How to write this for-loop using std::for_each or std::transform?

Question

This is more of a learning question. Is there a way I can write the following for-loop using std::for_each or std::transform? If not, is there anything in boost that can help on this? The loop simply flattens a vector of vectors into one long vector.

vector<vector<int> > int_vectors;

// ... fill int_vectors

vector<int> ints;

for (vector<vector<int> >::const_iterator iter = int_vectors.begin(); iter != int_vectors.end(); ++iter) {
   ints.insert(ints.end(), iter->begin(), iter->end());
}

I'm trying to learn more about STL algorithms and boost! – Haitham Gad Jan 21 '11 at 04:57 — Haitham Gad, Jan 21 '11 at 04:57

score 4 · Accepted Answer · edited May 23 '17 at 10:33

4

I wouldn't change this to use one of the algorithms unless you have a compiler that supports lambdas. It's completely clear as written. Even if your compiler did support lambdas, I'd probably not change this code.

One relatively straightforward option would be to write a flattening iterator. I wrote one for demonstration in an answer to another question.

If you really want a one-liner and can use bind (boost::bind from Boost, std::tr1::bind from TR1, and std::bind from C++0x will all work), then here is how that would look. I warn you in advance: it is horrible.

Edit: Technically this is also illegal. The type of a Standard Library member function is unspecified, so you cannot (portably or correctly) take the address of such a member function. If you could correctly take the address of a Standard Library member function, this is what it would look like:

typedef std::vector<int>::iterator (std::vector<int>::*IteratorGetter)();

std::for_each(int_vectors.begin(), int_vectors.end(),
    std::bind(
        std::bind(
            &std::vector<int>::insert<std::vector<int>::iterator>, 
            &ints, 
            std::bind((IteratorGetter)&std::vector<int>::end, &ints), 
            _1, 
            _2
        ),
        std::bind((IteratorGetter)&std::vector<int>::begin, _1),
        std::bind((IteratorGetter)&std::vector<int>::end, _1)
    )
);

(Yes, that is technically one "line of code" as it is a single statement. The only thing I have extracted is a typedef for the pointer-to-member-function type used to disambiguate the overloaded begin and end functions; you don't necessarily have to typedef this, but the code requires horizontal scrolling on Stack Overflow if I don't.)

edited May 23 '17 at 10:33

Community

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answered Jan 21 '11 at 04:26

James McNellis

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Thanks James, I did see your flattening iterator solution, but I was looking for a more straightforward one-line code. It's not that I don't like the above code, I'm just trying to learn! You mentioned compiler support for lambdas. Does it have to be supported by the compiler or can boost::lambda work? – Haitham Gad Jan 21 '11 at 04:50
@Haitham: I've posted what a pure one-liner would look like. Suffice to say, it isn't pretty. I don't know what a solution using Boost.Lambda would look like; I've never used it. – James McNellis Jan 21 '11 at 05:04
1

Is C++ the only prominent language where users torture themselves with 13-line "one line of code" replacements for 3-line for-loops? @Haitham: Scenarios using for_each, transform, or almost anything else are, when arbitrarily invented by someone who doesn't yet know them, very unlikely to be helpful. Instead, study how others use them, including how they are used in books, articles, etc., and always [deliberately](http://norvig.com/21-days.html) evaluate such uses. – Fred Nurk Jan 21 '11 at 05:08
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@Fred: I like writing torturous code in any programming language :-). I warned that this code was horrible and I strongly recommend not using it. It's fun to write, though. – James McNellis Jan 21 '11 at 05:15
@James: Thanks for taking the time to write this, really appreciate it. @Fred: Take it easy, I'm not gonna replace a 13-lines code for a 3-lines code. I was just curious how something like this could be done! Thanks guys! – Haitham Gad Jan 21 '11 at 05:23
@Haitham: Perhaps it comes across harsher than meant; but it's valuable to make this point about avoiding awkward learning situations, yet often hard to explain. What do you do when you see someone pounding on a screw with a hammer and insisting they're just doing it for "learning purposes"? :) – Fred Nurk Jan 21 '11 at 05:32

score 2 · Answer 2 · answered Jan 21 '11 at 06:00

2

If your compiler supports lambdas this is fairly simple. The typedefs help with readability.

typedef std::vector<int> VI;
typedef std::vector<VI> VVI;

VVI int_vectors;
VI ints;

std::for_each(int_vectors.begin(), int_vectors.end(), [&](VI &vec) {
    ints.insert(ints.end(), vec.begin(), vec.end());
});

answered Jan 21 '11 at 06:00

Blastfurnace

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Unfortunately my compiler doesn't support lambdas. It's gcc 3.4.6 ... pretty old! Nice code though! – Haitham Gad Jan 21 '11 at 07:22

score 0 · Answer 3 · answered Jan 22 '11 at 10:46

I kno macros aren't great practice but you can do some clever things with them:

#include <algorithm>
#include <vector>
#include <iostream>
#include <iterator>

using namespace std;
#define all(v) (v).begin(), (v).end()
#define foreach(it, v) for(auto it = (v).begin(); it != (v).end(); ++it)

void main() {
    vector<int> vi;
    vector<vector<int>> vvi;

    foreach(it, vvi) {
        copy(all(*it), back_inserter(vi));
    }
}

You can also use std::copy and back_inserter to do what you're trying to achieve.

How to write this for-loop using std::for_each or std::transform?

3 Answers3