloc( arr, r * c * sizeof(int) );
    if( arr == NULL ) {
        fprintf(stderr, "\n\n r=%d , c=%d , arr realloc() error! \n\n", r, c);
        exit(EXIT_FAILURE);
    }
    return arr;
} // reallocateMatrixSpace()

// Gets matrix element arr[i][j] of the matrix arr that has r rows and c columns
int getMij(int *arr, int r, int c, int i, int j){
    int error=0;
    // check that i and j are valid
    if( i<0 || i>r-1 ){
        printf("\n Index i = %d    for rows r = %d of matrix arr is not valid ! \n", i, r );
        error=1;
    }
    if( j<0 || j>c-1 ){
        printf("\n Index j = %d for columns c = %d of matrix arr is not valid ! \n", j, c );
        error=1;
    }
    if(error)
        return 0;
    else
        return( *(arr + i*c + j) );
} // getMij()

// Sets the matrix element arr[i][j] of the matrix arr that has r rows and c columns
// to the value newValue
int *setMij(int *arr, int r, int c, int i, int j, int newValue){
    int error=0;
    // check that i and j are valid
    if( i<0 || i>r-1 ){
        printf("\n Index i = %d    for rows r = %d of matrix arr is not valid ! \n", i, r );
        error=1;
    }
    if( j<0 || j>c-1 ){
        printf("\n Index j = %d for columns c = %d of matrix arr is not valid ! \n", j, c );
        error=1;
    }
    if(error)
        return 0;
    else{
        *(arr + i*c + j) = newValue;
        return(arr);
    }
} // setMij()


int main(){
    int r=3, c=4;                       // rows and columns of matrix arr[r][c]
    char *title = (char*)malloc( 100 * sizeof(char) );  // string for titles
    int *arr;                                           // matrix arr[r][c]

    // allocate sufficient space
    arr = (int *)malloc( r * c * sizeof(int) );
    if( arr == NULL ) {
        fprintf(stderr, "\n\n arr malloc() error! \n\n");
        exit(EXIT_FAILURE);
    }

    printf("\n -------------------------------------------------- \n");

    fillMatrix(arr,r,c);
    sprintf(title, "\n Matrix arr[%d][%d] is: \n\n", r, c );
    printMatrix(title,arr,r,c);
    printf("\n arr[%d][%d] = %d \n", r-1, c-1, getMij(arr,r,c,r-1,c-1) );

    printf("\n -------------------------------------------------- \n");

    // reallocate sufficient space for different matrix
    r = 5;
    c = 6;
    reallocateMatrixSpace(arr,r,c);
    fillMatrix(arr,r,c);
    sprintf(title, "\n Matrix arr[%d][%d] is: \n\n", r, c );
    printMatrix(title,arr,r,c);
    printf("\n arr[%d][%d] = %d \n", r-1, c-1, getMij(arr,r,c,r-1,c-1) );

    printf("\n -------------------------------------------------- \n");

    // reallocate sufficient space for different matrix
    r = 3;
    c = 3;
    reallocateMatrixSpace(arr,r,c);
    fillMatrix(arr,r,c);
    sprintf(title, "\n Matrix arr[%d][%d] is: \n\n", r, c );
    printMatrix(title,arr,r,c);
    printf("\n arr[%d][%d] = %d \n", r-1, c-1, getMij(arr,r,c,r-1,c-1) );

    setMij(arr,r,c,r-1,c-1,12);
    printMatrix(title,arr,r,c);
    printf("\n arr[%d][%d] = %d \n", r-1, c-1, getMij(arr,r,c,r-1,c-1) );

    printf("\n -------------------------------------------------- \n");


    /* Code for further processing and free the
      dynamically allocated memory */


    free(arr);  // free reserved memory for arr[r][c] with function malloc()

    return 0;
}