pointer to pointer and 2D arrays

This is a discussion on pointer to pointer and 2D arrays within the C Programming forums, part of the General Programming Boards category; is there a way to initialise a pointer to pointer to the 2D array? char **p; char (*q)[5]; I mean ...

  1. #1
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    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.

  2. #2
    Registered User trekker's Avatar
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    Re: pointer to pointer and 2D arrays

    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...

  3. #3
    and the hat of wrongness Salem's Avatar
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    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; 
    }

  4. #4
    End Of Line Hammer's Avatar
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    There's also this to read, compliments of Salem.
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  5. #5
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    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

  6. #6
    Code Goddess Prelude's Avatar
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    >"ANSI C++ forbids implicit conversion from 'void *' in assignment"
    Then compile this as C code instead of C++.

    >assignment to 'int (*)[20]' from 'int **'
    pa isn't declared as int **, it's declared as int (*)[20] and all usage should reflect that, including casts.

    -Prelude
    My best code is written with the delete key.

  7. #7
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    thanks

    will try it and let you know how it goes

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