create container from another container, applying each element some function in c++

Question

My question is simple, see example:

std::array<int,6> a = {{0,1,2,3,4,5}}; // -- given container. auto F = []( int i ) { return i*i; }; // -- given function. std::vector<int> v; // need create // my solution: v.reserve( a.size () ); for( std::size_t i = 0; i < a.size(); ++i ) v.push_back( F(a[i]) ); // but I need something like std::vector<int>v( a.begin(), a.end(), <|applying each element to F|> );

Can I create container something like above not calling reserve explicitly and any reallocation?

EDIT:

I want avoid reserve; because othercase my first solution is good for me :)
I want avoid any resize; because it's initialzed each element by default ctor.
I will use this in the real project which may included many 3-rd party libraries ( boost, Soft-STL, ...).

Will std::transform not do this?

Lightness Races in Orbit
– Lightness Races in Orbit

2014-02-21 19:22:58 +00:00
Commented Feb 21, 2014 at 19:22 — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Feb 21, 2014 at 19:22
transform iterators might be of use here.

Borgleader
– Borgleader

2014-02-21 19:23:36 +00:00
Commented Feb 21, 2014 at 19:23 — Borgleader
– Borgleader, Commented Feb 21, 2014 at 19:23
I want avoid v.reserve(a.size()), also.

Khurshid
– Khurshid

2014-02-21 19:23:46 +00:00
Commented Feb 21, 2014 at 19:23 — Khurshid
– Khurshid, Commented Feb 21, 2014 at 19:23

Lightness Races in Orbit · Accepted Answer · 2014-02-21 19:23:59Z

12

The standard algorithm std::transform does exactly this!

std::vector<int> v(a.size()); std::transform( std::begin(a), std::end(a), std::begin(v), F );

You can start with an empty vector and use std::back_inserter, if you like:

std::vector<int> v; std::transform( std::begin(a), std::end(a), std::back_inserter(v), F );

But you're subjecting yourself to needless re-allocations if you do that (unless you reserve first, as in your original attempt). You can decide for yourself what your priority is.

answered Feb 21, 2014 at 19:23

Lightness Races in Orbit

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12 Comments

Khurshid Over a year ago

I can't use v(a.size() ), because my real container element is not an int, some large class. and it's required me two lines.

Lightness Races in Orbit Over a year ago

@Khurshid: Sure, that's up to you. You have all options open to you here. What do you mean by requiring only two lines? Why? Space in source code is very cheap! :)

juanchopanza Over a year ago

@Khurshid Still, a call to reserve + the bqck_inserter example seems like a fine option.

Khurshid Over a year ago

does back_inserter call push_back ? if yes, it's not efficient memory usafe.

Khurshid Over a year ago

I was want, more readability solution without any cost to performance. explicit writing reserve - isn't readable, only for performance.

|

juanchopanza · Accepted Answer · 2014-02-21 19:23:53Z

5

Use std::transform:

#include <algorithm> // std::transform #include <iterator> // std::back_inserter .... transform(a.begin(), a.end(), back_inserter(v), F);

You may want to call v.reserve(asize()) first to avoid re-allocations.

answered Feb 21, 2014 at 19:23

juanchopanza

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5 Comments

Kerrek SB Over a year ago

Insidious use of ADL.

juanchopanza Over a year ago

@KerrekSB I was feeling particularly lazy. Friday is the new Sunday.

herohuyongtao Over a year ago

@KerrekSB What's ADL? :P

Felix Glas Over a year ago

@herohuyongtao It's what makes the unqualified call back_inserter(v) work.

Lightness Races in Orbit Over a year ago

@Snps: And to transform

Maxim Egorushkin · Accepted Answer · 2014-07-29 11:00:00Z

Another solution is to use boost::transform_iterator. The benefit is that you can pass iterators to the container constructor. That avoids memory reallocations compared to when using std::back_inserter or having to call reserve or resize on the destination. All in one statement:

std::vector<int> result( boost::make_transform_iterator(std::begin(a), F) , boost::make_transform_iterator(std::end(a), F) );

You can achieve terser syntax though, like this:

std::vector<int> result(transform_range(a, F));

transform_range implementation:

template<class Iterator> struct AutoSequence { Iterator const beg_, end_; template<class T> operator std::vector<T>() const { return {beg_, end_}; } }; template<class Function, class InSeq> auto transform_range(InSeq const& in) -> AutoSequence<decltype(boost::make_transform_iterator<Function>(in.begin()))> { return { boost::make_transform_iterator<Function>(std::begin(in)) , boost::make_transform_iterator<Function>(std::end(in)) }; } template<class Function, class InSeq> auto transform_range(InSeq const& in, Function&& f) -> AutoSequence<decltype(boost::make_transform_iterator(in.begin(), f))> { return { boost::make_transform_iterator(std::begin(in), f) , boost::make_transform_iterator(std::end(in), f) }; }

Completely unnecessary use of third-party library. What does this give you? One line of code saved here, a thousand added in a header. Awesome.
so, did you measure that performance benefit? and also did you measure compilation time impact?
@MaximYegorushkin: Aha! A rationale! It's a sound one, too, though you'd still have to try very hard to convince me that this tiny micro-optimisation is worth bringing in a third-party library to do the job. Your answer, unfortunately, doesn't make the OP aware of this whatsoever.

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