Vector manipulation

Question

I am not proficient in C++. I have a very short C++ script I am trying to convert to PHP which deals with vectors.

From online documentation I gather that vectors are lists of stuff, somewhat like arrays in PHP but with less features. However, I am struggling to find good documentation on vector manipulation.

For example

real32 test(std::vector<T>::iterator First, std::vector<T>::iterator Last)
{
    if(Last - First > 0) {
        // do stuff
    }
}

In this snippet I can hazard a guess at a number of outcomes to do with Last - First.

The difference in the number of elements between each vector
The difference in the sum of the elements between each vector
The difference between single elements in each vector (guessing from iterator - but this might be the for loop in the //do stuff part)

I am trying to RTFM but the some of the on-line resources I have stumbled across don't come across as particularly insightful (although maybe I am not able to ask/search the right question)

What is Last - First actually doing in this case?

Do note that two iterators may belong to two different vectors! — Ajay, Jun 01 '16 at 08:50
@Ajay If they do then that function will exhibit undefined behavior. — Galik, Jun 01 '16 at 08:56
@Galik, True. But such function is bound to raise runtime error. Try passing different iterators to `vector` and see it handles it well (well, at least in VC++) — Ajay, Jun 01 '16 at 09:07

score 6 · Accepted Answer · edited May 23 '17 at 11:44

The function does not take 2 vectors, but 2 iterators, which point at a particular place inside a vector. In this case, the first and one-past-the-last elements. This is a normal C++ idiom. (Note that the iterators might actually indicate some sub-part of an existing vector, doesn't actually have to be the whole thing).

Therefore the difference is the total number of elements in the vector. Usually, one would have a loop handling each element, and incrementing First until it hits Last.

1|2|3|4|5|
^First    ^Last

Note that Last does not point at 5, but one past it. Last is not a valid element of the vector, but a sentinel, indicating when to stop looping.

In this case Last-First == 5, since you would have to increment First 5 times to hit Last.

Reference documentation is not usually the best place to learn the language from. We do have the book list.

Your links:

isocpp.org: About the standardisation process and committee, not much actual language documentation here.
cplusplus.com: a non-official reference documentation site. In the past, was not very good about fixing errors. So many people prefer:
cppreference.com: a community-maintained wiki of reference documentation.

There is no "official" reference documentation other than The Standard, a draft of which can be found on isocpp.org, but really, don't try to read that, it is experts-only, aimed at compiler implementers. I'm not even going to link to it.

To be honest, I struggle to find a really good beginners tutorial online.

Strictly speaking we don't know that these two iterators point to the first and last elements of the vector, only that they point to a beginning and end portion of the vector. Their difference is the number of elements that lie between those two points. — Galik, Jun 01 '16 at 08:51

Vector manipulation

1 Answers1