C++11 provides multiple ways to iterate over containers. For example:
for(auto c : container) fun(c)
for_each(container.begin(),container.end(),fun)
However what is the recommended way to iterate over two (or more) containers of the same size to accomplish something like:
for(unsigned i = 0; i < containerA.size(); ++i) {
containerA[i] = containerB[i];
}
Rather late to the party. But: I would iterate over indices. But not with the classical for loop but instead with a range-based for loop over the indices:
for(unsigned i : indices(containerA)) {
containerA[i] = containerB[i];
}
indices is a simple wrapper function which returns a (lazily evaluated) range for the indices. Since the implementation – though simple – is a bit too long to post it here, you can find an implementation on GitHub.
This code is as efficient as using a manual, classical for loop.
If this pattern occurs often in your data, consider using another pattern which zips two sequences and produces a range of tuples, corresponding to the paired elements:
for (auto& [a, b] : zip(containerA, containerB)) {
a = b;
}
The implementation of zip is left as an exercise for the reader, but it follows easily from the implementation of indices.
(Before C++17 you’d have to write the following instead:)
for (auto&& items : zip(containerA, containerB))
get<0>(items) = get<1>(items);
boost::counting_range(size_t(0), containerA.size()) –
Tagliatelle indices implementation yields compiler output which is identical to using manual for loops. There is no overhead whatsoever. –
Amylum indices(std::initializer_list<T>&& cont)? Any rvalue should be "gone" by the time the range based for loop is entered. –
Religieux cont isn’t saved anywhere, and isn’t used inside the loop. –
Amylum containerA[i] = containerB[i]; a lot clearer than the line get<0>(items) = get<1>(items). Why would I even want to do away with the explicit index? And, please, don't say "Performance with list containers!": If you care about performance, there won't be any std::list<> in your code anyway. –
Find get may not be as pretty but it’s, by contrast, completely safe from UB. It‘s preventing a common cause of hard-to-debug bugs. –
Amylum zip() does defend against off-by-one errors. However, I prefer the better readability of the explicit index: In my experience, those off-by-one errors are extremely rare as writing correct loop control for just iterating over a container is an absolute no-brainer to me. –
Find zip is left as an exercise for the reader” –
Amylum i wonder why no one mentioned this:
auto itA = vectorA.begin();
auto itB = vectorB.begin();
while(itA != vectorA.end() || itB != vectorB.end())
{
if(itA != vectorA.end())
{
++itA;
}
if(itB != vectorB.end())
{
++itB;
}
}
PS: if the container sizes don't match, then you may need to put each container specific code into its corresponding if block.
while(itA != vectorA.end() && itB != vectorB.end())? –
Wrapping For your specific example, just use
std::copy_n(contB.begin(), contA.size(), contA.begin())
For the more general case, you can use Boost.Iterator's zip_iterator, with a small function to make it usable in range-based for loops. For most cases, this will work:
template<class... Conts>
auto zip_range(Conts&... conts)
-> decltype(boost::make_iterator_range(
boost::make_zip_iterator(boost::make_tuple(conts.begin()...)),
boost::make_zip_iterator(boost::make_tuple(conts.end()...))))
{
return {boost::make_zip_iterator(boost::make_tuple(conts.begin()...)),
boost::make_zip_iterator(boost::make_tuple(conts.end()...))};
}
// ...
for(auto&& t : zip_range(contA, contB))
std::cout << t.get<0>() << " : " << t.get<1>() << "\n";
However, for full-blown genericity, you probably want something more like this, which will work correctly for arrays and user-defined types that don't have member begin()/end() but do have begin/end functions in their namespace. Also, this will allow the user to specifically get const access through the zip_c... functions.
