Find all matching elements in std::list

问题

I was wondering if there's any built-in or well-established way (i.e. via lambda) to go through the elements of an std::list and find all the ones that match a given value? I know I can iterate through all of them, but I thought I'd ask if there's a way to get an iterator that iterates through just the elements that match a given criteria? My sample below only gives me the iterator to the first matching element.

#include <list>
#include <algorithm>
#include <stdio.h>

int main()
{
    std::list<int> List;
    List.push_back(100);
    List.push_back(200);
    List.push_back(300);
    List.push_back(100);
    int findValue = 100;

    auto it = std::find_if(List.begin(), List.end(), [findValue](const int value)
    {
        return (value == findValue);
    });

    if (it != List.end())
    {
        for (; it != List.end(); ++it)
        {
            printf("%d\n", * it);
        }
    }
    return 0;
}

Thanks for any feedback.

回答1:

std::find_if is a generalisation of std::find for when you need a function to check for the elements you want, rather than a simple test for equality. If you just want to do a simple test for equality then there's no need for the generalised form, and the lambda just adds complexity and verbosity. Just use std::find(begin, end, findValue) instead:

std::vector<std::list<int>::const_iterator> matches;
auto i = list.begin(), end = list.end();
while (i != end)
{
  i = std::find(i, end, findValue);
  if (i != end)
    matches.push_back(i++);
}

But rather than calling find in a loop I'd just write the loop manually:

std::vector<std::list<int>::const_iterator> matches;
for (auto i = list.begin(), toofar = l.end(); i != toofar; ++i)
  if (*i == findValue)
    matches.push_back(i);

回答2:

Using copy_if and iterators:

#include <list>
#include <algorithm>
#include <iterator>
#include <iostream>

int main()
{
    std::list<int> List;
    List.push_back(100);
    List.push_back(200);
    List.push_back(300);
    List.push_back(100);
    int findValue = 100;

    std::copy_if(List.begin(), List.end(), std::ostream_iterator<int>(std::cout, "\n"), [&](int v) {
        return v == findValue;
    });
    return 0;
}

If you don't want to directly output the results and want to fill another container with the matches:

std::vector<int> matches;
std::copy_if(List.begin(), List.end(), std::back_inserter(matches), [&](int v) {
    return v == findValue;
});

回答3:

boost::filter_iterator allows you to work with only the elements of a iterable that satisfy a predicate. Given a predicate Pred and a container Cont,

auto begin_iter = boost::make_filter_iterator(Pred, std::begin(Cont), std::end(Cont));
auto end_iter = boost::make_filter_iterator(Pred, std::end(Cont), std::end(Cont));

You can now use begin_iter and end_iter as if they were the begin and end iterators of a container containing only those elements of Cont that satisfied Pred. Another added advantage is that you can wrap the iterators in a boost::iterator_range and use it in places which expect a iterable object, like a range-based for loop like this:

auto range = boost::make_iterator_range(begin_iter, end_iter);
for(auto x : range) do_something(x);

In particular, setting Pred to a functor(could be a lambda) that checks for equality with your fixed value will give you the iterators you need.

来源：https://stackoverflow.com/questions/27454831/find-all-matching-elements-in-stdlist

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c++

c++11

stdlist