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  • 1 Post By scatterbrain
  • 2 Post By Salem

Passing PART of a 3D array

This is a discussion on Passing PART of a 3D array within the C Programming forums, part of the General Programming Boards category; Is there any way to pass only two dimensions of a 3D array. for arr[2][3][4], lets say I want to ...

  1. #1
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    Passing PART of a 3D array

    Is there any way to pass only two dimensions of a 3D array.

    for arr[2][3][4], lets say I want to use [3] and [4] to calculate something, and i only need those dimensions. Say i want to pass it to a function called calculate. I would try this...but it isn't working...

    Code:
    float calculate(float arr[3][4]);
    
    ...
    ...
    
    printf("We have found %f", calculate(arr[2]);

  2. #2
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    Pass the whole array... but only use the sections you need.

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    My profess. requires that we only pass portions of the array that are necessary for that particular task.

  4. #4
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    Your professor obviously isn't a programmer... You can't split up arrays like that.

  5. #5
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    Not splitting up an array. This is a "slice", and in some languages it's supported - like Go! uses them alot.

    In C, I wouldn't do it, but you could work it out using pointers I believe, (which is what the array passing degrades to anyway in C). Seems error prone, however.

  6. #6
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    Quote Originally Posted by scatterbrain View Post
    Is there any way to pass only two dimensions of a 3D array.

    for arr[2][3][4], lets say I want to use [3] and [4] to calculate something, and i only need those dimensions. Say i want to pass it to a function called calculate. I would try this...but it isn't working...

    Code:
    float calculate(float arr[3][4]);
    
    ...
    ...
    
    printf("We have found %f", calculate(arr[2]);
    This line below is wrong; because you only have two legal values for first dimension "0" and "1".

    Code:
    printf("We have found %f", calculate(arr[2]);
    I agree with adak "Seems error prone ...".

    You might try passing "&arr[0]" or "&arr[1]" instead of arr[2].

    Tim S.

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    Thanks for your help Adak and stahta01, I'll try that out.

    CommonTater, thanks for that typical condescending forum-programmer response. That's unnecessary and not helpful at all, obviously my programming teacher is a programmer. This is only a minor example here, but someone please explain to me why people on programming message boards are so ........ing arrogant and unhelpful at times.
    Salem likes this.

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    Make sure you're completely clear about your instructions (considering an assignment like this does not seem reasonable for any class in C). I would think it easier to "pass two dimensions" if you just passed one page of the array (this is still a third dimension, just a cross-section of it) rather than the third column of four pages. I think it would be error-prone as well, but perhaps less error-prone than what you wanted.

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    Quote Originally Posted by stahta01 View Post
    You might try passing "&arr[0]" or "&arr[1]" instead of arr[2].
    Tim S.
    stahta01 is correct. you pass in the address of the matrix you are concerned with, which is one element of the first index. then the calculate function should have type matching the type of the 2d matrix, which you have correct.
    [CODE]

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    I'm just guessing that Tater was looking at it from a production programmer point of view, and in C, I doubt if you it would be wise to do that - because again, we're just talking about a pointer being passed, no matter how large the array is.

    Your prof., on the other hand, is probably giving you this assignment as a tool to learn and practice how to work with the innards of an array. His point is an educational one. Do you have some class notes on this?

  11. #11
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    posting 2 threads for the same prob!!!????

  12. #12
    and the hat of wrongness Salem's Avatar
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    There's nothing wrong with passing around array slices.
    Code:
    #include <stdio.h>
    
    void foo ( int arr[3][4] ) {
        arr[0][0] = 1;
    }
    int main ( ) {
        int arr[4][3][4] = { { { 0 } } };
        foo(arr[0]);
        foo(arr[1]);
        printf("%d %d\n", arr[0][0][0], arr[1][0][0] );
        return 0;
    }
    I mean, you're doing the same thing if you're trying to tokenise a string, and passing around &myString[offset] to some other function accepting a char*.
    jimblumberg and iMalc like this.
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    Code:
    #include <stdio.h>
    
    #define ARRAY_MEMS(xx_a) (sizeof(xx_a)/sizeof(xx_a[0]))
    
    void
    print_one(int idx, float a[][9], int x,int y)
    {
       int j,k;
       printf("idx = %6d\n", idx);
       for(j=0; j<x; j++)
       {
          for(k=0; k<y; k++)
          {
             printf("%7.1f ", a[j][k]);
          }
          printf("\n");
       }
    }
    
    int
    main()
    {
       float a[5][7][9] ;
       int i,j,k;
       for(i=0; i<ARRAY_MEMS(a); i++)
       {
          for(j=0; j<ARRAY_MEMS(a[i]); j++)
          {
             for(k=0; k<ARRAY_MEMS(a[i][j]); k++)
             {
                a[i][j][k] = (i * 10000) + (j * 100) + k;
             }
          }
       }
       for(i=0; i<ARRAY_MEMS(a); i++)
       {
          print_one(i,a[i], ARRAY_MEMS(a[i]), ARRAY_MEMS(a[i][0]));
          printf("\n\n");
       }
       return(0);
    
    }
    Last edited by sparkomemphis; 12-22-2011 at 01:03 PM.

  14. #14
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    Quote Originally Posted by CommonTater View Post
    Your professor obviously isn't a programmer... You can't split up arrays like that.
    Yes you can. The code as posted should be working fine. Can we see real code, not snippets?
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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