Splitting std::vector based on some criteria - c++

I have a vector which contain some data. I want to split it into const number of vectors depending on some criteria. For example:
using Point=std::pair<int,int>;
std::array<std::vector<Point>,4> split_to_4(const std::vector<Point>& data,std::function<size_t(Point)> criteria);
int main(){
std::vector<Point> data;
//fill data
auto results=split_to_4(data,[](const Point& p){
if(cond1) return 0;
if(cond2) return 1;
if(cond3) return 2;
return 3;
});
}
What is the best way to implement split_to_4? My current attempt is:
std::array<std::vector<Point>,4> split_to_4(const std::vector<Point>& data,std::function<size_t(Point)> criteria){
std::array<std::vector<Point>,4> result;
for (const auto& p : data){
areas_regions[criteria(p)].emplace_back(p);
}
return result;
}
Any better.. More std way to do it?
By Better, I mean: more readable... depend on iterator... depend on some std functions...

You can do this in place with multiple calls to std::partition:
// Returns iterators to the three partition points in the range
template<class ForwardIt, class Which>
auto split4(ForwardIt first, ForwardIt last, Which which) {
std::array<ForwardIt, 3> ret;
ret[0] = std::partition(first, last,
[&](const auto &v){return which(v) == 0;});
ret[1] = std::partition(ret[0], last,
[&](const auto &v){return which(v) == 1;});
ret[2] = std::partition(ret[1], last,
[&](const auto &v){return which(v) == 2;});
return ret;
}
Of course you can also pass and use the conditions directly instead of proxiing through some which function if you so desire.
One could also trivially rewrite this with a loop to generalize it to splitN if necessary. (Watch out though, the complexity of this approach is O(N * n) for a range with n elements. This will probably be unreasonably slow for big N. On the other hand, we get swaps instead of copies, which may help if copying is expensive (compared to calling which). If performance is critical, measure.)
If you need the relative order of elements in each group preserved, std::stable_partition is your friend.
Just noticed the C++11 tag: The above code is C++14. For C++11 compatibility, simply change the autos I used to the explicit types, i.e. use std::array<ForwardIt, 3> as the return type and const std::iterator_traits<ForwardIt>::value_type& for the lambdas.
I'll leave that as is for brevity, this last paragraph completes the answer for the pre-C++14 folks.

update:
probably the most STL-like way:
Features:
Iterator-based so choice of source and destination containers is left to the caller
Source iterators may be move-iterators if move-partitioning is required, or leave as normal iterators to make a copy
Linear time complexity
Stable ordering of results (ref std::stable_partition)
-
#include <array>
#include <vector>
#include <utility>
#include <cassert>
using Point=std::pair<int,int>;
// example split function - could be a function object
extern std::size_t which_bucket(const Point&);
template<class Iter, class OutIter, class Which>
auto split_n(Iter first, Iter last,
OutIter outfirst, std::size_t N,
Which&& which)
{
while (first != last) {
auto index = which(*first);
assert (index < N);
std::next(outfirst, index) -> push_back(*first);
++ first;
}
}
template<class Iter, class OutIter, class Which>
auto split_to(Iter first, Iter last,
OutIter outfirst, OutIter outlast,
Which&& which)
{
return split_n(first, last, outfirst,
std::distance(outfirst, outlast),
std::forward<Which>(which));
}
int main(){
std::vector<Point> source;
std::array<std::vector<Point>, 4> dest { };
split_n(source.begin(), source.end(),
dest.begin(), dest.size(),
which_bucket);
// or
split_to(source.begin(), source.end(),
dest.begin(), dest.end(),
which_bucket);
// or with move request:
split_to(std::make_move_iterator(source.begin()),
std::make_move_iterator(source.end()),
dest.begin(), dest.end(),
which_bucket);
}
another way
#include <array>
#include <vector>
#include <utility>
using Point=std::pair<int,int>;
// example split function - could be a function object
extern std::size_t which_bucket(const Point&);
template<class Iter, class Which>
auto split4(Iter first, Iter last, Which&& which)
{
std::array<std::vector<Point>, 4> result {};
while (first != last) {
result[which(*first)].push_back(*first);
++first;
}
return result;
}
int main(){
std::vector<Point> data;
auto results = split4(data.begin(), data.end(), which_bucket);
}
Here's another way which honours any custom allocator in the vector:
#include <array>
#include <vector>
#include <utility>
using Point=std::pair<int,int>;
// example split function - could be a function object
extern std::size_t which_bucket(const Point&);
template<class T, class A, class Which>
auto split4(const std::vector<T,A>& v,
Which&& which)
{
using vec_type = std::vector<T,A>;
std::array<std::vector<T,A>, 4> result {
vec_type(v.get_allocator()),
vec_type(v.get_allocator()),
vec_type(v.get_allocator()),
vec_type(v.get_allocator())
};
for (auto& p : v) {
result[which(p)].push_back(p);
}
return result;
}
int main(){
std::vector<Point> data;
auto results = split4(data, which_bucket);
}

Related

How to use the range version of `transform()` with two ranges?

