why is the size of vector is out of range

Question

#include<bits/stdc++.h>
using namespace std;
int main()
{
    vector <int> v;
    vector<int> :: iterator ip;
    v.push_back(2);
    for(int i=3;i<=32000;i+=2)
    {
        int top;
        top = sqrt(i)+1;
        int flag=0;
        if(*ip>top) break;
        if(i%*ip==0)
        {
            flag=1;
            break;
        }
        if(flag==0)
        {
            v.push_back(i);
        }
    }
    int t;
    cin>>t;
    while(t--)
    {
        int n;
        cin>>n;
        cout<<v.at(n)<<endl;
    }
    return 0;
}

can you please explain me why my code is getting runtime error. is size of the vector is getting out of range ?? I dont know why i am getting wrong answer in this code although my code is perfectly fine.

Which line did the program stop on when you ran it in a debugger? — cdhowie, Oct 08 '14 at 16:50
Add a try catch and print out the exception, what does the exception say ? — Niall, Oct 08 '14 at 16:51
You don't initialize `ip`, so `*ip > top` does ... what? You also don't bounds check user input `n`. — crashmstr, Oct 08 '14 at 16:51
You do not initialize the iterator ip but try to get the value at the position this iterator points to. This can't work. — chrizke, Oct 08 '14 at 16:51
Found some info on that odd include: http://stackoverflow.com/q/25311011/10077 — Fred Larson, Oct 08 '14 at 17:03

score 2 · Answer 1 · answered Oct 08 '14 at 16:52

2

You're dereferencing an uninitialized iterator, you want something more like:

vector<int>::iterator ip = v.begin();

answered Oct 08 '14 at 16:52

Paul Evans

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That's part of the story. This will keep getting invalidated whenever the vector grows. – Mike Seymour Oct 08 '14 at 16:54
Only works if v contains a value, i.e. first the push_back(2) from above, then your line of code. – chrizke Oct 08 '14 at 16:54
1

Except that if the vector re-sizes, it will probably invalidate the iterator. Not even sure what they are trying to do, but an index is probably simpler. – crashmstr Oct 08 '14 at 16:54

why is the size of vector is out of range

1 Answers1