passing arrays/pointers to functions

This is a discussion on passing arrays/pointers to functions within the C++ Programming forums, part of the General Programming Boards category; let's say i have this code: Code: int main() { int size = 6; char ez[size][size]; //fill up 2d array ...

  1. #1
    Just because ygfperson's Avatar
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    passing arrays/pointers to functions

    let's say i have this code:
    Code:
    int main() {
    int size = 6;
    char ez[size][size];
    //fill up 2d array ez with some stuff
      deal_with(ez,size);
    }
    
    void deal_with( ????? , int size) {
    
    
    
    }
    what kind of thing do i pass to the function? i've tried passing it as a 2d array, but since size is not predetermined, it can't guess the size of the array. i've tried a char** but it gives me an error. any ideas?

  2. #2
    Code Goddess Prelude's Avatar
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    >i've tried a char** but it gives me an error.
    Understandable. When someone sees that an array passed to a function can be passed as either

    char array[]
    or
    char *array

    They feel that a similar method can be taken with multidimensional arrays:

    char array[][10]
    or
    char **array

    But, in reality, two dimensional arrays are degraded into a pointer to an array:

    char array[][10]
    becomes
    char (*array)[10]

    In your case, the use of a structure would minimize headaches. Create the array in a structure, along with the size of each dimension and then allocate the required space at run-time. When you want to pass the array, all you have to do is pass the struct instance and not have to worry about the annoying syntax of 2D arrays as function parameters.

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

  3. #3
    and the hat of wrongness Salem's Avatar
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    Not quite perfect, but it seems to work

    Code:
    #include <iostream.h>
    
    void foo ( int size, void *p ) {
        int (*arr)[size] = (int(*)[size])p;
        cout << "Size is " << size << endl;
        int r, c;
        for ( r = 0 ; r < size ; r++ ) {
            for ( c = 0 ; c < size ; c++ ) {
                cout << arr[r][c] << " ";
            }
            cout << endl;
        }
    }
    
    int main ( ) {
        int r,c;
    
        int size = 5;
        int arr[size][size];
        for ( r = 0 ; r < size ; r++ ) {
            for ( c = 0 ; c < size ; c++ ) {
                arr[r][c] = r * c;
            }
        }
        foo ( size, arr );
    
        int size2 = 8;
        int arr2[size2][size2];
        for ( r = 0 ; r < size2 ; r++ ) {
            for ( c = 0 ; c < size2 ; c++ ) {
                arr2[r][c] = r * c;
            }
        }               
        foo ( size2, arr2 );
    
        return 0;
    }
    The obvious
    void foo ( int size, int (*arr)[size] );
    results in complaints about scope.

    From a type point of view, each size would be a different function. You can't pass a [2][2] array to a function which is expecting a [3][3] array.

    The downside is, the array is no longer checked, since its being passed to a void* parameter.

  4. #4
    Registered User
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    Hrrm I still have to look up some reference on structures (going to use this site's tutorial)
    but - Prelude you're suggesting something like..

    struct ...
    {
    int min=MIN;
    int max=MAX;
    char array[MIN][MAX];
    }

    yes?

  5. #5
    Registered User
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    Ohh never mind... I just went through the tutorial and finished looking through the message board more...

    struct name{
    char [5][25];
    }
    OS: Windows ME, Windows XP Pro
    Development Environment: MS VC++ 6.0 (SP3)
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    none

  6. #6
    Guest Sebastiani's Avatar
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    The thing is, you want the function to accept either a char** or a char*[]. You're just thinking too much

    EZ:

    Code:
    
    void deal_with(char **strings, int max)
    {  
      int i = 0;
      
      printf("Printing From Within...");
    
      for(i = 0; i < max; i++)  printf(strings[i], i + 1);   
    }
    
    
    
    
    
    
    int main(int argc, char *argv[])
    {
      int size = 100;
    
      char ez[ size ][ size ], *pointers[ size ];
    
      int i = 0;
    
      for(i = 0; i < size; i++) pointers[i] = strcpy(ez[i],"String #%i\n");
    
      deal_with( pointers, size );
    
      getch();
      
    return 0;
    }
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

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