Push_back method works differently in this code. Why?

Question

#include<bits/stdc++.h>
#define MAX 20
using namespace std;
int main()
{
//Creating a vector
vector <int> v;
std::vector<int>::iterator vIterator;
int i;

for(i=1;i<MAX;i++)
{
    v.push_back(i);
}

cout<<"Numbers:"<<endl;
for(vIterator = v.end();vIterator>v.begin();vIterator--)
{

    cout<<*vIterator<<endl;
}

int el_count = v.size();
cout<<"Size="<<el_count;

return 0;
}

Here's the output of the code:

Numbers: 0 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 Size=19

Why am I getting this "0" at the start? And why does my list begin with a 2?

Note that `vIterator = v.end()` points at one beyond the actually valid data in the vector. So dereferencing in the 1st cycle is undefined behavior. — πάντα ῥεῖ, Aug 13 '16 at 18:51
Use `rbegin` and `rend` instead to iterate a container backwards without this hassle. — Baum mit Augen, Aug 13 '16 at 18:53
Don't ever include anything under `bits`, it's libstdc++ implementation-specific stuff, non portable and subject to change. — Matteo Italia, Aug 13 '16 at 18:56
@MatteoItalia There's even a canonical: http://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h — πάντα ῥεῖ, Aug 13 '16 at 19:04
`#include` is not standard c++. Who is teaching people to do this? — Richard Hodges, Aug 13 '16 at 20:42
It is recommended to use `!=` rather than `<` or `>` to compare iterators while iterating containers. This is to maintain uniformity among all containers. — Shubham, Aug 13 '16 at 20:45

GoodDeeds · Accepted Answer · 2016-08-18T20:29:10.377

1

This is because the range of indices of a vector is from v.begin() to v.end()-1. You are instead using it from v.begin()+1 to v.end().

The statement in the loop should be,

for(vIterator = v.end()-1; vIterator>=v.begin(); --vIterator)

Accessing v.end() would be undefined behaviour, here giving a 0, and clearly you are skipping the first element.

Edit: (Thanks to @Revolver_Ocelot)

As can be seen from the comments, in the last iteration, when vIterator=v.begin(), and then vIterator-- is called, the location pointed by the iterator will result in undefined behaviour. This is because vIterator>=v.begin() may or may not be true when vIterator is decremented below v.begin(). An alternative could be,

for(vIterator = v.end()-1; vIterator>v.begin(); --vIterator)
{
    cout<<*vIterator<<endl;
}
cout<<*vIterator<<endl;

Another way would be using reverse iterators,

std::vector<int>::reverse_iterator vIterator;
for(vIterator = v.rbegin(); vIterator!=v.rend(); ++vIterator)

edited Aug 18 '16 at 20:29

answered Aug 13 '16 at 18:51

GoodDeeds

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Decrementing iterator equal to `begin()` is UB. Reverse iterators use special care to avoid this. You have to use `break` in the loop, use reverse iterators or handle `.begin()` out of the loop. – Revolver_Ocelot Aug 13 '16 at 18:54
1

@Revolver_Ocelot Can you please explain how it invokes undefined behavior? – Shubham Aug 13 '16 at 19:38
@Shubham Creatin iterator which are not pointing inside the container or are one-past-the-end iteratirs is illegal for all standard iterators. At the last iteration you are decrementing iterator equal to `begin()`, which takes you outside container. – Revolver_Ocelot Aug 13 '16 at 20:01
@Revolver_Ocelot True, but after decrementing the iterator in the last cycle, we are not using it or dereferencing it. So how does this affect the program? – Shubham Aug 13 '16 at 20:13
@Shubham mere act of creating iterator outside of container is UB. It does not matter if you do not dereference it. For complex containers, iterator creation might access container. For arrays/vectors and segmented memory, you migh find that `--begin > begin` – Revolver_Ocelot Aug 13 '16 at 20:26

Push_back method works differently in this code. Why?

1 Answers1