The header <algorithm> contains a version of std::transform() taking a two input sequences, an output sequence, and a binary function as parameters, e.g.:
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
int main()
{
std::vector<int> v0{1, 2, 3};
std::vector<int> v1{4, 5, 6};
std::vector<int> result;
std::transform(v0.begin(), v0.end(), v1.begin(), std::back_inserter(result),
[](auto a, auto b){ return a + b; });
std::copy(result.begin(), result.end(),
std::ostream_iterator<int>(std::cout, " "));
std::cout << '\n';
}
C++20 introduced range algoirthms which does include std::ranges::views::transform(R, F) and its implementation std::ranges::views::transform_view. I can see how to use this transform() with one range, e.g.:
#include <algorithm>
#include <iostream>
#include <iterator>
#include <ranges>
#include <vector>
int main()
{
std::vector<int> v0{1, 2, 3};
for (auto x: std::ranges::views::transform(v0, [](auto a){ return a + 3; })) {
std::cout << x << ' ';
}
std::cout << '\n';
}
However, there is no version supporting more than one range. So, the question becomes: How to use the range version of transform() with two (or more) ranges? On objective of this approach is to benefit from the lazy evaluation of views and avoid the creation of an intermediate sequence (result in the non-ranges version above). A potential use could look like this (putting the function argument in front to make it easier for a potential solution allowing even more than two ranges):
for (auto v: envisioned::transform([](auto a, auto b){ return a + b; }, v0, v1) {
std::cout << v << ' ';
}
std::cout << '\n';
The way you would like to use transform, where you take an arbitrary number of input ranges is not possible directly with what's available in <algorithm> as of C++20. You can of course write such an algorithm yourself without too much effort.
The example with 2 input ranges can be implemented in C++20 like this:
std::ranges::transform(v0, v1,
std::ostream_iterator<int>(std::cout, " "),
std::plus{});
Here's a demo, and this is specifically the last overload of transform listed here.
There is unfortunately no way to write the equivalent version like this:
for (auto v : std::views::transform(v0, v1, std::plus{})) // no, unfortunately
std::cout << v << " ";
but the above implementation does the same thing. It certainly satisfies your requirements of not having to store the results separately; they can be printed as they're generated.
What you're looking for is the algorithm that range-v3 calls zip_with and what we are proposing in P2214 to add to C++23 under the name zip_transform. There is no such algorithm in C++20.
Until then, the range-v3 version is exactly your use-case:
for (auto v : zip_with([](auto a, auto b){ return a + b; }, v0, v1)) {
std::cout << v << ' ';
}
It can handle an arbitrary number of ranges.
Note that there is no piping version here, just as there is not with regular zip.
The answer blow is how I envisioned to answer the question and I think it still contains some interesting bits on how to actually implement a view. It turns out that P2214 mentioned in #Barry's answer has an interesting view (zip_transform) which does an intermediate step of the solution posted below but actually fully covers the functionality needed to do a multi-range transform!
It seems there are essentially two ingredients to using std::ranges::views::transform() with multiple ranges:
Some way to zip the objects at the corresponding positions of the ranges into a std::tuple, probably retaining the value category of the respective values.
