How to erase elements in STL vector without copying iterator and without memory leakage?

Question

This is the edited code

vector<vector<int> > vec; 
vector<vector<int> >::iterator vit; 
vector<int>::iterator it;
for ( vit = vec.begin(); vit != vec.end(); ++vit)
{
    it = vit->begin();
    while(it != vit->end()) 
    {
        while(it != vit->end())
        {

            if( condition )
            {
                while( condition )
                {
                    //going back to certain it
                    //erase an element
                    it  = vit->erase(it);
                }
            }
            else
                ++it;
        }
        ++it;
    }
}

The inner while loops back to certain point. Link missing. Erasing element without copying iterator and without memory leakage in back loop and again forward?

Possible duplicate of [How to delete an element from a vector while looping over it?](http://stackoverflow.com/questions/8597240/how-to-delete-an-element-from-a-vector-while-looping-over-it) — Pixelchemist, Apr 12 '16 at 10:32

Pixelchemist · Answer 1 · 2016-04-12T10:59:29.793

If you want to remove certain elements of the inner vectors use the erase-remove idiom on them:

for (auto & intv : vec)
{
  intv.erase( 
    std::remove_if( intv.begin(), intv.end(), 
      [](int value) -> bool { return value == 2; }), 
    intv.end() );
}

Notes:

You can apply certain conditions depending on vec or other local variables by capturing them in the lambda expression.
This example would remove all elements from the inner vectors that are equal to 2.
For int it is preferable to pass the lambda paramter by value. For bigger objects you'd probably want to have a const reference lambda paramter.

Mayhem · Answer 2 · 2016-04-12T15:58:45.910

3

You can use the erase-remove idiom for that purpose.

Simple example:

#include <iostream>
#include <vector>
#include <algorithm>

bool is_odd(const int &i)
{
    return (i % 2) != 0;  
}

int main() 
{
    std::vector<int> v = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
    // removes all odd numbers
    v.erase(std::remove_if(v.begin(), v.end(), is_odd), v.end());
    for(const auto &i:v)
    {
        std::cout << i << ' ';
    }
    return 0;
}

Of course you can achieve the same thing using a lambda like this:

#include <iostream>
#include <vector>
#include <algorithm>

int main() 
{
    std::vector<int> v = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
    // removes all odd numbers
    v.erase( std::remove_if(v.begin(), v.end(),
        [](const int &i) -> bool {return (i % 2) != 0;}),
        v.end() );
    for(const auto &i:v)
    {
        std::cout << i << ' ';
    }
    return 0;
}

edited Apr 12 '16 at 15:58

answered Apr 12 '16 at 10:31

Mayhem

487
2
5
13

1

Would be good to include a snippet of code on what the idiom is. – Niall Apr 12 '16 at 10:33
Completely agree with @Niall. Links tend to break at some point – 463035818_is_not_an_ai Apr 12 '16 at 11:02
You are absolutely right @Niall. I've added a simple example. :) – Mayhem Apr 12 '16 at 11:08

score 1 · Answer 3 · answered Apr 12 '16 at 10:33

1

You can use STL vector's erase function. Just use vector.remove(*it) to remove the element

answered Apr 12 '16 at 10:33

N6t9N

21
3

marcinj · Accepted Answer · 2016-04-12T12:05:48.100

You dont need to worry about memory leaks, in your case you use std::vector which uses RAII to make sure no memory is leaked. With STL containers you can "leak" in such case on;y if you do not shrink your vector after using remove algorithm.

Below is fix to your code, and not entire rewrite. This code removes all values equal to 1 [LIVE]:

vector<vector<int> > vec = { {0,1,2,3}, {0,1,2,3,4,5}, {0,1} }; 
vector<vector<int> >::iterator vit = vec.begin(); 
vector<int>::iterator it;

while(vit != vec.end()) {
    it = vit->begin();
    while(it != vit->end()) {
     if( *it == 1 ) {
        it = vit->erase(it);

         // Not sure why you want this loop here
         //while( condition )
         //{
         //     **erase an element;**
         //}
     }
     else {
       ++it;
     }
    }
    ++vit;
}  

for (auto it1 = vec.begin(); it1 != vec.end(); ++it1){
    for (auto it2 = it1->begin(); it2 != it1->end(); ++it2)
       std::cout<<*it2<< ", ";
    std::cout << "\n";
}

produces:

g++ -std=c++14 -O2 -Wall -pedantic -pthread main.cpp && ./a.out
0, 2, 3, 
0, 2, 3, 4, 5, 
0,

Thanks. The inner 'while' loops back to certain point while going back elements are erased on a condition. There is a problem in going backward and forward again, missing links. — Raghu G, Apr 12 '16 at 12:08
@RaghuG I probably dont understand, but you can reassign `it` to new iterator, as long as it is in the range of the vector on which it operates. In my example you can assign `it` back to `vit->begin()`, then iterate it again in your inner loop, or better use another iterator. — marcinj, Apr 12 '16 at 12:22
Marcin Jędrzejewski Thank you. Solved problem by assigning new iterator. — Raghu G, Apr 12 '16 at 14:57

How to erase elements in STL vector without copying iterator and without memory leakage?

4 Answers4