And if you're an advocate of nice error messages, like me, then you probably want this, which checks if any temporary containers were passed to any of the zip_... functions, and prints a nice error message if so.
auto works exactly the same as a template parameter, and T&& in a template is a universal reference as explained in the first link, so auto&& v = 42 will be deduced as int&& and auto&& w = v; will then be deduced as int&. It allows you to match lvalues aswell as rvalues and let both be mutable, without making a copy. –
Vasculum zip_range. –
Vasculum auto& is an lvalue reference - it simply cannot bind to temporaries. auto&&, on the other hand, can bind to either temporaries or references (lvalues). –
Vasculum auto& != auto const&. The former only binds to lvalues. –
Vasculum #include <algorithm>
#include <forward_list>
#include <ranges>
#include <array>
#include <iostream>
int main()
{
auto foos = std::to_array({ 1, 2, 3, 4, 5 });
auto woos = std::to_array({ 6, 7, 8, 9, 10 });
auto fooswoos = std::views::zip(foos,woos);
for(auto [foo, woo] : fooswoos) {
woo += foo;
}
std::ranges::for_each(woos, [](const auto& e) { std::cout << e << '\n'; });
return 0;
}
We are constructing a special "view". This view allows us to look at containers as if they were other structures without actually doing any copying or anything like that. Using a structured binding we are able to take a reference to each aligning element per iteration and do whatever we want to it (and safely)
There are a lot of ways to do specific things with multiple containers as provided in the algorithm header. For instance, in the example you've given, you could use std::copy instead of an explicit for loop.
On the other hand, there isn't any built-in way to generically iterate multiple containers other than a normal for loop. This isn't surprising because there are a lot of ways to iterate. Think about it: you could iterate through one container with one step, one container with another step; or through one container until it gets to the end then start inserting while you go through to the end of the other container; or one step of the first container for every time you completely go through the other container then start over; or some other pattern; or more than two containers at a time; etc ...
However, if you wanted to make your own "for_each" style function that iterates through two containers only up to the length of the shortest one, you could do something like this:
template <typename Container1, typename Container2>
void custom_for_each(
Container1 &c1,
Container2 &c2,
std::function<void(Container1::iterator &it1, Container2::iterator &it2)> f)
{
Container1::iterator begin1 = c1.begin();
Container2::iterator begin2 = c2.begin();
Container1::iterator end1 = c1.end();
Container2::iterator end2 = c2.end();
Container1::iterator i1;
Container2::iterator i2;
for (i1 = begin1, i2 = begin2; (i1 != end1) && (i2 != end2); ++it1, ++i2) {
f(i1, i2);
}
}
Obviously you can make any kind of iterations strategy you want in a similar way.
Of course, you might argue that just doing the inner for loop directly is easier than writing a custom function like this ... and you'd be right, if you are only going to do it one or two times. But the nice thing is that this is very reusable. =)
for (Container1::iterator i1 = c1.begin(), Container2::iterator i2 = c2.begin(); (i1 != end1) && (i2 != end2); ++it1, ++i2) but the compiler yells. Can anyone explain why this is invalid? –
Fash for (int x = 0, y = 0; ... works, but for (int x = 0, double y = 0; ...) does not. –
Coif typename... –
Coif I personally prefer using what is already in the STL (in the <algorithm> header) if possible. std::transform has a signature that can take two input iterators. So at least for the case of two input containers you could do:
std::transform(containerA.begin(), containerA.end(), containerB.begin(), outputContainer.begin(), [&](const auto& first, const auto& second){
return do_operation(first, second);
});
Note that the outputContainer can also be one of the input containers. But one limitation is that you cannot do a post update operation if you are modifying one of the containers in place.
std::back_inserter(outputContainer) as third parameter makes life easier. –
Coati In case when you need to iterate simultaneously over 2 containers only, there is an extended version of standard for_each algorithm in boost range library, e.g:
#include <vector>
#include <boost/assign/list_of.hpp>
#include <boost/bind.hpp>
#include <boost/range/algorithm_ext/for_each.hpp>
void foo(int a, int& b)
{
b = a + 1;
}
int main()
{
std::vector<int> contA = boost::assign::list_of(4)(3)(5)(2);
std::vector<int> contB(contA.size(), 0);
boost::for_each(contA, contB, boost::bind(&foo, _1, _2));
// contB will be now 5,4,6,3
//...
return 0;
}
When you need to handle more than 2 containers in one algorithm, then you need to play with zip.
another solution could be capturing a reference of the iterator of the other container in a lambda and using post increment operator on that. for example simple copy would be:
vector<double> a{1, 2, 3};
vector<double> b(3);
auto ita = a.begin();
for_each(b.begin(), b.end(), [&ita](auto &itb) { itb = *ita++; })
inside lambda you can do whatever with ita and then increment it. This easily extends to the multiple containers case.