Instead of using an n-ary function to take the elements of the range as parameters the function would rather use a std::tuple and possibly use that to call a corresponding n-ary function.
Using this idea would allow creating a version of transform() dealing with an arbitrary number of ranges, although it is easier to take the function object first rather than extract the last element of a parameter pack:
auto transform(auto&& fun, auto&&... ranges)
{
return std::ranges::views::transform(zip(std::forward<decltype(ranges)>(ranges)...),
[fun = std::forward<decltype(fun)>(fun)]
(auto&& t){ return std::apply(fun, std::forward<decltype(t)>(t)); });
}
The zip view used by this implementation can be implemented in terms of std::tuple:
template <typename... Range>
struct zip_view
: std::ranges::view_base
{
template <typename V>
struct rvalue_view
{
std::shared_ptr<std::decay_t<V>> view;
rvalue_view() = default;
rvalue_view(V v): view(new std::decay_t<V>(std::move(v))) {}
auto begin() const { return this->view->begin(); }
auto end() const { return this->view->end(); }
};
template <typename T>
using element_t = std::conditional_t<
std::is_rvalue_reference_v<T>,
rvalue_view<T>,
T
>;
using storage_t = std::tuple<element_t<Range>...>;
using value_type = std::tuple<std::ranges::range_reference_t<std::remove_reference_t<Range>>...>;
using reference = value_type;
using difference_type = std::common_type_t<std::ranges::range_difference_t<Range>...>;
storage_t ranges;
template <typename> struct base;
template <std::size_t... I>
struct base<std::integer_sequence<std::size_t, I...>>
{
using value_type = zip_view::value_type;
using reference = zip_view::value_type;
using pointer = value_type*;
using difference_type = std::common_type_t<std::ranges::range_difference_t<Range>...>;
using iterator_category = std::common_type_t<std::random_access_iterator_tag,
typename std::iterator_traits<std::ranges::iterator_t<Range>>::iterator_category...>;
using iterators_t = std::tuple<std::ranges::iterator_t<Range>...>;
iterators_t iters;
reference operator*() const { return {*std::get<I>(iters)...}; }
reference operator[](difference_type n) const { return {std::get<I>(iters)[n]...}; }
void increment() { (++std::get<I>(iters), ...); }
void decrement() { (--std::get<I>(iters), ...); }
bool equals(base const& other) const {
return ((std::get<I>(iters) == std::get<I>(other.iters)) || ...);
}
void advance(difference_type n){ ((std::get<I>(iters) += n), ...); }
base(): iters() {}
base(const storage_t& s, auto f): iters(f(std::get<I>(s))...) {}
};
struct iterator
: base<std::make_index_sequence<sizeof...(Range)>>
{
using base<std::make_index_sequence<sizeof...(Range)>>::base;
iterator& operator++() { this->increment(); return *this; }
iterator operator++(int) { auto rc(*this); operator++(); return rc; }
iterator& operator--() { this->decrement(); return *this; }
iterator operator--(int) { auto rc(*this); operator--(); return rc; }
iterator& operator+= (difference_type n) { this->advance(n); return *this; }
iterator& operator-= (difference_type n) { this->advance(-n); return *this; }
bool operator== (iterator const& other) const { return this->equals(other); }
auto operator<=> (iterator const& other) const {
return std::get<0>(this->iters) <=> std::get<0>(other.iters);
}
friend iterator operator+ (iterator it, difference_type n) { return it += n; }
friend iterator operator+ (difference_type n, iterator it) { return it += n; }
friend iterator operator- (iterator it, difference_type n) { return it -= n; }
friend difference_type operator- (iterator it0, iterator it1) {
return std::get<0>(it0.iters) - std::get<0>(it1.iters);
}
};
zip_view(): ranges() {}
template <typename... R>
zip_view(R&&... ranges): ranges(std::forward<R>(ranges)...) {}
iterator begin() const { return iterator(ranges, [](auto& r){ return std::ranges::begin(r); }); }
iterator end() const { return iterator(ranges, [](auto& r){ return std::ranges::end(r); }); }
};
auto zip(auto&&... ranges)
-> zip_view<decltype(ranges)...>
{
return {std::forward<decltype(ranges)>(ranges)...};
}
This implementation makes some decisions about the value_type and the reference type and how to keep track of the different ranges. Other choices may be more reasonable (P2214 makes slightly different, probably better, choices). The only tricky bit in this implementation is operating on the std::tuples which requires a parameter pack containing indices or a suitable set of algorithms on std::tuples.
