Thread: need help!!!

For this program, I want to generate a floating point number in the interval [-1, 1] using the formular X(n+1) = (X(n) + X(n-1)) % 1.0, however, when I print out the numbers it generated, it printed out some number less than -1. The function I used here is fmod(), this function generate a random number between 0 to 1, then it is multiplied by 2 and minus 1, which should give me a floating number between -1 to 1.

The array for storing these random values are dynamically allocated using pointer.

I spent a lot of time fixing this but couldn't find out where the error is. Very appreciated if anyone can help. Thanks!!

Code:

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

#define MAX_NUM 10

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

  float init_x1 = 0.122840678;
  float init_x2 = 0.102683;
  int i;
  int j;
  
  int count[] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};      /* counter for histogram */

  float *random_floating, *nodePtr;
  float *temp_x1;
  float *temp_x2;
  
  random_floating = (float *) malloc (MAX_NUM * sizeof(float));  /* allocate enough space for random number generated */
  nodePtr = random_floating;
  
  *nodePtr++ = init_x1;
  *nodePtr++ = init_x2;

  for (; nodePtr < (MAX_NUM + random_floating); nodePtr++) {
    temp_x1 = nodePtr - 2;
    temp_x2 = nodePtr - 1;
    
    *nodePtr = fmod(*temp_x1 + *temp_x2, 1.0) * 2.0 - 1.0; /* generate a floating number in [-1, 1] */
   
    printf ("%f\n", *nodePtr);
    
    count[(int)(*nodePtr * 10) + 10]++;
  }
  
  for (i = 0; i < 20; i++)
    printf ("%d   ", count[i]);
}

Thanks, I see.
Now my problem is what I should modify so it will generate floating numbers between -1 to 1 using X(n+1) = (X(n) + X(n-1)) % 1.0. The initial two points must be in the interval [0, 1).

this will work, but the numbers generated are not uniformaly distributed in the interval [-1, 1), almost all the points lies in [-1, -0.9) if 1000000 numbers are generated.

what I did was first generated 1000000 numbers in the interval [0, 1), then in another for loop traverse through the list and mulitply each number by 2 and subtract 1. This gives me an unifomaly distrubuted number in [-1, 1).

thanks Dave_Sinkula, really appreciated your help!!