SIGSEGV Error Cause??????

[khirodkantnaik]

Help me figure out why I am getting a SIGSEGV error while submitting at SPOJ.
Also is there a utility by which we can check the cause of such signals in our program. For eg: - i use "g++ -Wall -ansi -pedantic filename.cpp"

Okay so my code is:

#include <iostream>
#include <vector>

using namespace std;

void merge(vector<int> &a, int, int, int);

void merge_sort(vector<int> &A, int left, int right)
{
	int mid;
	if (left >= right) {
		return;
	}
	else {
		mid = (left + right) / 2;
		merge_sort(A, left, mid);
		merge_sort(A, mid + 1, right);

		merge(A, left, mid, right);
	}
}

void merge(vector<int> &a, int left, int mid, int right)
{
	int b[100];

	int i;
	int j;
	int k = left;

	for (i = left; i <= right; i++) {
		b[i] = a[i];
	}

	i = left;
	j = mid + 1;

	while (i <= mid && j <= right) {
		if (b[i] < b[j]) {
			a[k] = b[i];
			i = i + 1;
		}

		else {
			a[k] = b[j];
			j = j + 1;
		}
		k = k + 1;
	}

	while (i <= mid) {
		a[k] = b[i];
		i = i + 1;
		k = k + 1;
	}

	while (j <= right) {
		a[k] = b[j];
		j = j + 1;
		k = k + 1;
	}
} 

void answer(vector<int> a, int left, int right, int k, vector<int> &v)
{
	int b[100];
	int ctr = 0;		// ctr is counter for b[100];	
	merge_sort(a, left, right);
	for (int i = left; i <= right; i++) {
		b[ctr] = a[i];
		ctr = ctr + 1;
	}
	
	v.push_back(b[k]);
}		
int main()
{
	int size, temp;
	int num_ques;

	int left, right, k;

	vector<int> v;

	cin >> size;
	cin >> num_ques;

	vector<int> arr;

	for (int i = 0; i < size; i++) {
		cin >> temp;
		arr.push_back(temp);
	}
	

	for (int i = 0; i < num_ques; i++) {
		cin >> left;
		cin >> right;
		cin >> k;
		
		answer(arr, left - 1, right - 1, k - 1, v);
	}

	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << endl;
	}

	return 0;
}

[Azagaros]

My first guess is your running into a memory issue with the recursion. The other option I see could be something in the Vector class are loosing track of something. Have you traced through the program to see where it crashes?

The other note I make is most places you pass by reference except on the Answer function, it makes a copy each time it goes through things. This might be the source of the memory overload, considering how many times it copies the for each call of Answer.

[khirodkantnaik]

Yes it may be a cause!!! i will check it once again

[Moschops]

What values are you entering? A quick look through indicates that for some input values you'll read and write unallocated memory.

[khirodkantnaik]

SAMPLE INPUT
7 3
1 5 2 6 3 7 4
2 5 3
4 4 1
1 7 3

[khirodkantnaik]

The above was the sample input given for the spoj question

https://www.spoj.pl/problems/MKTHNUM/

My program runs absolutely fine for the test case.

Now i also replaced vectors with array still i'm facing the same problem.

Here' s the new code

#include <iostream>
#include <vector>

using namespace std;

void merge(int a[], int, int, int);

void merge_sort(int A[], int left, int right)
{
	int mid;
	if (left >= right) {
		return;
	}
	else {
		mid = (left + right) / 2;
		merge_sort(A, left, mid);
		merge_sort(A, mid + 1, right);

		merge(A, left, mid, right);
	}
}

void merge(int a[], int left, int mid, int right)
{
	int b[100];

	int i;
	int j;
	int k = left;

	for (i = left; i <= right; i++) {
		b[i] = a[i];
	}

	i = left;
	j = mid + 1;

	while (i <= mid && j <= right) {
		if (b[i] < b[j]) {
			a[k] = b[i];
			i = i + 1;
		}

		else {
			a[k] = b[j];
			j = j + 1;
		}
		k = k + 1;
	}

	while (i <= mid) {
		a[k] = b[i];
		i = i + 1;
		k = k + 1;
	}

	while (j <= right) {
		a[k] = b[j];
		j = j + 1;
		k = k + 1;
	}
} 

void answer(int a[], int left, int right, int k, vector<int> &v)
{
	int b[100];
	int ctr = 0;		// ctr is counter for b[100];	
	merge_sort(a, left, right);
	for (int i = left; i <= right; i++) {
		b[ctr] = a[i];
		ctr = ctr + 1;
	}
	
	v.push_back(b[k]);
}		
int main()
{
	int size;
	int num_ques;

	int left, right, k;

	vector<int> v;

	cin >> size;
	cin >> num_ques;

	int arr[size];
	int temp[size];

	for (int i = 0; i < size; i++) {
		cin >> arr[i];
	}

	for (int i = 0; i < size; i++) {
		temp[i] = arr[i];
	}
	
	for (int i = 0; i < num_ques; i++) {
		cin >> left;
		cin >> right;
		cin >> k;
		
		answer(arr, left - 1, right - 1, k - 1, v);

		for (int j = 0; j < size; j++) {
			arr[j] = temp[j];
		}
	}

	for (int i = 0; i < v.size(); i++) {
		cout << v[i] << endl;
	}

	return 0;
}

[Moschops]

My program runs absolutely fine for the test case.

To fix the bug, it helps if we get to see the bug. Giving us the case which does not exhibit the bug isn't going to help.

Also is there a utility by which we can check the cause of such signals in our program.

Build it with debugging symbols included and then run it under valgrind.