Thread: Random Number Generator - Error

Hey guys. I know little about programming. I have made this code with some help, but there is still a problem with it. I am basically trying to create a random number generator that will calculate the average of 1000 numbers and then go on and produce the average of square of those numbers, the cube, etc, up to power of 15. In this case I think I am missing a bracket around the j loop, but I am not sure where. Can anyone see it? I have to learn to really see the code through as well. Thank you.

Code:

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

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

  unsigned int iseed = (unsigned int)time(NULL);
  srand (iseed);
  int i,average; //defines i and average as integers
  int j;
  double x, y;
  double ran;
  double sum [15];
  for (i=0; i<15; i++)
     sum [i]=0;
  for (j=0; j<1000; j++)


  {
    //printf ("%u\n", rand ());
    x=y=(rand() / (RAND_MAX + 1.0));
    for (i=0; i<15; i++)     //error here averages less than 1
    sum[i]+=y;
    y=y*x;

  }
printf("The Sum of Random Numbers : %d ", sum);
for (i=0; i<15; i++)
{
    average=sum[i]/j; //j values retained from the loop (1000); alternatively: sum[15]
    printf("The Average of Random Numbers : %d i=%d",average, i);
}

  return 0;
}

I don't understand the / RAND_MAX + 1

Why not rand() % LargestNumberYouWant + 1?

That's a separate issue, but it's because I don't know much about C. I use Code Blocks and used the help files to extract the format written there.

Code:

    for (i=0; i<15; i++)     //error here averages less than 1
    sum[i]+=y;
    y=y*x;

Since your indentation of subordinate code lines is missing, I can't tell for sure what you want here - but without braces around the y=y*x;, that line will execute just ONE time, instead of every time around the loop.

Hope you learn to indent - your eye WILL become trained to use it, quite naturally.

Could you be more specific about including y=y*x in the loop? I thought it is already in the loop.

Ok, let's see if this makes it clear:

I take 1000 random numbers between 0 and 1. I need two things from here:

1. Generate the sum of all these numbers.
2. Generate the AVERAGE of these numbers like this:

a. First do a regular average: add up all 1000 numbers together and then divide by 1000.
b. Now do the same, except SQUARE all the random numbers, add them up, and only then divide by 1000 to get the average.
c. Do the same ...... except do the cube this time.
...
...
...
continue like this until you hit the power of 15. So raise all of the random numbers to the power of 15 and then divide by 1000 again to get an average.

Please note that the average in all these cases is going to be less than one because you're adding numbers that are less than one and get progressively smaller as you increase the power to which they are raised. This means that the generated output for the average must ALWAYS be smaller than 1, and get progressively smaller for larger and larger powers raised.

Does this make sense?

And thanks a lot brack. I will try to see if your code corrected what I was hoping. Meanwhile, if you think my explanation above changes things, I'd appreciate your advice.

Thanks!

Originally Posted by Excelisus

Ok, let's see if this makes it clear:

I take 1000 random numbers between 0 and 1. I need two things from here:

1. Generate the sum of all these numbers.
2. Generate the AVERAGE of these numbers like this:

a. First do a regular average: add up all 1000 numbers together and then divide by 1000.

this cover some questions...now the only thing that remains is to understand how pow() works...

Code:

#include <stdio.h>

#include <stdlib.h>

#include <time.h>



main(){

     int i;

     float x, sum=0, m;



               srand(time(NULL));

               for(i=0; i<1000; i++){

                    x = rand();
                    //printf("x: %f \n", x);

                    x = (x/RAND_MAX);

                    printf("x: %f \n", x);

                    sum+=x;

               }

               m=sum/1000;

               printf("Average: %f\n", m);


return 0;

}

I still haven't look at the code - I can't until I have access on my other computer when I can run the program + debugging mode. But basically the power portion is inherent in the loop itself. This is what x=x*y does.

Basically first you have x. Then when the loop runs once more, you get x^2 just by the nature of the loop and definition. Then when the loop runs again this time x=x^2. Now when the program does x=x*y, the value becomes x=x^2*x which is the same as x^3. The loop continues like this until it reaches the power of 15 - which is what i<15 takes care of.

So basically what you meant by your correction is this? I have to try this.

Code:

  {
    //printf ("%u\n", rand ());
    x=y=(rand() / (RAND_MAX + 1.0));

1)    for (i=0; i<15; i++){     //basically adding a bracket after this line
2)    sum[i]+=y;
3)    y=y*x;
4)    }

Ideally I would like to keep my own code with some modifications because that's what I understand best, even if barely. Otherwise I would have to start from scratch to understand a different code well enough.

Originally Posted by iMalc

brack's code is wrong because it doesn't use the same random numbers for each power.
BTW, you can shorten y = y * x to just y *= x

You' re right....i wasn' t thinking something like that...my sorry