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    Standard Library Range UtilitiesStandard Library Range UtilitiesRange Utilities

    New to C++'s standard library algorithms?  ⇒  Short Introduction

    distance distance

    returns the size of an iterator range

    distance(@range_begin, @element_in_range) → index_of_element_in_range

    #include <vector>
#include <iterator>  // std::distance
std::vector<int> v {0,1,2,3,4,5,6,7,8};
// size of subrange (as shown in image)
auto n = distance(begin(v)+2, begin(v)+7);    // int n = 5
// size of entire container
auto m = distance(begin(v), end(v));          // int m = 9

    std::vector<int> w {4,5,1,9,8};
// get index of smallest element in w:
auto argmin = distance(     begin(w), min_element(begin(w),end(w)) );  
// int argmin = 2

    Avoid using distance with iterators into non-random access containers like std::list, because the runtime will be proportional to the size of the input range!

    for_each / for_each_n for_each/for_each_n for_each

    • convenient if you already have a function(object) to be applied to each element
    • works for all standard sequence and associative containers
    • container agnostic ⇒ easy to change container type
    • no signed/unsigned index type hassle
    • self-documenting name
    • out-of-bounds access bugs possible with iterator ranges
     C++20
    • no out-of-bounds access possible
    #include <ranges>  // std::ranges::for_each
namespace ranges = std::ranges;  // alias
ranges::for_each(v,   [](auto x){ cout << x; }); 
ranges::for_each(v,   [](auto const& x){ cout << x; });
ranges::for_each(v,   [](auto& x){ cin >> x; });
    • out-of-bounds access bugs possible
    #include <algorithm>  // std::for_each
// read-only, type cheap to copy or copy needed:
for_each(begin(v), end(v),         [](Type x){ cout << x; });
for_each(begin(v), end(v),         [](auto x){ cout << x; });
for_each(begin(v)+2, begin(v)+5,   [](auto x){ cout << x; });
for_each(begin(v)+5, end(v),       [](auto x){ cout << x; });
// read-only, type expensive to copy:
for_each(begin(v), end(v),   [](Type const& x){ cout << x; });
for_each(begin(v), end(v),   [](auto const& x){ cout << x; });
// modify values:
for_each(begin(v), end(v),   [](Type& x){ cin >> x; });
for_each(begin(v), end(v),   [](auto& x){ cin >> x; });
    • out-of-bounds access bugs possible
    #include <algorithm>  // std::for_each_n
auto const n = container.size() / 2;
for_each_n(begin(v), n,   [](auto x){ cout << x; });
for_each_n(begin(v), n,   [](auto const& x){ cout << x; });
for_each_n(begin(v), n,   [](auto& x){ cin >> x; });

    Iterator Operations Iterator

    #include <iterator>

    Functions std::prev and std::next provide a universal way of incrementing/decrementing iterators even if the iterator type does not support random access (e.g., it += 5).

    However, be aware that advancing non-random access iterators (e.g., those from std::list) by N steps might be costly, i.e., involve on the order of N memory operations.

    C++11
    std::vector<int> v {1,2,3,4,5,6};
auto i = next(v.begin());
auto j = next(i, 3);
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    C++11
    std::vector<int> v {1,2,3,4,5,6};
auto i = prev(v.end());
     i = prev(i);
auto j = prev(i, 3);
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    Related …

    Last updated: 2021-01-10