[RESOLVED] removing an element from a vector?

[lab1]

Hello,

Say I have a vector of something called "vec" And I use this for loop to access the elements of vec

for(size_t i=0; i<vec.size(); i++)
{
}

How can I remove the element from the vector without an iterator?

Thanks!

[laserlight]

You can get an iterator from a vector's index by adding the index to the iterator returned by begin(). That said, have you considered using std::remove() or std::remove_if() with the version of erase() that takes iterators that denote a range? That approach is less error prone and likely to be faster.

[lab1]

Hello Laserlight..

So to remove I might do this:

vec.erase(vec.begin()+i); ?

Is that right??

Thanks!

[Philip Nicoletti]

Yes, but you need to be careful about the loop index variable "i"
if you erase an element.

[lab1]

when will this code not work then? I guess what you mean is that i will actually change right?

say I have these elements in the list: a a b c d

and I do an erase on b, which is index i=2 my code would look like this:

vec.erase(vec.begin()+i);

however, in my for loop if it continues it will next grab index=3 which is now d, skipping the element c.

Is that a correct analysis?

Thanks!

[laserlight]

Yes, unless you make it such that you only increment the loop index when you do not erase anything. So, why not use std::remove() or std::remove_if()?

[lab1]

hey laser,

maybe I will look at those methods. I break form the loop when I find the element I want to erase, so the loop is not an issue.

Thanks though!!!

[JohnW@Wessex]

Iterators could be more flexible than an index as erase returns an iterator to the element after the last element erased.

[laserlight]

I break form the loop when I find the element I want to erase, so the loop is not an issue.

In that case, use std::find() or std::find_if() with erase().

[lab1]

Heya laser,

I would have done find() but the list is of sockaddr_in's and all I have is an ip address and a port. so I have to go through each sockaddr_in and compare those internal values.

Thanks though!

[laserlight]

I would have done find() but the list is of sockaddr_in's and all I have is an ip address and a port. so I have to go through each sockaddr_in and compare those internal values.

That is where std::find_if() would come in handy.

[JohnW@Wessex]

That is where std::find_if() would come in handy.

I second that! Just write a predicate functor and use find_if.

[STLDude]

Not exactly to original question but when removing elements from std::vector, always keep this in mind.