A range-library provides this and other very helpful functionality. The following example uses Boost.Range. Eric Niebler's rangev3 should be a good alternative.
#include <boost/range/combine.hpp>
#include <iostream>
#include <vector>
#include <list>
int main(int, const char*[])
{
std::vector<int> const v{0,1,2,3,4};
std::list<char> const l{'a', 'b', 'c', 'd', 'e'};
for(auto const& i: boost::combine(v, l))
{
int ti;
char tc;
boost::tie(ti,tc) = i;
std::cout << '(' << ti << ',' << tc << ')' << '\n';
}
return 0;
}
C++17 will make this even better with structured bindings:
int main(int, const char*[])
{
std::vector<int> const v{0,1,2,3,4};
std::list<char> const l{'a', 'b', 'c', 'd', 'e'};
for(auto const& [ti, tc]: boost::combine(v, l))
{
std::cout << '(' << ti << ',' << tc << ')' << '\n';
}
return 0;
}
delme.cxx:15:25: error: no match for 'operator=' (operand types are 'std::tuple<int&, char&>' and 'const boost::tuples::cons<const int&, boost::tuples::cons<const char&, boost::tuples::null_type> >') std::tie(ti,tc) = i; ^ –
Richmound 19.13.26132.0 and Windows SDK version 10.0.16299.0): error C2679: binary '<<': no operator found which takes a right-hand operand of type 'const boost::tuples::cons<const char &,boost::fusion::detail::build_tuple_cons<boost::fusion::single_view_iterator<Sequence,boost::mpl::int_<1>>,Last,true>::type>' (or there is no acceptable conversion) –
Jam boost::combine: https://mcmap.net/q/151802/-boost-combine-range-based-for-and-structured-bindings/8414561 –
Suellensuelo I'm a bit late too; but you can use this (C-style variadic function):
template<typename T>
void foreach(std::function<void(T)> callback, int count, ...) {
va_list args;
va_start(args, count);
for (int i = 0; i < count; i++) {
std::vector<T> v = va_arg(args, std::vector<T>);
std::for_each(v.begin(), v.end(), callback);
}
va_end(args);
}
foreach<int>([](const int &i) {
// do something here
}, 6, vecA, vecB, vecC, vecD, vecE, vecF);
or this (using a function parameter pack):
template<typename Func, typename T>
void foreach(Func callback, std::vector<T> &v) {
std::for_each(v.begin(), v.end(), callback);
}
template<typename Func, typename T, typename... Args>
void foreach(Func callback, std::vector<T> &v, Args... args) {
std::for_each(v.begin(), v.end(), callback);
return foreach(callback, args...);
}
foreach([](const int &i){
// do something here
}, vecA, vecB, vecC, vecD, vecE, vecF);
or this (using a brace-enclosed initializer list):
template<typename Func, typename T>
void foreach(Func callback, std::initializer_list<std::vector<T>> list) {
for (auto &vec : list) {
std::for_each(vec.begin(), vec.end(), callback);
}
}
foreach([](const int &i){
// do something here
}, {vecA, vecB, vecC, vecD, vecE, vecF});
or you can join vectors like here: What is the best way to concatenate two vectors? and then iterate over big vector.
This variant zips different sequences to a vector of tuple's that can be used in structured bindings.
You can use any amount of sequences.
Very simple and efficient.
