C++ output producing random long integers - c++

Question

I recently coded a question, and I have debugged and figured out where the problem is but cannot quite place a finger on it. My program aims to merge groups of sectors if they overlap or are next to each other, but the last section is scrambling the output.
#include <cstdio>
using namespace std;
int main() {
freopen("beachin.txt", "r", stdin);
freopen("beachout.txt", "w", stdout);
int n, u, k, x;
scanf("%d %d %d %d", &n, &u, &k, &x);
int umbrellas[u][2];
for (int i = 0; i < u; i++) {
scanf("%d %d", &umbrellas[i][0], &umbrellas[i][1]);
}
int groups[u][2];
int len = 0;
for (int i = 0; i < u; i++) {
int s1 = umbrellas[i][0];
int e1 = umbrellas[i][1];
int found = false;
for (int a = 0; a < len; a++) {
int s2 = groups[a][0];
int e2 = groups[a][1];
if ((s1 <= e2 && s1 >= s2) || (e1 >= s2 && e1 <= e2) || (s1 <= s2 && e1 >= e2)) {
int start, end;
if (s1 < s2) start = s1;
else start = s2;
if (e1 > e2) end = e1;
else end = e2;
groups[a][0] = start;
groups[a][1] = end;
found = true;
}
}
if (found == false) {
len++;
groups[len][0] = s1;
groups[len][1] = e1;
}
}
int largest = 0; // scramble bit here from for loop
for (int i = 0; i < len; i++) {
int current = groups[i][1]-groups[i][0]+1;
if (current >= largest) largest = current;
}
printf("%d\n", largest);
return 0;
}
My input for this is
7 2 5 2
4 5
5 6
The expected answer is 2, but the output varies between 2 and random long numbers like 1550870207.
Does anyone know why this is happening?
Note: If it's any help, problem statement is here

Answer 1

Pointing out some issues with the code above, many of which you can get reported by using compiler options:
umbrellas[u][2] and groups[u][2]: these are variable length arrays; they're not standard C++.
n, k, and x are declared, read from, but never used later on.
groups is 1) not initialized, 2) filled only for groups[1], so that groups[0] values remain undefined (at if (found == false) block), and 3) accessed both for groups[0] and groups[1] (at bottom for loop).
len is incremented before being used; in the second iteration of the top for loop, the if (found == false) loop is executed twice, so the second time you access groups[2]; that's an out of bounds access.
Check it here. I've modified your code initializing umbrellas at the point of declaration, and using fixed length arrays for umbrellas and groups.
#include <fmt/core.h>
int main() {
int umbrellas[2][2]{ { 4, 5 }, { 5, 6 } };
int groups[2][2]{};
int len{};
for (int i = 0; i < 2; i++) {
fmt::print("*** i = {}\n", i);
int s1 = umbrellas[i][0];
int e1 = umbrellas[i][1];
bool found{ false };
for (int a = 0; a < len; a++) {
int s2 = groups[a][0];
int e2 = groups[a][1];
if ((s1 <= e2 && s1 >= s2) ||
(e1 >= s2 && e1 <= e2) ||
(s1 <= s2 && e1 >= e2)) {
int start{ (s1 < s2) ? s1 : s2 };
int end{ (e1 > e2) ? e1 : e2 };
groups[a][0] = start;
groups[a][1] = end;
found = true;
}
}
if (not found) {
len++;
fmt::print("\tAccessing groups[{}]\n", len);
groups[len][0] = s1;
groups[len][1] = e1;
}
}
int largest = 0; // scramble bit here from for loop
for (int i = 0; i < len; i++) {
int current = groups[i][1] - groups[i][0] + 1;
if (current >= largest) {
largest = current;
}
}
fmt::print("==> largest = {}\n", largest);
}
// Outputs:
//
// *** i = 0
// Accessing groups[1]
// *** i = 1
// Accessing groups[2]
// ==> largest = 2