Missing array data in C

Question

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;
}

The code that you have posted looks reasonable. Can you post the code for generate_random()? The issue could be with how you create your array. — Degustaf
– Degustaf, Commented Sep 19, 2014 at 19:05
Sorry, why, if you are using a compiler that allows non-constant array size automatic declarations (as you use them in generate_random() (declaration double random_numbers[n]; with n a parameter to the function) don't do the same with int count[n]; ---instead of int count[100];--- in the first function group()? This way you won't make dimensioning mistakes. — Luis Colorado
– Luis Colorado, Commented Sep 22, 2014 at 6:05
This is just a very rough sketch of what I want to final code to look like. The compiler gave an error when I tried to set int count[n], so I figured I'd just get it to compile and work first and then worry about stuff like that. — Lukas Bystricky
– Lukas Bystricky, Commented Sep 22, 2014 at 17:11

DrC · Accepted Answer · 2014-09-19 19:13:59Z

4

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.

edited Sep 19, 2014 at 19:13

answered Sep 19, 2014 at 19:05

DrC

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3 Comments

Jonathan Leffler Over a year ago

…and when you allocate the memory, you have to remember to free it too, so as to prevent memory leaks.

barak manos Over a year ago

Change the second occurrence of "stack" to "heap".

Lukas Bystricky Over a year ago

Excellent, thanks. Easy fix I guess; still getting used to memory management in C.

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