With all of that in place a multi-range transform can be used nicely, e.g.:
#include <algorithm>
#include <iostream>
#include <iterator>
#include <memory>
#include <ranges>
#include <utility>
#include <tuple>
#include <type_traits>
#include <vector>
// zip_view, zip, and transform as above
int main()
{
std::vector<int> v0{1, 2, 3};
std::vector<int> v1{4, 5, 6};
std::vector<int> v2{7, 8, 9};
for (auto x: transform([](auto a, auto b, auto c){ return a + b + c; }, v0, v1, v2)) {
std::cout << x << ' ';
}
std::cout << '\n';
}

What STL algorithm can determine if exactly one item in a container satisfies a predicate?

I need an STL algorithm that takes a predicate and a collection and returns true if one and only one member of the collection satisfies the predicate, otherwise returns false.
How would I do this using STL algorithms?
E.g., to replace the following with STL algorithm code to express the same return value.
int count = 0;
for( auto itr = c.begin(); itr != c.end(); ++itr ) {
if ( predicate( *itr ) ) {
if ( ++count > 1 ) {
break;
}
}
}
return 1 == count;
Two things come to my mind:
std::count_if and then compare the result to 1.
To avoid traversing the whole container in case eg the first two elements already match the predicate I would use two calls looking for matching elements. Something along the line of
auto it = std::find_if(begin,end,predicate);
if (it == end) return false;
++it;
return std::none_of(it,end,predicate);
Or if you prefer it more compact:
auto it = std::find_if(begin,end,predicate);
return (it != end) && std::none_of(std::next(it),end,predicate);
Credits goes to Remy Lebeau for compacting, Deduplicator for debracketing and Blastfurnance for realizing that we can also use none_of the std algorithms.
You can use std::count_if† to count and return if it is one.
For example:
#include <iostream>
#include <algorithm> // std::count_if
#include <vector> // std::vector
#include <ios> // std::boolalpha
template<class Iterator, class UnaryPredicate>
constexpr bool is_count_one(Iterator begin, const Iterator end, UnaryPredicate pred)
{
return std::count_if(begin, end, pred) == 1;
}
int main()
{
std::vector<int> vec{ 2, 4, 3 };
// true: if only one Odd element present in the container
std::cout << std::boolalpha
<< is_count_one(vec.cbegin(), vec.cend(),
[](const int ele) constexpr noexcept -> bool { return ele & 1; });
return 0;
}
†Update: However, std::count_if counts entire element in the container, which is not good as the algorithm given in the question. The best approach using the standard algorithm collections has been mentioned in #formerlyknownas_463035818 's answer.
That being said, OP's approach is also good as the above mentioned best standard approach, where a short-circuiting happens when count reaches 2. If someone is interested in a non-standard algorithm template function for OP's approach, here is it.
#include <iostream>
#include <vector> // std::vector
#include <ios> // std::boolalpha
#include <iterator> // std::iterator_traits
template<class Iterator, class UnaryPredicate>
bool is_count_one(Iterator begin, const Iterator end, UnaryPredicate pred)
{
typename std::iterator_traits<Iterator>::difference_type count{ 0 };
for (; begin != end; ++begin) {
if (pred(*begin) && ++count > 1) return false;
}
return count == 1;
}
int main()
{
std::vector<int> vec{ 2, 3, 4, 2 };
// true: if only one Odd element present in the container
std::cout << std::boolalpha
<< is_count_one(vec.cbegin(), vec.cend(),
[](const int ele) constexpr noexcept -> bool { return ele & 1; });
return 0;
}
Now that can be generalized, by providing one more parameter, the number of N element(s) has/ have to be found in the container.
