70 lines
1.7 KiB
Markdown
70 lines
1.7 KiB
Markdown
# `erase by iterator` vs. `erase by key` in C++
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## Overview
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erasing by iterator in std::maps is typically faster than erasing by key.
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## Erasing by Iterator:
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- When you erase an element using an iterator, the map already has a direct reference to the node that contains the element to be erased.
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- The iterator is a pointer to the element (node) in the underlying tree structure (which is usually a red-black tree for std::map).
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## Erasing by Key:
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- When you erase an element by key, std::map first has to search for the key.
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- It does this by traversing the red-black tree to find the node that matches the key.
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- After finding the node, it performs the same operation as erase(iterator), but since it had to perform a search to locate the key, this is slower than directly accessing the iterator.
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## Benchmarks
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Compile and run these 2 programs to see the performance difference.
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[map_erase_slow.cpp](map_erase_slow.cpp) [map_erase_fast.cpp](map_erase_fast.cpp)
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Both programs just create a map containing 1 millions integers, and then erase these 1 millions integers. As can be seen from the following results, erasing by iterators is much faster than erasing by key.
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```console
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$g++ map_erase_slow.cpp -o map_erase_slow
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$g++ map_erase_fast.cpp -o map_erase_fast
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$time ./map_erase_slow
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real 0m0.640s
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user 0m0.624s
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sys 0m0.016s
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$time ./map_erase_fast
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real 0m0.406s
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user 0m0.386s
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sys 0m0.020s
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$time ./map_erase_fast
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real 0m0.382s
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user 0m0.374s
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sys 0m0.008s
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$time ./map_erase_slow
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real 0m0.629s
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user 0m0.617s
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sys 0m0.012s
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$time ./map_erase_slow
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real 0m0.632s
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user 0m0.623s
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sys 0m0.009s
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$time ./map_erase_fast
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real 0m0.383s
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user 0m0.366s
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sys 0m0.017s
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```
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