Remove elements from a list while iterating through it in C++

This post will discuss how to remove elements from a list while iterating inside a loop in C++.

The idea is to iterate the list using iterators and call list::erase on the desired elements. But we can’t directly call the erase() function inside a for-loop since calling it invalidates the iterator. We can handle this in many ways:

1. We can reset the iterator to the next element in the sequence using the return value of erase(). Note that this will only work with C++11 and above.

#include <iostream>

#include <list>

#include <algorithm>

#include <iterator>

using namespace std;

int main()

{

list<string> input = { "red", "green", "blue", "gray", "black" };

list<string>::const_iterator itr = input.cbegin();

while (itr != input.cend())

{

// remove strings having length 5

if (itr->length() == 5) {

itr = input.erase(itr);

}

else {

++itr;

}

copy(input.begin(), input.end(), ostream_iterator<string>(cout, "\n"));

return 0;

}

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Output:

red
blue
gray

2. We can also decrement the iterator inside the function arguments using the postfix decrement operator. This way, a copy of the iterator is passed to the erase() function, and the actual iterator is incremented.

#include <iostream>

#include <list>

#include <iterator>

using namespace std;

int main()

{

list<string> input = { "red", "green", "blue", "gray", "black" };

for (auto itr = input.cbegin(); itr != input.end(); itr++)

{

// remove strings having length 5

if (itr->length() == 5) {

input.erase(itr--);

}

copy(input.begin(), input.end(), ostream_iterator<string>(cout, "\n"));

return 0;

}

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Output:

red
blue
gray

3. Like the second solution, we can call the erase() function on a copy of the original iterator after advancing the original iterator to the next element.

#include <iostream>

#include <list>

#include <iterator>

using namespace std;

int main()

{

list<string> input = { "red", "green", "blue", "gray", "black" };

list<string>::const_iterator itr = input.cbegin();

while (itr != input.cend())

{

list<string>::const_iterator curr = itr++;

if (curr->length() == 5) {

input.erase(curr);

}

copy(input.begin(), input.end(), ostream_iterator<string>(cout, "\n"));

return 0;

}

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Output:

red
blue
gray

That’s all about removing elements from a list while iterating through it in C++.

Remove elements from a list while iterating through it in C++

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