Printing randomly generated numbers?

**arcadedragon** · 10-12-2012

Hello, I have a program here that must generate 256 random numbers between 20 and -20 and write them to a file, numbers.txt. Then I must print the first 10 numbers generated. However with the code I have so far, all I get is the number 7, printed indefinitely on a loop. I'm thinking the j++ counter should be moved elsewhere, but I can't figure out where. Any help is appreciated.

Code:

#include <stdio.h>
#include <stdlib.h> 


int rand( void ) ;


int main( int argc, char *argv[] ) {


  int x, i = 0, j = 0 ;
  FILE *fin ;
  
  fin = fopen("numbers.txt", "a+") ;


  while ( i < 255 ) {


    x = rand() ;
    
    if (( x <= 20 ) && ( x >= -20 )){


    fprintf(fin, "%d ", x ) ;


    i++ ;


    j++ ;


      while( j <= 10 ){


	printf("\n%d", x ) ;
      
      }
    }
  }


  fclose( fin ) ;  


  return 0 ;


}

**camel-man** · 10-12-2012

Look into seeding. There is a function known as srand() and you can include time.h to use time(NULL) inside the srand parameter.

**arcadedragon** · 10-12-2012

Ah, I should've mentioned for this assignment I'm not allowed to use srand for this assignment. but thank you.

**camel-man** · 10-12-2012

Are you suppose to make the random function? Where is your function definition for rand?

**iMalc** · 10-12-2012

If you're not allowed to use srand, then rand() is of no use to you either. You can't use one without the other.

It probably means that you have to implement another random number generator by yourself, or what I would recommend is getting the XORShift algorithm code from Wikipedia.

**oogabooga** · 10-12-2012

Originally Posted by iMalc

If you're not allowed to use srand, then rand() is of no use to you either. You can't use one without the other.

It probably means that you have to implement another random number generator by yourself, or what I would recommend is getting the XORShift algorithm code from Wikipedia.

Another interpretation is that he's not supposed to use srand so that the output it easier to mark. Either that or he's mistaken.

**Salem** · 10-12-2012

Code:

    j++ ;


      while( j <= 10 ){


	printf("\n%d", x ) ;
      
      }

Can you see INSIDE the loop where j is being modified?

If you can't see where j is modified inside the loop, how do you expect j<=10 to become false at some point and break the loop.

When you understand this, can you think where a good place to put j++ would be?

**iMalc** · 10-13-2012

Originally Posted by Salem

Code:

    j++ ;


      while( j <= 10 ){


	printf("\n%d", x ) ;
      
      }

Can you see INSIDE the loop where j is being modified?

If you can't see where j is modified inside the loop, how do you expect j<=10 to become false at some point and break the loop.

When you understand this, can you think where a good place to put j++ would be?

I suspect that the inner while loop is supposed to be just an if-statement rather than a loop. In that case it would be possible for the increment to stay where it is. Of course in that case j is also redundant as i could be used instead.

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