87 lines
2.8 KiB
Markdown
87 lines
2.8 KiB
Markdown
---
|
||
title: Erase–remove idiom
|
||
---
|
||
|
||
## Desctiprion
|
||
How to remove elements from container is a common C++ interview question, so you can earn some brownie points, if you read this page carefully. The erase–remove idiom is a C++ technique to eliminate elements that fulfill a certain criterion from a container. Howerever, it is possible to eliminate elements with traditional hand-written loop, but the erase–remove idiom has several advantages.
|
||
|
||
### Comparison
|
||
|
||
```cpp
|
||
// Using a hand-written loop
|
||
std::vector<int> v = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
|
||
for (auto iter = v.cbegin(); iter < v.cend(); /*iter++*/)
|
||
{
|
||
if (is_odd(*iter))
|
||
{
|
||
iter = v.erase(iter);
|
||
}
|
||
else
|
||
{
|
||
++iter;
|
||
}
|
||
}
|
||
|
||
// Using the erase–remove idiom
|
||
std::vector<int> v = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
|
||
v.erase(std::remove_if(v.begin(), v.end(), is_odd), v.end());
|
||
```
|
||
|
||
As you can see, the code with hand-written loop requires a bit more typing, but it also has a performance issue. Each `erase` call has to move forward all the elements after the deleted one, to avoid "gaps" in the collection. Calling `erase` multiple times on the same container generates lots of overhead of moving the elements.
|
||
|
||
On the other hand, the code with the erase–remove idiom is not only more expressive, but it also is more efficient. First, you use `remove_if/remove` to move all elements which don't fit the remove criteria to the front of the range, keeping the relative order of the elements. So after calling `remove_if/remove`, a single call of `erase` deletes all remaining elements at the end of the range.
|
||
|
||
### Example
|
||
|
||
```cpp
|
||
#include <vector> // the general-purpose vector container
|
||
#include <iostream> // cout
|
||
#include <algorithm> // remove and remove_if
|
||
|
||
bool is_odd(int i)
|
||
{
|
||
return (i % 2) != 0;
|
||
}
|
||
|
||
void print(const std::vector<int> &vec)
|
||
{
|
||
for (const auto& i : vec)
|
||
std::cout << i << ' ';
|
||
std::cout << std::endl;
|
||
}
|
||
|
||
int main()
|
||
{
|
||
// initializes a vector that holds the numbers from 1-10.
|
||
std::vector<int> v = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
|
||
print(v);
|
||
|
||
// removes all elements with the value 5
|
||
v.erase(std::remove(v.begin(), v.end(), 5), v.end());
|
||
print(v);
|
||
|
||
// removes all odd numbers
|
||
v.erase(std::remove_if(v.begin(), v.end(), is_odd), v.end());
|
||
print(v);
|
||
|
||
// removes multiples of 4 using lambda
|
||
v.erase(std::remove_if(v.begin(), v.end(), [](int n) { return (n % 4) == 0; }), v.end());
|
||
print(v);
|
||
|
||
return 0;
|
||
}
|
||
|
||
/*
|
||
Output:
|
||
1 2 3 4 5 6 7 8 9 10
|
||
1 2 3 4 6 7 8 9 10
|
||
2 4 6 8 10
|
||
2 6 10
|
||
*/
|
||
```
|
||
|
||
### Sources
|
||
"Erase–remove idiom" Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc. [en.wikipedia.org/wiki/Erase-remove_idiom](https://en.wikipedia.org/wiki/Erase%E2%80%93remove_idiom)
|
||
|
||
Meyers, Scott (2001). Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library. Addison-Wesley.
|