Thread: pointer to pointer and 2D arrays

is there a way to initialise a pointer to pointer to the 2D array?

char **p;
char (*q)[5];

I mean a pointer to an array to a 2D array, I know that. but what about the above? I was trying it out but I kept getting errors.

In case anybody is wondering, I'm just curious whether it's possible so that I could use or understand it when the time comes.

Please feedback.

Originally posted by rfbu
is there a way to initialise a pointer to pointer to the 2D array?

char **p;
char (*q)[5];

Please feedback.

the above definitions are almost comletely different

char **p;
p is a pointer to pointer to char
you may use that as a 2D array with the proper
memory allocations.

char (*q)[5];
q is pointer to an 1-D 5 element char array.

hope this helps...

Any use?

Code:

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

int main ( ) {
    int     arr[10][20];    // a 2D array
    int     (*pa)[20];      // a pointer to a 2D array
    int     **b;            // a pointer to a pointer
    int     cols;

    // simple point at the given array
    pa = arr;

    // you can also call malloc to initialise
    // this synthesises arr[30][20]
    // the major size can be anything you want, but
    // the minor size is stuck at 20 at compile time
    pa = malloc ( 30 * sizeof(pa[0]) );

    // for total flexibility
    // this also allocates arr[30][20]
    // but you can choose all dimensions at run-time
    b = malloc( 30 * sizeof(b[0]) );
    for ( cols = 0 ; cols < 30 ; cols++ ) {
        b[cols] = malloc( 20 * sizeof(b[0][0]) );
    }

    return 0; 
}

thanks for the hints and tips.

however, I compiled the code but I got an error

"ANSI C++ forbids implicit conversion from 'void *' in assignment"

for this line

/code/ pa = malloc( 30 * sizeof(pa[0]) ); /*code/

by the way, my compiler is Dev-C++ 4.9.3.0

so I tried this...this may sound silly, but get ready ...

/code/ pa = (int **)malloc( 30 * sizeof(pa[0]) ); /*code/

and I got this error,

/errmesg/ assignment to 'int (*)[20]' from 'int **'/errmesg/

I'm still trying to get a 'firm' grasp of pointers, any help is appreciated.

thanks in advance

thanks

will try it and let you know how it goes