Unique Random Numbers in 2D Array

This is a discussion on Unique Random Numbers in 2D Array within the C++ Programming forums, part of the General Programming Boards category; I am a beginer. I have been able to place unique random numbers into an array but I am having ...

  1. #1
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    Aug 2003
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    Question Unique Random Numbers in 2D Array

    I am a beginer. I have been able to place unique random numbers into an array but I am having trouble placing them in 2D array. How do I do that? Can u show me?

    Thanks in advance.

    here is my simple code

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <time.h>
    
    int main()
    {
            int array[7];
            int i;
    
            for (i=0; i<=7; i++)
                    array[i] = i;
    
            srand( (unsigned)time( NULL ) );
            for (i=0; i<=7; i++)
            {
                    int index1 = i;
                    int index2 = rand()%7;
    
                    int temp;
                    temp = array[index1];
                    array[index1] = array[index2];
                    array[index2] = temp;
            }
    
            for (i=0; i<=7; i++)
                    printf("array[%d] = %d\n",i,array[i]);
    
            return(0);
    }

  2. #2
    Cat
    Cat is offline
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    For a 2D array, you do the same thing as for a 1D. It might be easier to think of the 2D array as "lined up" as a 1D array. That is, imagine assigning each element in the array with a single index. Then you take the index, and extract the row and column numbers. One way to do this is to use / and % operators. For example, to randomly pick any position in a 5x8 array and set it to zero, you could use this:

    int pos = rand % 40; // We imagine the array is a 1-D array of 40 (5*8) elements

    int r = pos / 8; // we get the row by division by the # of columns
    int c = pos % 8; // We get the column by modulus by the # of columns

    array[r][c] = 0;

    If we think of our array, this is how our imaginary indices map to real ones:

    Code:
    real col: 00 01 02 03 04 05 06 07  
    real row -------------------------
          00| 00 01 02 03 04 05 06 07  
          01| 08 09 10 11 12 13 14 15 
          02| 16 17 18 19 20 21 22 23 
          03| 24 25 26 27 28 29 30 31 
          04| 32 33 34 35 36 37 38 39
    For example, notice 29 of our "imaginary" index maps to row # 3 (29/8) and col # 5 (29 % 8)

    You can use any algorithm that works on a 1D array on a 2D array if you use this method. There is a way you can use a similar method for 3D, 4D, or any arbitrary number of dimentions, but it's more complex for those. The 2D case is pretty straightforward.
    Last edited by Cat; 08-05-2003 at 10:21 PM.

  3. #3
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    Code:
    int my_array[5][10];
    
    for(int i=0; i<5; i++)
    {
          for(int j=0; j<10; j++)
          {  
                   my_array[i][j] = 10;   //or a random number you generate
           }
    }

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