template<typename Iterator>
using diff_type = typename std::iterator_traits<Iterator>::difference_type;
template<class Iterator, class UnaryPredicate>
bool has_exactly_n(Iterator begin, const Iterator end, UnaryPredicate pred, diff_type<Iterator> N = 1)
{
diff_type<Iterator> count{ 0 };
for (; begin != end; ++begin) {
if (pred(*begin) && ++count > N) return false;
}
return count == N;
}
Starting from formerlyknownas_463035818's answer, this can be generalized to seeing if a container has exactly n items that satisfy a predicate. Why? Because this is C++ and we're not satisfied until we can read email at compile time.
template<typename Iterator, typename Predicate>
bool has_exactly_n(Iterator begin, Iterator end, size_t count, Predicate predicate)
{
if(count == 0)
{
return std::none_of(begin, end, predicate);
}
else
{
auto iter = std::find_if(begin, end, predicate);
return (iter != end) && has_exactly_n(std::next(iter), end, count - 1, predicate);
}
}
Using std::not_fn to negate a predicate
As the core of the algorithm of this question (as has been elegantly covered by combining std::find_if and std::none_of in the accepted answer), with short-circuiting upon failure, is to scan a container for a unary predicate and, when met, continue scanning the rest of the container for the negation of the predicate, I will mention also the negator std::not_fn introduced in C++17, replacing the less useful std::not1 and std::not2 constructs.
We may use std::not_fn to implement the same predicate logic as the accepted answer (std::find_if conditionally followed by std::none_of), but with somewhat different semantics, replacing the latter step (std::none_of) with std::all_of over the negation of the unary predicate used in the first step (std::find_if). E.g.:
// C++17
#include <algorithm> // std::find_if
#include <functional> // std::not_fn
#include <ios> // std::boolalpha
#include <iostream>
#include <iterator> // std::next
#include <vector>
template <class InputIt, class UnaryPredicate>
constexpr bool one_of(InputIt first, InputIt last, UnaryPredicate p) {
auto it = std::find_if(first, last, p);
return (it != last) && std::all_of(std::next(it), last, std::not_fn(p));
}
int main() {
const std::vector<int> v{1, 3, 5, 6, 7};
std::cout << std::boolalpha << "Exactly one even number : "
<< one_of(v.begin(), v.end(), [](const int n) {
return n % 2 == 0;
}); // Exactly one even number : true
}
A parameter pack approach for static size containers
As I’ve already limited this answer to C++14 (and beyond), I’ll include an alternative approach for static size containers (here applied for std::array, specifically), making use of std::index_sequence combined with parameter pack expansion:
#include <array>
#include <ios> // std::boolalpha
#include <iostream>
#include <utility> // std::(make_)index_sequence
namespace detail {
template <typename Array, typename UnaryPredicate, std::size_t... I>
bool one_of_impl(const Array& arr, const UnaryPredicate& p,
std::index_sequence<I...>) {
bool found = false;
auto keep_searching = [&](const int n){
const bool p_res = found != p(n);
found = found || p_res;
return !found || p_res;
};
return (keep_searching(arr[I]) && ...) && found;
}
} // namespace detail
template <typename T, typename UnaryPredicate, std::size_t N,
typename Indices = std::make_index_sequence<N>>
auto one_of(const std::array<T, N>& arr,
const UnaryPredicate& p) {
return detail::one_of_impl(arr, p, Indices{});
}
int main() {
const std::array<int, 5> a{1, 3, 5, 6, 7};
std::cout << std::boolalpha << "Exactly one even number : "
<< one_of(a, [](const int n) {
return n % 2 == 0;
}); // Exactly one even number : true
}
This will also short-circuit upon early failure (“found more than one”), but will contain a few more simple boolean comparisons than in the approach above.
However, note that this approach could have its draw-backs, particularly for optimized code for container inputs with many elements, as is pointed out by #PeterCordes in a comment below. Citing the comment (as comments are not guaranteed to persist over time):
Just because the size is static doesn't mean that fully unrolling the loop with templates is a good idea. In the resulting asm, this needs a branch every iteration anyway to stop on found, so that might as well be a loop-branch. CPUs are good at running loops (code caches, loopback buffers). Compilers will fully unroll static-sized loops based on heuristics, but probably won't roll this back up if a is huge. So your first one_of implementation has the best of both worlds already, assuming a normal modern compiler like gcc or clang, or maybe MSVC

How to combine back_inserter with a transformation, C++

How can I wrap an OutputIterator such as back_inserter_iterator with a transformation?
