generating 10 unique random numbers in C

This is a discussion on generating 10 unique random numbers in C within the C Programming forums, part of the General Programming Boards category; Code: #include<stdio.h> #include<stdlib.h> int main() { int i=0,j=0,n=0,a[10],flag=-1; for(i=0;i<10;i++) { a[i]=-1; } while(i<10) { n=rand()%10; for(j=0;j<10;j++) { if(n==a[j]) { flag=0; ...

  1. #1
    apprentiCe
    Join Date
    Oct 2008
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    Hyderabad,India
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    Question generating 10 unique random numbers in C

    Code:
    #include<stdio.h>
    #include<stdlib.h>
    
    int main()
    {
        int i=0,j=0,n=0,a[10],flag=-1;
        for(i=0;i<10;i++)
        {
            a[i]=-1;
        }
        while(i<10)
        {
             n=rand()%10;
             for(j=0;j<10;j++)
             {
                   if(n==a[j])
                   {
                       flag=0;
                       break;
                   }
                   else
                  {
                       flag=1;
                  }
             }
             if(flag==1)
             {
                  a[i]=n;
                  i++;
              }  
         }
        for(i=0;i<10;i++)
        {
             printf("%d\n",a[i]);
         }  
    }
    when i compile and run the above code I get -1.
    it should generate 10 unique random numbers between 10, but it does not...

    i use gcc this is the output
    Code:
    $ gcc random.c
    $ ./a.out 
    -1
    -1
    -1
    -1
    -1
    -1
    -1
    -1
    -1
    -1

  2. #2
    Kernel hacker
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    Of course it does. What is the value of "i" after the first for-loop?

    Also, you don't need to set flag = 1 each time in the loop for(j) - set it before the loop, and remove the else-branch.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  3. #3
    apprentiCe
    Join Date
    Oct 2008
    Location
    Hyderabad,India
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    136
    its really pathetic of me...

    please mods lock and delete this thread...

    i didnt set i to 0 before while loop

  4. #4
    Woof, woof! zacs7's Avatar
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    And you should seed the generator really... with srand().

  5. #5
    and the hat of wrongness Salem's Avatar
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    It might take a while to get the 10th slot filled (random guesses, with only one answer).

    Start with an array filled with 1 to 10, then use a shuffle.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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