SIGSEGV(signal 11) 'segmentation fault' in FCTRL2 codechef - c++

Question

I am facing SIGSEGV error on submitting solution for codechef small factorial problem code FCTRL2 though the code works fine on ideone
coding language C++ 4.3.2
Example
Sample input:
4
1
2
5
3
Sample output:
1
2
120
6
#include <iostream>
using namespace std;
void fact(int n) {
int m = 1, a[200];
for (int j = 0; j < 200; j++) {
a[j] = 0;
}
a[0] = 1;
for (int i = 1; i <= n; i++) {
int temp = 0;
for (int j = 0; j < m; j++) {
a[j] = (a[j] * i) + temp;
temp = a[j] / 10;
a[j] %= 10;
if (temp > 0) {
m++;
}
}
}
if (a[m - 1] == 0) {
m -= 1;
}
for (int l = m - 1; l >= 0; l--) {
cout << a[l];
}
}
int main() {
int i;
cin >> i;
while (i--) {
int n;
cin >> n;
fact(n);
cout << endl;
}
return 0;
}

Answer 1

Caveat I'm not going to just fix up your code for you straight up, but I will highlight where it's going wrong and why you get the seg fault.
Your problem is with your implementation of how you're trying to handle the digit by digit multiplication - specifically with what happens to your m value. Test it out by outputting m each time it's incremented - you'll find it's incrementing more often than you intend. You're right to realise you need to use an approach to get to 158 digits and your basic concept could be made to work.
The first clue is by testing with n = 6 when you get a leading 0 that you shouldn't even though you try to get rid of that problem with the if block that contains m-=1
Try with n = 25 and you will see a lot of leading zeros.
Any value greater than this will fail with a Segmentation error. The Seg fault is because, with this error, you try to set values of the array a beyond the max index (as m gets greater than 200)
N.B. Your assertion that the code works on Ideone.com is only true up to a point - it will fail with n > 25.

Answer 2

(Erased code computing a factorial using int)
The problem in your code is that you increment m each time temp is not 0 for each digit multiplication. You may then get a SIGSEGV when computing big factorials because m becomes too big. You probably saw it because 0 shows up in front of your result. I guess this is why you added the
if (a[m - 1] == 0) {
m -= 1;
}
You should only increment m when the inside loop is finished and term is not null. Once fixed you can get rid of the above code.
void fact(int n) {
int m = 1, a[200];
for (int j = 0; j < 200; j++) {
a[j] = 0;
}
a[0] = 1;
for (int i = 1; i <= n; i++) {
int temp = 0;
for (int j = 0; j < m; j++) {
a[j] = (a[j] * i) + temp;
temp = a[j] / 10;
a[j] %= 10;
}
// if (temp > 0) {
// a[m++] = temp;
// }
while (temp > 0)
{
a[m++] = temp%10;
temp /= 10;
}
}
for (int l = m - 1; l >= 0; l--) {
cout << a[l];
}
}