#include <iostream>
#include <utility>
#include <algorithm>
#include <cstddef>
#include <tuple>
#include <vector>
#include <string>
#include <array>
#include <map>
template<typename... Container>
auto zip(Container&... containers) noexcept {
using tuple_type = std::tuple<std::decay_t<decltype(*std::begin(containers))>&...>;
std::size_t container_size = std::min({ std::size(containers)... });
std::vector<tuple_type> result;
result.reserve(container_size);
auto iterators = std::make_tuple(std::begin(containers)...);
for (std::size_t i = 0; i < container_size; ++i) {
std::apply([&result](auto&... it) {
result.emplace_back(*it++...);
}, iterators);
}
return result;
}
int main() {
std::vector v1 = { 1.1, 1.2, 1.3 };
std::array v2 = { 1, 2, 3 };
std::map<std::string, int> v3;
v3["key1"] = 1;
v3["key2"] = 2;
v3["key3"] = 3;
int arr[] = { 3, 2, 1 };
for (const auto& [val1, val2, val3, val4] : zip(v1, v2, v3, arr)) {
std::cout << "vector -> " << val1;
std::cout << "\narray -> " << val2;
std::cout << "\nmap -> " << val3.first << '|' << val3.second;
std::cout << "\nC-array -> " << val4;
std::cout << "\n\n";
}
std::cout.flush();
return 0;
}
If you want something more efficient you can also try this constexpr approach:
(note that the structured binding must always be auto&& to forward the refences correctly)
#include <iostream>
#include <utility>
#include <algorithm>
#include <iterator>
#include <cstddef>
#include <tuple>
#include <vector>
#include <string>
#include <array>
#include <map>
template<typename... Container>
class zippable {
public:
using tuple_type_t = std::tuple<std::decay_t<decltype(*std::begin(std::declval<Container&>()))>&...>;
using iterator_tuple_t = std::tuple<decltype(std::begin(std::declval<Container&>()))...>;
constexpr zippable(Container&... containers) noexcept :
iterators_begin(std::make_tuple(std::begin(containers)...)),
iterators_end(std::make_tuple(std::end(containers)...)) {}
class iterator {
public:
constexpr iterator(const iterator_tuple_t& it) noexcept : iterators(it) {}
constexpr iterator(iterator_tuple_t&& it) noexcept : iterators(std::move(it)) {}
constexpr iterator& operator++() {
std::apply([](auto&... it) {
((it++), ...);
}, iterators);
return *this;
}
constexpr tuple_type_t operator*() const {
return std::apply([](const auto&... it) -> tuple_type_t {
return std::forward_as_tuple(*it...);
}, iterators);
}
constexpr bool operator==(const iterator& other) const {
return iterators == other.iterators;
}
constexpr bool operator!=(const iterator& other) const {
return iterators != other.iterators;
}
private:
iterator_tuple_t iterators;
};
constexpr iterator begin() noexcept {
return iterator(iterators_begin);
}
constexpr iterator end() noexcept {
return iterator(iterators_end);
}
private:
iterator_tuple_t iterators_begin;
iterator_tuple_t iterators_end;
};
int main() {
std::vector v1 = { 1.1, 1.2, 1.3 };
std::array v2 = { 1, 2, 3 };
std::map<std::string, int> v3;
v3["key1"] = 1;
v3["key2"] = 2;
v3["key3"] = 3;
int arr[] = { 3, 2, 1 };
for (auto&& [val1, val2, val3, val4] : zippable(v1, v2, v3, arr)) {
std::cout << "vector -> " << val1;
std::cout << "\narray -> " << val2;
std::cout << "\nmap -> " << val3.first << '|' << val3.second;
std::cout << "\nC-array -> " << val4;
std::cout << "\n\n";
}
std::cout.flush();
return 0;
}
Here is one variant
template<class ... Iterator>
void increment_dummy(Iterator ... i)
{}
template<class Function,class ... Iterator>
void for_each_combined(size_t N,Function&& fun,Iterator... iter)
{
while(N!=0)
{
fun(*iter...);
increment_dummy(++iter...);
--N;
}
}
Example usage
void arrays_mix(size_t N,const float* x,const float* y,float* z)
{
for_each_combined(N,[](float x,float y,float& z){z=x+y;},x,y,z);
}
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transformpresent in#include <algorithm>? – ChopscontainerA = containerB;instead of the loop. – Tussock