CODEWITHSUNDEEP
carraysmemory-management
Lukas Bystricky
Lukas Bystricky

Reputation: 1272

Missing array data in C

I have a code which creates an array of 1000 random entries and a routine which bins them in to a histogram. When I look at the results of the histogram I see that there are a lot more 0s than there should be. I can debug the code and I find that while my original array seems correct, after I pass the array to the next routine the last few entries are now 0. Furthermore, if I run a few more lines in the same routine, even more of the entries turn to 0. Does anyone have an idea as to what might be causing this?

Here is the code for reference:

int n = 1000;
int m = 100;
double r;
double *p;//line 22
p = generate_random( n ); //returns array of length n containing random numbers between 0 and 1
r = group(p, m, n);  
...
double group(double* rnd, int m, int n)
{
   int count[100] = { 0 };
   int length = n;
   int i, bin;//line 66
   double dx = 1/(double) m;//length of each interval

   for (i = 0; i < length; i++)
   {
       bin = (int) floor(*(rnd+i)/dx);//line 71
       count[bin]++;//count[0] should be the number of random elements between 0 and 0.1
   }
...//some more code that sets return double

When I enter the debugger at line 22 after creating p, I can see that p contains 1000 random numbers:

print *p = 0.78846
print *(p+851) = 0.3475
print *(p+999) = 0.9325

At line 66 it contains less:

print *rnd = 0.78846
print *(rnd+851) = 0.3475
print *(rnd+999) = 0

And at line 71 (i = 0) even less:

print *rnd = 0.78846
print *(rnd+851) = 6.9533e-310
print *(rnd+999) = 0

Edit: Here is the code for generate_random:

double * generate_random( int  n )
{
   const unsigned int a = 1664525;
   const unsigned int b = 1013904223;
   const unsigned int M = pow(2,32);

   int i;
   double random_numbers[n];

   unsigned int rnd = time(NULL)%M; //number of seconds since 00:00 hours, 
                                // Jan 1, 1970 UTC

   for (i = 0; i < n; i++)
   {
      rnd = (a*rnd + b)%M;
      random_numbers[i] = (double) rnd;

      //map to [0,1]
      random_numbers[i] = random_numbers[i]/M;
   }


   return random_numbers;
}

Upvotes: 0

Views: 61

Answers (1)

DrC
DrC

Reputation: 7698

You are allocating random_numbers as a local which places it on the stack. When you return it, that storage is no longer reserved and so the results are not defined. Instead, either allocate the array on the heap malloc(sizeof(double)*n) or, better, allocate the array in the caller and pass the array in to the generate_random function which can just fill it in. If you allocate on the heap, then be sure to free it when done.

Upvotes: 4

Related Questions