Consider
std::vector<double> xx;
std::vector<double> yy;
std::vector<double> diff;
auto ba = std::back_inserter(diff);
std::set_difference(xx.begin(), xx.end(), yy.begin(), yy.end(), ba);
I would like to apply a free function f(double) or g(std::vector<double>::iterator) before pushing back to the diff vector:
Specifically, how can I store the addresses of the diff elements (or iterators) instead of the elements themeselves.
std::vector<double&> diff;
auto baAdr = ??? std::back_inserter( ??? (diff));
std::set_difference(xx.begin(), xx.end(), yy.begin(), yy.end(), baAdr);
For performance reasons (the real data is big) I do not want to construct a temporary vector and std::transform from it. It would also not work for non-copyable, movable types.
I can use boost.
With boost::function_output_iterator:
#include <vector>
#include <algorithm>
#include <boost/function_output_iterator.hpp>
int main()
{
std::vector<double> xx;
std::vector<double> yy;
std::vector<const double*> diff; // const pointers, or else you
// need a const_cast in lambda
std::set_difference(xx.begin(), xx.end(), yy.begin(), yy.end(),
boost::make_function_output_iterator(
[&diff](const double& d) { diff.push_back(&d); }
)
);
}
There's probably something built in to boost, but here's my hacky attempt to write my own iterator:
template <typename T, typename FN>
struct transform_iterator {
transform_iterator(T &t, FN fn)
: _t{t}
, _fn{std::move(fn)} { }
transform_iterator<T, FN>& operator * () { return *this; }
transform_iterator<T, FN>& operator ++ () { return *this; }
template <typename V>
transform_iterator<T, FN>& operator = (V const &v) {
_t.push_back(_fn(v));
return *this;
}
T &_t;
FN _fn;
};
This will take a function and execute it whenever something tries to assign to the iterator (I think this is how things like back_inserter usually work). A trivial helper function can create the iterators:
template <typename T, typename FN>
auto make_transform_iterator(T &t, FN fn) {
return transform_iterator<T, FN>{t, std::move(fn)};
};
Lastly, iterator_traits needs to be specialized so transform_iterator will work with algorithms.
namespace std {
template <typename T, typename FN>
struct iterator_traits<transform_iterator<T, FN>> {
using value_type = typename T::value_type;
};
}
There are more types that need to be set in iterator_traits, but this was sufficient for my testing; your mileage will vary.
My main looks like this:
int main() {
std::vector<int> xx{1, 2, 3};
std::vector<int> yy{1, 3, 5};
std::vector<int> diff;
auto ba = make_transform_iterator(diff, [](auto v) { return v + 10; });
std::set_difference(std::begin(xx), std::end(xx),
std::begin(yy), std::end(yy),
ba);
for(auto const &v: diff) {
std::cout << v << '\n';
}
return 0;
}
You could expand this to work with generic output iterators instead of just types that support push_back.

Binary predicate for `std::count_if` is not working

I am currently trying to use a lambda function to std::count_if the sum of two consecutive elements in an array equal to a number. A sample code is given below.
#include <iostream>
#include <vector>
#include <algorithm>
int main()
{
const int Number = 3;
std::vector<int> vec = {1,1,2,4,5,6};
auto count = std::count_if( vec.begin(), vec.end(),
[&](int A, int B) -> bool
{ return A+B == Number; });
std::cout << count << '\n';
}
The output should be 1, since we have one possible case( 1 + 2 ).
However, I could not succeed. Can anybody tell me what do I miss?
Here is the error msg:
|234|error: no match for call to '(main()::<lambda(int, int)>) (int&)'|
Problem is that std::count_if uses unary predicate. What compiler tells you: "You gave me a lambda with 2 arguments, I expected lambda with one argument".
I believe what you are looking for is std::adjacent_find. It compares every two adjacent elements of a container (possibly using a binary predicate).
