Segmentation Fault [2D array]

Question

#include <iostream>

using namespace std;

void input_Array(char (&A)[10][10], int x, int y);
int calc_Collision(char (&A)[10][10]);
void display(char (&A)[10][10]);
void init_Array(char (&A)[10][10]);

int main()
{
  int m,n,test;
  char A[10][10];
  init_Array(A);
  cin>>test;
  while (test>0)
  {
    cin>>m>>n;
    input_Array(A,m,n);
    display(A);
    cout<<"FLAG";
    cout<<calc_Collision(A);
    test--;
  }
}

//Calculates no. of ways to select two 1's in each column
int calc_Collision(char (&A)[10][10])
{
  int count=0;
  int sum=0;
  int select(int x, int y);
  for (int j = 0; j<10; j++)
  {
   count=0;
    for(int i = 0; i<10; i++)
    {
      if (A[i][j]=='1')
      {
        count++;
      }
    }
    sum=sum + select(count,2);
  }
  return sum;
}

//Returns no. of ways to select y items from x items
int select(int x, int y)
{
  int fact(int a);
  return (fact(x)/(fact(y)*fact(x-y)));
}

//Returns a!
int fact(int a)
{
  if (a==0)
  {
    return 1;
  }
  else
  {
    return (a*fact(a-1));
  }
}

void display(char (&A)[10][10])
{
  for (int i=0; i<10; i++)
  {
    for (int j=0; j<10; j++)
    {
      cout<<A[i][j]<<"\t";
    }
    cout<<"\n";
  }
}

Input Format:

no. of trials

no. of rows (whitespace) no. of columns

row1 (No space, 1's or 0's only)

row2...

Output:

The 2D array

Total no. of ways to select two one's in each column

Problem:

The program displays the array fine enough.

But upon coming across calc_Collision(A), the code outputs:

Segmentation fault (core dumped)

"FLAG" is NOT displayed.

Still a beginner here, so ANY help would be appreciated.

The right tool to solve this is your debugger. Debugging is an essential skill, you can read more about it here. — Rakete1111
– Rakete1111, Commented Sep 6, 2016 at 12:58
Note that fact will overflow on inputs greater than 12. You probably need a way to compute select that doesn't directly involve the factorial. (Hint: use maths to derive a formula.) — molbdnilo
– molbdnilo, Commented Sep 6, 2016 at 13:09
Thanks for the replies! Any valid input gives Segmentation fault. Eg: 1 3 3 101011111 — Rahul R
– Rahul R, Commented Sep 6, 2016 at 13:30

Benson Lin · Accepted Answer · 2016-09-06 13:35:51Z

2

int select(int x, int y){
  int fact(int a);
  return (fact(x)/(fact(y)*fact(x-y)));
}


int fact(int a){
  if (a==0)  {
    return 1;
  }
  else  {
    return (a*fact(a-1));
  }
}

Notice that if x in select is less than 2, then fact(x-y) will call itself indefinitely. This is because the variable a in fact will be negative. This occurs when the size of the input array has less than 10 columns, resulting in the last column becoming empty. This causes the iteration of count to become 0 in calc_Collision. The segmentation fault does not occur if the input has 10 columns. Since you are only computing nC2 (N choose 2), the select function can be rewritten as:

int select(int x){
  return (x*(x-1))/2;
}

edited Sep 6, 2016 at 13:35

answered Sep 6, 2016 at 13:30

Benson Lin

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