Solutions for z^2 = x^2 + y^2 in C. Code works for small number but segfaults for bigger numbers

Question

I wrote some code in python and C to find solutions for z^2 = x^2 + y^2, x, y and z being all integers. From the python code, which seems to work fine, I know how many solutions there are for some given range. For x and y ranging from 1 to 10000, there should be 14471 unique solutions, or double that, 28942 counting duplicates.

In the following code if I use the ranges from 1 to 1000 for x and y, and set the array solutions to have an appropriate size(2064 in this case) the code works fine. But when I try extend the range of x and y to 1 to 10000, I get a segfault, but don't know why so.

int main()
{
  int solutions[28942][2]={0};
  int counter = 0;

  for(int x=1;x<10000;++x)
  {
    for(int y=1; y<10000;++y)
    {
      float z = sqrt(x*x + y*y);
      if(z == (int) z) 
      {
        solutions[counter][0] = x;
        solutions[counter][1] = y;
        ++counter;
      }
    }
  }

  for(int i = 0;i<28942;++i)
  {
    printf("This is a solution: {%d,%d}\n",solutions[i][0],
                                           solutions[i][1]);
  }

  printf("value of counter is %d\n",counter );
}

access violations are best dealt with by writing robust code: e.g. use `assert` to check your assumptions when accessing into a static array — BeyelerStudios, Aug 18 '15 at 14:50
Your `counter` gets larger than your array bounds... Segfault happens for me (using `gdb`) when `counter = 29652` (wayyyyy out of bounds!) — Nidhoegger, Aug 18 '15 at 14:52
Add `if (counter >= 28942) exit(1);` before `solutions[counter][0]`. — chux - Reinstate Monica, Aug 18 '15 at 15:09

score 4 · Accepted Answer · edited Aug 19 '15 at 08:49

4

Maybe your code accepts solutions that are not legit. Comparing floats to int is a tricky business.

try doing it like this:

int z = (int) sqrt(x*x + y*y);

if(z*z == (x*x + y*y)) {
    solutions[counter][0] = x;
    solutions[counter][1] = y;
    ++counter;
}

that way you're comparing integers only

edited Aug 19 '15 at 08:49

answered Aug 18 '15 at 14:54

Tawcharowsky

615
4
18

Good answer. I just tried your suggestion and now the code seems to output 28942 solutions as hoped. – aristotle85 Aug 18 '15 at 15:27

score 2 · Answer 2 · edited May 23 '17 at 10:26

Avoid declaring large arrays on the stack, as it may cause stack overflows. A stack overflow might manifest itself as a seg fault. solutions should be declared dynamically with malloc() instead.
Never do floating point comparisons using the == operator. This is because of the inaccurate nature of floating points and nothing unique to C. See this. Instead you need to check
```
if ( abs(z - (int)z) >= EPSILON)
```
where EPSILON is some appropriate constant, for example 0.0001f.

user1635881 · Answer 3 · 2015-08-18T15:06:40.853

1

Most likely, your stack is not big enough to hold nearly 60000 ints.

Try to allocate it as a global variable (in the data segment), or use dynamic memory allocation.

edited Aug 18 '15 at 15:06

answered Aug 18 '15 at 14:51

user1635881

239
3
11

Solutions for z^2 = x^2 + y^2 in C. Code works for small number but segfaults for bigger numbers

3 Answers3