Segmentation fault while initializing vector by an initialized array in C++

Question

I wanted to use the sort() in the algorithm library in C++. I could find examples for sorting vectors only, thus I am trying to initialize a vector by an initialized array. When executing I am getting a segmentation fault and couldn't figure out what is wrong here in the code I wrote.

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;

int main() {
int n,k,packet[1000],min=0;
scanf("%d",&n);
scanf("%d",&k);

for (int i = 0; i < n; ++i)
{
    scanf("%d",&packet[i]);
    cout<<i<<endl;
}
cout<<"debug";
vector<int> packets(packet,packet+n);
vector<int>::iterator start,stop;
sort(packets.begin(),packets.begin()+n);

min=*(packets.begin())- *(packets.end());
cout<<min;
for (vector<int>::iterator it=packets.begin(); it!=packets.end()-k; ++it)
{
    printf("%d  ",*it );
    if((*(it+k) - *it)<min)
    {
        start=it;
        stop=it+k;
    }
}
printf("%d\n",*stop- *start );

return 0;

}

You can use std::sort with an array just fine. The same iterators you initialize the vector with are what you pass to the algorithm. — Qaz
– Qaz, Commented Oct 19, 2013 at 17:11
For sort, use "sort(packets.begin(), packets.end());" if that's what you mean. — kfsone
– kfsone, Commented Oct 19, 2013 at 17:12
If you are getting a segfault while compiling, you have a compiler error, then you need to post: which compiler, which operating system, what command line arguments and all of the compiler output from your compilation. — kfsone
– kfsone, Commented Oct 19, 2013 at 17:15

awesoon · Accepted Answer · 2013-10-19 17:15:12Z

2

*(packets.end())

packets.end() returns an iterator to the element, following the last element of the vector.

Attempting to derefenrence it causes Undefined Behavior.

answered Oct 19, 2013 at 17:15

awesoon

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3 Comments

Shivendra Over a year ago

I have always used as you mentioned except when I didn't want the dereferenced value.

Shivendra Over a year ago

Sorry, got it. Thanks. packet.end()-1 will do?

digital_revenant Over a year ago

@Shivendra why not use packet.back() if you want reference to the last element.

thelamb · Accepted Answer · 2013-10-19 17:18:38Z

The comments explain that you can use sort with an array just fine (if you look at http://en.cppreference.com/w/cpp/algorithm/sort you'll see that sort takes two arguments that: -RandomIt must meet the requirements of ValueSwappable and RandomAccessIterator.. Plain pointers fulfill this requirement).

In your example, the segfault happens because you try to dereference a valid but undereferencable iterator (the iterator returned by 'end()' in: min=*(packets.begin())- *(packets.end());. Basically it returns an iterator that points to after the last element of the vector. If you want to get an iterator to the last element, you can use rbegin() but of course you need to make sure that the vector is not empty first).

You could have seen this quite easily by running your code under a debugger, you'd see that the segmentation fault had nothing to do with the call to sort

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