Another possible option is to use std::inner_product. First I'd write a little helper function:
#include <numeric>
#include <functional>
#include <iterator>
template <typename ForwardIterator, typename BinaryPredicate>
auto count_pairs_if(ForwardIterator first, ForwardIterator last,
BinaryPredicate pred)
{
const auto n = std::distance(first, last);
if (n < 2) return std::size_t {0};
return std::inner_product(first, std::next(first, n - 1), std::next(first),
std::size_t {0}, std::plus<> {}, pred);
}
template <typename Range, typename BinaryPredicate>
auto count_pairs_if(const Range& values, BinaryPredicate pred)
{
return count_pairs_if(std::cbegin(values), std::cend(values), pred);
}
Then you can use it like:
auto count = count_pairs_if(vec, [=] (auto lhs, auto rhs) { return lhs + rhs == Number; });
Here's a demo.
As #Yksisarvinen explained, the std::count_if is designed for the unary predicate. Therefore the compiler can not accept the lambda, I passed.
After a while, I have found another solution to this problem. If I provide a templated function, which takes
iterators(i.e. start and end) of the container (on which I need to do the adjacent element check), and
the binary predicate, which will be used for checking adjacent relationship
that could be a more natural solution, like any other standard algorithm. (See a live demo online)
template <typename Iterator, typename BinaryPred = std::equal_to<>>
constexpr std::size_t count_adjacent_if(
Iterator beginIter,
const Iterator endIter,
const BinaryPred pred = {})
{
if (beginIter == endIter) return 0; // nothing to do!
std::size_t count{};
for (Iterator nextIter{ beginIter }; ++nextIter != endIter; beginIter = nextIter)
if (pred(*beginIter, *nextIter))
++count;
return count;
}
and can be called like:
const auto count = ::count_adjacent_if(
vec.cbegin(), vec.cend(), [number](const int lhs, const int rhs) { return lhs + rhs == number; }
);
Or like #bipil mentioned in the comments, let the predicate remember the previous element. Which is less recommended, since it is a non-generic solution and needs the non-empty container. (See a live demo online)
int lhs = vec[0];
const auto count = std::count_if(vec.cbegin() + 1, vec.cend(),
[&](const int rhs) {
const bool condition = (lhs + rhs == number); // check for the condition
lhs = rhs; // change the lhs = rhs (i.e. current element = next element)
return condition; // return the condition
});

Initializing a vector of auto (unknown) type inside a template function in C++

I have a template function inside which I want to generate a vector which is of an unknown type. I tried to make it auto, but compiler says it is not allowed.
The template function gets either iterators or pointers as seen in the test program inside the followed main function. How can the problem be fixed?
template<class Iter>
auto my_func(Iter beg, Iter end)
{
if (beg == end)
throw domain_error("empty vector");
auto size = distance(beg, end);
vector<auto> temp(size); // <--HERE COMPILER SAYS CANNOT BE AUTO TYPE
copy(beg, end, temp->begin);
.
.
return ....
}
int main()
{
int bips[] = {3, 7, 0, 60, 17}; // Passing pointers of array
auto g = my_func(bips, bips + sizeof(bips) / sizeof(*bips));
vector<int> v = {10, 5, 4, 14}; // Passing iterators of a vector
auto h = my_func(v.begin(), v.end());
return 0;
}
You cannot use a std::vector of auto. You might use std::iterator_traits instead:
std::vector<typename std::iterator_traits<Iter>::value_type> temp(size);
If you have a C++17-compatible compiler, you may profit from class template argument deduction.
So unless you have a specific reason to fill your vector with std::copy, you could write your code like this:
template<class Iter>
auto my_func(Iter beg, Iter end)
{
if (beg == end)
throw domain_error("empty vector");
vector temp(beg, end);
// do the remaining stuff
return ....
}
If this feature is not available on your compiler, then I'd vote for
vector<typename iterator_traits<Iter>::value_type> temp(beg, end);
like in Jonathan's answer
You might be looking for something like
std::vector<typename std::remove_reference<decltype(*beg)>::type> temp(beg, end);
Demo
The reason auto doesn't work is because it's not allowed in that context. You may not provide auto in place of a template argument. The correct course of action when you want the compiler to deduce a template argument automatically is to not provide an argument at all. However, in this case, there is no way for the compiler to deduce what that type should be. You must provide the type explicitly.
