Problem creating unique random numbers

This is a discussion on Problem creating unique random numbers within the C++ Programming forums, part of the General Programming Boards category; I'm not sure whats wrong... Code: #include <iostream> #include "math.h" #include "time.h" char *shapes[2] = {"ball", "cube"}; // two arrays ...

  1. #1
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    Problem creating unique random numbers

    I'm not sure whats wrong...

    Code:
    #include <iostream>
    #include "math.h"
    #include "time.h"
    
    
    
    char *shapes[2] = {"ball", "cube"}; // two arrays for color and shape
    char *colors[4] = {"red", "blue", "orange", "green"};
    
    int selectObject(int, int); //function prototype
    
    int main(){
        int color = 0; //numbers to be used for selection and display
        int shape = 0;
        
        
        
        int loopNumber;
        for (loopNumber = 0; loopNumber <= 100; loopNumber++){
            selectObject(color, shape);
            std::cout<< colors[color]<< " "<< shapes[shape];
        }
        
        std::cin.get();
    }
    
    int selectObject(int color, int shape){
        std::srand(time(NULL)); // I thought this was to make each random number different?
        
        color = std::rand() % 3; // gets random number to decide member of array to display
        shape = std::rand() % 1;
        
        return color;
        return shape;
    }

  2. #2
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    You might find this article interesting: Eternally Confuzzled - Using rand()

  3. #3
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    1. Two return statements unconditionally after each other are useless.
    2. Modulo 1 is useless.
    3. I think modulo 3 isn't what you intended either...

  4. #4
    the hat of redundancy hat nvoigt's Avatar
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    Code:
    #include <iostream>
    #include <math.h>
    #include <time.h>
    
    const char* shapes[2] = {"ball", "cube"}; // two arrays for color and shape
    const char* colors[4] = {"red", "blue", "orange", "green"};
    
    void selectObject(int& color, int& shape)
    {
        color = std::rand() % 4; 
        shape = std::rand() % 2;
    }
    
    int main()
    {
        std::srand(time(NULL)); 
    
        int color = 0;
        int shape = 0;
       
        for ( int loopNumber = 0; loopNumber <= 100; loopNumber++)
       {
            selectObject(color, shape);
            std::cout<< colors[color]<< " "<< shapes[shape];
        }
        
        std::cin.get();
    }
    This is the code you probably meant to write. Note that the function is now able to change it's parameters back in the main method because they are passed by reference and doesn't return anything. srand is called just once to seed the random number generator. Note that it's still random. 1,1,1,2,1,3,1,4,1,1,5,3 is a perfectly random sequence. Compare ordinary dice. They don't automatically roll a six once you rolled a 1,2,3,4,5.

    You need to use the array size for your modulo operation (read the article posted why this might be bad if you need perfect random numbers... I guess it's fine for now). I replaced 3 and 1 by 4 and 2 but you might want to keep that numbers somewhere else in a central place as constants or #defines.

    What did you expect to get anyway? There are 8 possible combinations and you get 100 randoms, there is no way to get 100 unique values out of 8 possible variations. If you need to get 8 combinations in random order, I'd suggest to generate all 8 and shuffle them.
    hth
    -nv

    She was so Blonde, she spent 20 minutes looking at the orange juice can because it said "Concentrate."

    When in doubt, read the FAQ.
    Then ask a smart question.

  5. #5
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    Thanks nvoigt.. I now understand... And the only reason it was set to 100 was to make sure it wasn't just sptting the same random numbers out then they change

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