Is there any way to iterate over only a part of an array in the C++11 range-based loop? Say I have an int someArray[10000000] but I only want to iterate over the first n elements. Thus, I can't simply use
for(auto elem: someArray) {//doStuff}
Any way to limit the scope of the loop while still using the range-based goodies?
Simply adapt the range to be a different type where begin() and end() do the right thing.
struct Slice {
int* arr;
size_t n;
int* begin() { return arr; }
int* end() { return arr + n; }
};
for(auto elem : Slice{someArray, 100}) {/*doStuff*/}
size_t for the size?
for is almost certainly global or automatic, so is unlikely to be too big for an int index. A system that can create an array of 10e7 probably has at least 32-bit int. If not, use a bigger type.There is no C++11 way to do this without writing your own wrapper for it. An alternative option could be to use Range-V3 which has view::slice (see it live):
#include <iostream>
#include <range/v3/view.hpp>
int main() {
int arr[10] = {1,2,3,4,5,6,7,8,9,10} ;
using namespace ranges;
auto rng = arr | view::slice(3, 6);
for( auto &item : rng )
{
std::cout << item << ", " ;
}
std::cout << std::endl ;
}
If on the other hand C++14 was available then GSL array_view would be a viable option:
gsl::array_view<int> av(arr+3,3) ;
for( auto &item : av )
{
std::cout << item << ", " ;
}
std::cout << std::endl ;
gsl-lite offers a minimal GSL implementation that works in C++11.
With C++20 std::span can be used.
for (auto elem : std::span(someArray).first(n))
std::cout << elem;
No. Not in the current standard.
There are some things in the making for C++17 that would make this possible, e.g. std::range and std::array_view.
But until then, just use the good old iterator or index for, it will be the most readable:
for(auto i = 0u; i < n; ++i)
{
// do the stuffs on someArray[i]
}
The range-based for loop iterates over the entire range. What you could do is store an additional index that you check in each look.
size_t index = 0;
for(auto elem: someArray)
{
//doStuff
if(index++ > n) break;
}
However, I don't see what you'd gain with this over a regular for loop.
Ofc, you could also use std::for_each on the desired sub-range, but I'd still prefer a traditional for loop.
forloop?