87 lines
2.8 KiB
Markdown
87 lines
2.8 KiB
Markdown
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---
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title: Erase–remove idiom
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---
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## Desctiprion
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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.
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### Comparison
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```cpp
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// Using a hand-written loop
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std::vector<int> v = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
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for (auto iter = v.cbegin(); iter < v.cend(); /*iter++*/)
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{
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if (is_odd(*iter))
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{
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iter = v.erase(iter);
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}
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else
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{
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++iter;
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}
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}
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// Using the erase–remove idiom
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std::vector<int> v = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
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v.erase(std::remove_if(v.begin(), v.end(), is_odd), v.end());
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```
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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.
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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.
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### Example
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```cpp
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#include <vector> // the general-purpose vector container
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#include <iostream> // cout
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#include <algorithm> // remove and remove_if
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bool is_odd(int i)
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{
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return (i % 2) != 0;
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}
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void print(const std::vector<int> &vec)
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{
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for (const auto& i : vec)
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std::cout << i << ' ';
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std::cout << std::endl;
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}
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int main()
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{
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// initializes a vector that holds the numbers from 1-10.
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std::vector<int> v = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
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print(v);
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// removes all elements with the value 5
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v.erase(std::remove(v.begin(), v.end(), 5), v.end());
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print(v);
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// removes all odd numbers
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v.erase(std::remove_if(v.begin(), v.end(), is_odd), v.end());
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print(v);
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// removes multiples of 4 using lambda
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v.erase(std::remove_if(v.begin(), v.end(), [](int n) { return (n % 4) == 0; }), v.end());
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print(v);
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return 0;
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}
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/*
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Output:
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1 2 3 4 5 6 7 8 9 10
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1 2 3 4 6 7 8 9 10
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2 4 6 8 10
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2 6 10
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*/
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```
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### Sources
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"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)
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Meyers, Scott (2001). Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library. Addison-Wesley.
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