subtract consecutive set elements

Question

I am trying to implement a set version of the solLoops function:

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <ctime>
#include <vector>
#include <algorithm>
#include <unordered_set>

int solLoops(std::vector<int> A)
{

    int count = 0;
    for (int i = 0; i < A.size() - 1; ++i)
    {
        if (A[i + 1] - A[i] <= 1)
        {
            count++;
        }
    }
    return count;
}

int solSet(std::vector<int> A)
{
    std::unordered_set<int> S;

    for (int i = 0; i  < A.size(); ++i)
    {
        S.insert(A[i]);
    }

    int count = 0;
    for (auto it = S.begin(); it != std::prev(S.end()); ++it)
    {
        auto it2 = std::next(it, 1);
        if (*it2 - *it <= 1)
        {   
            count++;
        }
    }

    return count;
}



int main() {

    const unsigned int N = 4;

    std::vector<int> A;
    A.reserve(N);
    A.emplace_back(11);
    A.emplace_back(12);
    A.emplace_back(13);
    A.emplace_back(1);
    A.emplace_back(2);

    //int result = solLoops(A);
    int result = solSet(A);
    std::cout << "Result is: " << result << "\n";
    std::cout << "\n";

    return 0;
}

but I am getting a seg fault. I am trying to implement the loop:

for (int i = 0; i < A.size() - 1; ++i)

by

for (auto it = S.begin(); it != std::prev(S.end()); ++it)

and make the subtraction:

if (A[i + 1] - A[i] <= 1)

as

auto it2 = std::next(it, 1); if (*it2 - *it <= 1)

(I know that the set contains the elements in random order. I am just trying to figure how to do the subtraction here)

Answer 1

std::unordered_set have forward iterators (which can only go forward). But std::prev requires a bidirectional iterator.

You can solve your problem by checking if the next iterator is the end or not: std::next(it) != S.end(). Or use std::set instead, which have bidirectional iterators.