c_matrices_with_pointers_with_dynamic_memory_allocation_v3.c

/*
    c_matrices_with_pointers_with_dynamic_memory_allocation_v3.c

    Updated: 2021.05.11  ,  by Dragan Milicev, https://www.facebook.com/dmilicev

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

3) Using pointer to a pointer
We can create an array of pointers also dynamically using a double pointer.
Once we have an array pointers allocated dynamically,
we can dynamically allocate memory and for every row like method 2.


    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[r][c] that has r rows and c columns.
// title describes the shown matrix arr[r][c].
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][j]);   // Note that arr[i][j] is same as *(*(arr+i)+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=1;
    for(i=0; i<r; i++)
        for(j=0; j<c; j++)
            arr[i][j] = count++;    // Note that arr[i][j] is same as *(*(arr+i)+j)
} // fillMatrix()

// Reallocate matrix space of the matrix arr[i][j].
// The new matrix has r_new rows and a c_new columns.
// The old matrix had r_old rows and a c_old columns.
int **reallocateMatrixSpace( int **arr, int r_new, int c_new, int r_old, int c_old ){
    int i;
    for(i=0; i<r_new; i++){
        if( r_new < r_old ){            // reallocate sufficient space
            arr[i] = (int *)realloc( arr[i], c_new * sizeof(int) );
            if( arr[i] == NULL ) {
                fprintf(stderr, "\n\n arr[%d] realloc() error! \n\n", i);
                exit(EXIT_FAILURE);
            }
        }else{                          // allocate sufficient space
            arr[i] = (int *)malloc( c_new * sizeof(int) );
            if( arr[i] == NULL ) {
                fprintf(stderr, "\n\n arr[%d] malloc() error! \n\n", i);
                exit(EXIT_FAILURE);
                }
        }
    }
    // if there are extra rows, we free the memory that was reserved for them
    for(i=r_new; i<r_old; i++)
        free( arr[i] );
    return arr;
} // reallocateMatrixSpace()

// Gets matrix element arr[i][j] of the matrix arr[r][c] 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 M 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 M is not valid ! \n", j, c );
        error=1;
    }
    if(error)
        return 0;
    else
        return arr[i][j];       // or  return *(*(arr+i)+j);
} // getMij()

// Sets the matrix element arr[i][j]
// of the matrix arr[r][c] 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][j] = newValue;   // or  *(*(arr+i)+j) = newValue;
        return(arr);
    }
} // setMij()


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

    // allocate sufficient space for r rows of matrix arr[r][c]
    arr = (int **)malloc( r * sizeof(int *) );
    if( arr[i] == NULL ) {
        fprintf(stderr, "\n\n arr[%d] malloc() error! \n\n", i);
        exit(EXIT_FAILURE);
    }

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

    // for each row r, allocate sufficient space for c columns, of matrix arr[r][c]
    for (i=0; i<r; i++){
        arr[i] = (int *)malloc( c * sizeof(int) );
        if( arr[i] == NULL ) {
            fprintf(stderr, "\n\n arr[%d] malloc() error! \n\n", i);
            exit(EXIT_FAILURE);
        }
    }

    fillMatrix(arr,r,c);    // fill matrix arr[i][j] with ordinal numbers
    sprintf(title, "\n Matrix arr[%d][%d] is: \n\n", r, c );    // make title
    printMatrix(title, arr, r, c);                  // print matrix 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 new different matrix
    r_old = r;  // we remember the old values
    c_old = c;
    r = 6;      // new values
    c = 7;
    reallocateMatrixSpace( arr, r, c, r_old, c_old );   // reallocate memory space
    fillMatrix(arr,r,c);        // fill matrix arr[i][j] with ordinal numbers
    sprintf(title, "\n Matrix arr[%d][%d] is: \n\n", r, c );    // make title
    printMatrix(title,arr,r,c);                     // print matrix 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 new different matrix
    r_old = r;  // we remember the old values
    c_old = c;
    r = 5;      // new values
    c = 6;
    reallocateMatrixSpace( arr, r, c, r_old, c_old );   // reallocate memory space
    fillMatrix(arr,r,c);        // fill matrix arr[i][j] with ordinal numbers
    sprintf(title, "\n Matrix arr[%d][%d] is: \n\n", r, c );    // make title
    setMij(arr, r, c, r-1, c-1, 111 );
    printMatrix(title,arr,r,c);                     // print matrix 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 memory reserved by malloc() for matrix arr[r][c]
    for(i=0; i<r; i++)
        free( arr[i] );

    return 0;
}