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I have a quick question that's giving me some grief. As part of a much larger project, I want to scan a vector for elements that are 0 and, when I find them, delete them. I'm curious as to why the following is alright:

if (playerVec[5] == 0)

But this is not:

for(vector<Player>::iterator it = playerVec.begin(); it != playerVec.end(); ++it) {

if(playerVec[it] == 0) { //Invalid if condition

}

I assume it has to do with the factor that it is an iterator, but how else could I approach the deletion within a for loop? Do I need another variable for indexing?

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3 Answers 3

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2

it is an iterator, not an index. If you want to get the index from a (random access) iterator it, just do:

 it - playerVec.begin()

However, in your case you just want to dereference the iterator to get the value of the pointed element:

if (*it == 0) { 
    // ...
}
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7 Comments

Alright, mostly understood. The auto keyword is somewhat fuzzy to me, though I have heard of its use before. Is it strictly necessary in this case? If so, why?
@Rome_Leader: It is not strictly necessary, and in fact I edited the answer, because what I was suggesting was somewhat stupid. Please disregard it :)
Haha, fair enough! I have a feeling I will need to go elsewhere to overload the == operator though, since the compiler currently complains it has no definition for Player Type == Int Type. Am I right here?
I think is is not always right to compare a Player object with 0.(depends on how Player class is defined)
@ta.speot.is: lol, leftover from a previous attempt to cover more than I actually wanted to. I should definitely get some sleep and stop answering :)
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2

You can use the remove_if function to remove elements within a container that meet some criteria:

#include <algorithm>

bool isZero(Player p) {
  return p == 0;
}

playerVec.erase(std::remove_if(playerVec.begin(), playerVec.end(), isZero),
                playerVec.end());
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3 Comments

For this case, you can just use std::remove and pass the value 0. No need for a custom function. Also, in order to actually delete the elements from the container, you need to capture the return value of the algorithm and pass it to vector::erase.
Yeah, I definitely want the elements gone altogether. So erase seems to be the way to go. I think I can do that, still concerned about the condition for deletion, though
@Benjamin Thanks, I completely forgot about using erase.
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Use the erase-remove idiom:

std::erase(std::remove(playerVec.begin(), playerVec.end(), 0), playerVec.end());

The reason your condition is invalid is because an iterator isn't an index. It's like a pointer to an element, and you can use it as such (with * and -> to access the element). For example, the correct condition in your code would be if(*it == 0).

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2 Comments

I understand remove, but trying to wrap it in erase like you suggested gives a syntax error saying it is undefined. I have included both string and algorithm libraries, so I'm wondering what the issue is. EDIT: Actually, when I build it, the compiler throws an error then, even for the remove line itself. I have a feeling it's due to the multiple data member nature of Player.
Nvm, I got it with this, just needed to edit the syntax a little. Inside the outer brackets, after the remove operation, should be ,playerVec.end(). Thanks for the point in the right direction!

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