C++11 for each loop with more than one variable - c++

Question

I would like to translate the following traditional for loop into a C++11 for-each loop without extra looping constructs:
int a[] = { 5, 6, 7, 8, 9, 10 };
int b[] = { 50, 60, 70, 80, 90, 100 };
// Swap a and b array elements
for (int i = 0; i < sizeof(a)/sizeof(a[0]); i++)
{
a[i] ^= b[i]; b[i] ^= a[i]; a[i] ^= b[i];
}
Does there exist any way by which it is possible to provide more than one variable in the C++11 for-each loop like:
for (int i, int j : ...)

Answer 1

There is no built-in way to do this. If you can use Boost, boost::combine will work for iterating two (or more) ranges simultaneously (Does boost offer make_zip_range?, How can I iterate over two vectors simultaneously using BOOST_FOREACH?):
for (boost::tuple<int&, int&> ij : boost::combine(a, b)) {
int& i = boost::get<0>(ij);
int& j = boost::get<1>(ij);
// ...
}
Unfortunately accessing the elements within the tuple elements of the zipped range is highly verbose. C++17 will make this much more readable using structured binding:
for (auto [&i, &j] : boost::combine(a, b)) {
// ...
}
Since you don't need to break out of the loop or return from the enclosing function, you could use boost::range::for_each with the body of your loop as a lambda:
boost::range::for_each(a, b, [](int& i, int& j)
{
// ...
});

Answer 2

zip or combine ranges are common in many range libraries.
Writing one strong enough for a for(:) loop isn't hard however.
First we write a basic range type:
template<class It>
struct range_t {
It b,e;
It begin() const{ return b; }
It end() const{ return e; }
range_t without_front( std::size_t count = 1 ) const {
return {std::next(begin()), end()};
}
bool empty() const { return begin()==end(); }
};
template<class It>
range_t<It> range( It b, It e ) { return {b,e}; }
template<class C>
auto range( C& c ) {
using std::begin; using std::end;
return range( begin(c), end(c) );
};
Then we write an iterator that works with ranges (easier than with iterators):
template<class R1, class R2>
struct double_foreach_iterator {
R1 r1;
R2 r2;
void operator++() { r1 = r1.without_front(); r2 = r2.without_front(); }
bool is_end() const { return r1.empty() || r2.empty(); }
auto operator*()const {
return std::tie( *r1.begin(), *r2.begin() );
}
using self=double_foreach_iterator;
auto cur() const {
return std::make_tuple( r1.begin(), r2.begin() );
}
friend bool operator==( self const& lhs, self const& rhs ) {
if (lhs.is_end() || rhs.is_end())
return lhs.is_end() == rhs.is_end();
return lhs.cur() == rhs.cur();
}
friend bool operator!=( self const& lhs, self const& rhs ) {
return !(lhs==rhs);
}
};
now we double iterate:
template<class A, class B>
auto zip_iterate(
A& a, B& b
) {
auto r1 = range(a);
auto r2 = range(b);
auto r1end = range(r1.end(), r1.end());
auto r2end = range(r2.end(), r2.end());
using it = double_foreach_iterator<decltype(r1), decltype(r2)>;
return range( it{r1, r2}, it{r1end, r2end} );
}
which gives us:
for (auto tup : zip_iterate(a, b)) {
int& i = std::get<0>(tup);
int& j = std::get<1>(tup);
// ...
}
or in C++17:
for (auto&& [i, j] : zip_iterate(a, b)) {
// ...
}
My zip iterate does not assume the two containers are of the same length, and will iterate to the length of the shorter one.
live example.

Answer 3

Just for fun.
The following isn't intended to be a serious answer to the question but just an exercise to try to understand the potentiality of C++11 (so, please, be patient).
The following is an example of a class (a draft of a class) that receive a couple of container (with size() method), with the same size (exception otherwise), and of a custom iterator that return a std::pair of std::reference_wrapper to n-position elements.
With a simple use example that show that it's possible to change the value in the starting containers.
Doesn't work with old C-style arrays but works with std::array. We're talking about C++11 so I suppose we could impose the use of std::array.
#include <array>
#include <vector>
#include <iostream>
#include <functional>
template <typename T1, typename T2>
class pairWrapper
{
public:
using V1 = typename std::remove_reference<decltype((T1().at(0)))>::type;
using V2 = typename std::remove_reference<decltype((T2().at(0)))>::type;
using RW1 = std::reference_wrapper<V1>;
using RW2 = std::reference_wrapper<V2>;
class it
{
public:
it (pairWrapper & pw0, std::size_t p0): pos{p0}, pw{pw0}
{ }
it & operator++ ()
{ ++pos; return *this; }
bool operator!= (const it & it0)
{ return pos != it0.pos; }
std::pair<RW1, RW2> & operator* ()
{
static std::pair<RW1, RW2>
p{std::ref(pw.t1[0]), std::ref(pw.t2[0])};
p.first = std::ref(pw.t1[pos]);
p.second = std::ref(pw.t2[pos]);
return p;
}
private:
std::size_t pos;
pairWrapper & pw;
};
it begin()
{ return it(*this, 0U); }
it end()
{ return it(*this, len); }
pairWrapper (T1 & t10, T2 & t20) : len{t10.size()}, t1{t10}, t2{t20}
{ if ( t20.size() != len ) throw std::logic_error("no same len"); }
private:
const std::size_t len;
T1 & t1;
T2 & t2;
};
template <typename T1, typename T2>
pairWrapper<T1, T2> makePairWrapper (T1 & t1, T2 & t2)
{ return pairWrapper<T1, T2>(t1, t2); }
int main()
{
std::vector<int> v1 { 1, 2, 3, 4 };
std::array<long, 4> v2 { { 11L, 22L, 33L, 44L } };
for ( auto & p : makePairWrapper(v1, v2) )
{
std::cout << '{' << p.first << ", " << p.second << '}' << std::endl;
p.first += 3;
p.second += 55;
}
for ( const auto & i : v1 )
std::cout << '[' << i << ']' << std::endl;
for ( const auto & l : v2 )
std::cout << '[' << l << ']' << std::endl;
return 0;
}
p.s.: sorry for my bad English