How to generate vector like list comprehension
15,328
Solution 1
Do you want to use third-party libraries? Eric Niebler's range-v3 allows for:
std::vector<string> blockPathList =
view::ints(0, blockNum)
| view::transform([&desPath](int i) {
return desPath + "part" + std::to_string(i);
});
That's about as functional list-comprehension-y as you're going to get in C++.
Solution 2
Not pretty either, but should also get the job done:
int cur = 0;
std::vector<std::string> blockPathList(blockNum);
std::generate(blockPathList.begin(), blockPathList.end(),
[&](){ return destPath + "part" + std::to_string(cur++); });
Unfortunately this
- Requires the vector to be pre-sized
- Requires an external iteration variable (since the
std::generate
Generator
does not take any arguments.
You can also use std::for_each
:
std::vector<int> nums(blockNum);
std::iota(nums.begin(), nums.end(), 0);
std::for_each(nums.begin(), nums.end(), [&](int c) {
blockPathList.push_back(destPath + "part" + std::to_string(c));
});
but again this is uglified because std::iota
doesn't generate ranges. It populates an existing range with an iterator, rather than acting as a numeric iterator in-itself (of course you can solve that by implementing or using something which generates those iterators)
Solution 3
Another example (c++14):
#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
template<typename CONTAINER, typename LAMBDA>
auto comprehension(CONTAINER&& container, LAMBDA&& lambda){
std::vector<decltype(lambda(*container.begin()))> w;
std::transform(container.begin(),container.end(),std::back_inserter(w), lambda);
return w;
}
int main(){
auto&& ints = {1,2,3,4,5};
auto&& squares = comprehension(ints,[](auto i){ return i*i; });
for( auto s : squares){ std::cout << s << ' '; }
std::cout << '\n';
}
Output:
1 4 9 16 25
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Author by
chenzhongpu
Updated on September 14, 2022Comments
-
chenzhongpu over 1 year
In C++11,
vector<string> blockPathList; for(int i = 0; i < blockNum; i++) { blockPathList.push_back(desPath + "part" + to_string(i)); }
Is it possible to re-write the code above like list comprehension, or shorter and more concise?
-
Vincent Savard almost 8 years@ChenZhongPu: My point is those information should be in your question so it can be answered appropriately. Read more about minimal reproducible example.
-
ForceBru almost 8 yearsDoes C++ support list comprehensions? You're probably looking for Haskell or Python
-
Mark Nunberg almost 8 yearsIs
std::generate
something you'd be interested in (though in this case you'd need to pre-size the vector)? -
Vincent SavardWhat's
blockNum
anddesPath
? -
CarcigenicateIt looks like you're turning 1 type of list (vector) into another. This seems like a job for a
map
function, which I'm fairly sure C++ has.
-
-
Max Lybbert almost 8 yearsYou don't have to pre-size the vector if you use
generate_n
(C++11) andback_inserter
.