There are many ways of finding out what the correct type for your vector is. You can use std::iterator_traits to obtain information about an iterator, including the type of value it refers to. You would use typename std::iterator_traits<Iter>::value_type.
#include <algorithm>
#include <iterator>
#include <stdexcept>
#include <vector>
template<class Iter>
auto my_func(Iter beg, Iter end)
{
if (beg == end)
throw std::domain_error("empty vector");
auto size = std::distance(beg, end);
using t_value = typename std::iterator_traits<Iter>::value_type;
std::vector<t_value> temp(size);
std::copy(beg, end, temp.begin());
return temp;
}
int main()
{
int bips[] = { 3,7,0,60,17 };//Passing pointers of array
auto g = my_func(bips, bips + sizeof(bips) / sizeof(*bips));
std::vector<int> v = { 10,5,4,14 };//Passing iterators of a vector
auto h = my_func(v.begin(), v.end());
return 0;
}
I would like to point out that there is no reason to check for 0 size ranges. It would correctly return an empty vector.
You can also simplify the body of my_func quite a bit by taking advantage of the fact that std::vector has a constructor that accepts a pair of iterators and copies that range.
template<class Iter>
auto my_func(Iter beg, Iter end)
{
using t_value =typename std::iterator_traits<Iter>::value_type;
return std::vector<t_value>(beg, end);
}
You can extract a pointer/iterator's type information using iterator_traits. value_type is the specific trait that you are interested in, so you can do:
const vector<typename iterator_traits<Iter>::value_type> temp(beg, end);
Live Example
It is not true that the type is not known. The type of the vector you want to create is of the same kind of Iter.
Just get iter Underlying type either using decltype or using iterator type trait as follows:
decltype -> std::vector<typename remove_reference<decltype(*beg)>::type> temp(beg, end);
iterator type trait
as follows
using Type = std::iterator_traits<Iter>::value_type;
std::vector<Type>...
I would solve this slightly differently than your question seems to be asking for.
First, I find ranges to be a better fundamental type than taking two iterators. The two iterators are coupled, they should be one argument. A range is a simple struct of two iterators, with some utility methods:
template<class It>
struct range_t:
std::iterator_traits<It>
{
It b{}, e{};
It begin() const { return b; }
It end() const { return e; }
bool empty() const { return begin()==end(); }
auto size() const { return std::distance(begin(), end()); }
// etc
range_t()=default;
range_t(range_t const&)=default;
range_t(range_t &&)=default;
range_t& operator=(range_t const&)=default;
range_t& operator=(range_t &&)=default;
};
template<class It>
range_t<It> make_range( It s, It f ) { return {std::move(s), std::move(f)}; }
range_ts correctly couple the begin end iterator together.
Now
template<class Range>
auto my_func(Range&& range) {
// todo
}
template<class Iter>
auto my_func(Iter beg, Iter end)
{
return my_func(make_range(std::move(beg), std::move(end)));
}
is the first step. Or just eliminate the two-iterator version entirely, and expect the caller to package up their iterators for you.
template<class Range>
auto my_func(Range&& range) {
if (range.empty())
throw domain_error("empty vector");
// todo
}
Ok, now you want to do this:
auto size = range.size();
vector<auto> temp(size);//<--HERE COMPILER SAYS CANNOT BE AUTO TYPE
copy(range.begin(), range.end(), temp->begin);
but that is a common operation. So we write it for range:
template<class Range>
auto as_vector( Range const& r ) {
using value_type = typename Range::value_type;
std::vector<value_type> v( range.begin(), range.end() );
return v;
}
giving us:
template<class Range>
auto my_func(Range&& range) {
if (range.empty())
throw domain_error("empty vector");
auto v = as_vector(range);
// ...
return ...;
}
We have decomposed your problem into simple primitives that have meaning, and moved implementation complexity into those primitives. The "business logic" of your my_func no longer cares what steps you take to make a range into a vector.
This makes your my_func more readable, so long as you have some trust that as_vector(range) actually returns that range as a vector.