Question

I am trying to solve Project Euler Problem 88, and I did it without too much effort; however, I find that some seemingly irrelevant code in my program is affecting the result. Here’s my complete code (it’s not short, but I cannot locate the error. I believe it would be obvious to more experienced eyes, so please read my description first):

#include <iostream>
#include <set>

using namespace std;

bool m[24001][12001];
bool p[24001]; // <------------ deleting this line will cause error in result!
long long answer[12001];

int main() {
  long long i;
  long long j;
  long long l;
  set<long long> all;
  long long s = 0;
  for (i = 0; i <= 24000; i++) {
        for (j = 0; j <= 12000; j++) {
      m[i][j] = false;
    }
  }
  m[1][1] = true;
  for (i = 2; i <= 24000; i++) {
    m[i][1] = true;
    for (j = 2; (j <= i) && (i * j <=24000); j++) {
      for (l = 1; l <= i; l++) {
    if (m[i][l]) {
      m[i * j][l + 1 + (i * j) - i - j] = true;
    }
      }
    }
  }
  for (i = 0; i <= 24000; i++) {
    for (j = 0; j <= 12000; j++) {
      if (m[i][j] && (answer[j] == 0)) {
    answer[j] = i;
      }
    }
  }
  for (i = 2; i <= 12000; i++) {
    cout << answer[i] << endl;
    all.insert(answer[i]);
  }
  cout << all.size() << endl;
  for (set<long long>::iterator it = all.begin(); it != all.end(); it++) {
    //cout << *it << endl;
    s += *it;
  }
  cout << s << endl;
}

With the “useless” bool array, all the answers are right, between 0 and 24000; but without it, some answers in the middle got corrupted and become very large numbers.

I am completely confused now; why would that unused array affect the middle of the answer array?

Thanks and sorry for the long code! I will be grateful if someone could edit the code into a better example, I simply son’t know what is with the code.

Answer 1

As requested, adding this to an answer.

You are definitely writing beyond the bounds of the array m somewhere, when the unused array p exists, m overwrites in to its contents which doesn’t affect the answer array but once p is removed the overwriting happens in to answer array showing up the problems.

Overwriting beyond the bounds of the array is an Undefined Behavior and it causes your program to be ill-formed. With Undefined Behavior all safe bets are off and any behavior is possible. While your program may work sometimes or crash sometimes or give incorrect results.Practically, Anything is possible and the behavior may or even may not be explainable.