Why does the code below causes Segmentation Fault (SIGSEGV)? - c++

Question

PROBLEM STATEMENT
You are given a strictly increasing sequence of integers A1,A2,…,AN. Your task is to compress this sequence.
The compressed form of this sequence is a sequence of ranges separated by commas (characters ','). A range is either an integer or a pair of integers separated by three dots (the string "..."). When each range a...b in the compressed form is decompressed into the subsequence (a,a+1,…,b), we should obtain the (comma-separated) sequence A again.
For each maximal contiguous subsequence (a,a+1,…,b) of A such that b≥a+2, the compressed form of A must contain the range a...b; if b≤a+1, such a sequence should not be compressed into a range. A contiguous subsequence is maximal if it cannot be extended by at least one element of A next to it. It can be proved that the compressed form of any sequence is unique (i.e. well-defined).
Input
The first line of the input contains a single integer T denoting the number of test cases. The description of T test cases follows.
The first line of each test case contains a single integer N.
The second line contains N space-separated integers A1,A2,…,AN.
Output
For each test case, print a single line containing one string ― the compressed form of the given sequence.
Constraints
1≤T≤100
1≤N≤100
1 ≤ Ai ≤ 1000 for each valid i
A1 < A2 < …... <AN
Subtasks
Subtask #1 (100 points): Original constraints
Example Input
3
12
1 2 3 5 6 8 9 10 11 12 15 17
4
4 5 7 8
1
4
Example Output
1...3,5,6,8...12,15,17
4,5,7,8
4
MY Code:
#include <bits/stdc++.h>
using namespace std;
bool b[1005];
int a[1005];
int main()
{
int test, i, j, size, count;
cin >> test;
while (test--)
{
for (i = 0; i < 1005; i++)
b[i] = false;
cin >> size;
for (i = 0; i < size; i++)
{
cin >> a[i];
b[a[i]] = true;
}
for (i = 0; i < 1005; i++)
{
if (b[i] == true)
{
cout << i;
j = i;
count = 0;
while (b[j] == true)
{
count++;
j++;
}
if (count > 2)
{
i = j;
if ((j - 1) != a[size - 1])
cout << "..." << i - 1 << ",";
else
cout << "..." << i - 1;
}
if (count == 2)
{
i = j;
if ((j - 1) != a[size - 1])
cout << "," << i - 1 << ",";
else
cout << "," << i - 1;
}
if (count == 1 && ((j - 1) != a[size - 1]))
cout << ",";
}
}
}
return 0;
}
}
MY Question:
Above code runs perfectly on my device giving desired output. But when I am submitting this solution to
Online Judge , it says segmentation fault. It's sure that fundamentally I am accessing the memory incorrectly. Could you please show me where it is?

Answer 1

b is defined a bool[1005]
In this part
for(i=0 ; i<4000 ; i++)
b[i] = false;
You are writing false value 4000 times, exceeding the array size.
Overwriting past the array is allowed on the compiler but will have undefined behaviour in runtime.
In short: it can or can not cause a segfault.

Answer 2

Here is another approach given that the input data is in a file input.txt:
#include <fstream>
#include <iostream>
#include <string>
#include <vector>
class Reader {
public:
Reader(const std::string& filename) :
filename_(std::move(filename)), is_(filename_)
{
is_.exceptions( std::ifstream::failbit | std::ifstream::badbit );
}
int get_N() {
int N;
is_ >> N;
return N;
}
std::vector<int> get_ints(int N) {
std::vector<int> v;
v.reserve(N);
for (int i = 0; i < N; i++ ) {
int value;
is_ >> value;
v.push_back(value);
}
return v;
}
int get_num_cases() {
int num_cases;
is_ >> num_cases;
return num_cases;
}
private:
std::string filename_;
std::ifstream is_;
};
bool start_range_cur( std::vector<int> &v, int j, int N )
{
if ( j>= (N - 2) ) return false;
return ((v[j+1] - v[j]) == 1) && ((v[j+2] - v[j+1]) == 1);
}
bool in_range_cur( std::vector<int> &v, int j )
{
return (v[j+1] - v[j]) == 1;
}
void print_range( int min, int max, bool print_comma)
{
std::cout << min << ".." << max;
if (print_comma) std::cout << ",";
}
void print_single(int val, bool print_comma)
{
std::cout << val;
if (print_comma) {
std::cout << ",";
}
}
int main() {
Reader is {"input.txt"};
int num_cases = is.get_num_cases();
for (int i = 0; i < num_cases; i++) {
int N = is.get_N();
std::vector<int> v = is.get_ints(N);
bool in_range = false;
int range_start;
for( int j = 0; j< N; j++ ) {
if (in_range) {
if (j == (N - 1)) {
print_range(range_start, v[j], false);
}
else if (in_range_cur(v, j)) {
continue;
}
else {
print_range(range_start, v[j], true);
in_range = false;
}
}
else {
if (j == (N - 1)) {
print_single(v[j], false);
}
else if (start_range_cur(v, j, N)) {
in_range = true;
range_start = v[j];
}
else {
print_single(v[j], true);
}
}
}
std::cout << '\n';
}
return 0;
}