Related
I'm a beginner in c++ i was searching for templates that could check if a vector / map independent of their data type, contains a given value, I have found these:
template <typename Container, typename Value>
bool vector_contains(const Container& c, const Value& v)
{
return std::find(std::begin(c), std::end(c), v) != std::begin(c);
}
template< typename container, typename key >
auto map_contains(container const& c, key const& k)
-> decltype(c.find(k) != c.end())
{
return c.find(k) != c.end();
}
My doubt is, does using templates to do this kind of verification impact performance somehow?
I have found these
Ok, but do analyze them. They are sub optimal and/or plain wrong.
template <typename Container, typename Value>
bool vector_contains(const Container& c, const Value& v)
{
return std::find(std::begin(c), std::end(c), v) != std::begin(c);
}
This will return true as long as v is not the first value found. It'll also return true if v is not found at all.
A vector, without any other information, is unsorted, which means that contains will have to search from the first element to the last if the value is not found. Such searches are considered expensive.
If you on the other hand std::sort the vector and use the same Comparator when using std::binary_search, it'll have a quicker lookup. Sorting takes time too, though.
template< typename container, typename key >
auto map_contains(container const& c, key const& k) -> decltype(c.find(k) != c.end())
{
return c.find(k) != c.end();
}
This looks like it may work for types matching the function template. It should use map::contains instead - if it's meant to be used with maps.
What is the C++ way of checking if an element is contained in an array/list, similar to what the in operator does in Python?
if x in arr:
print "found"
else
print "not found"
How does the time complexity of the C++ equivalent compare to Python's in operator?
The time complexity of Python's in operator varies depending on the data structure it is actually called with. When you use it with a list, complexity is linear (as one would expect from an unsorted array without an index). When you use it to look up set membership or presence of a dictionary key complexity is constant on average (as one would expect from a hash table based implementation):
https://wiki.python.org/moin/TimeComplexity
In C++ you can use std::find to determine whether or not an item is contained in a std::vector. Complexity is said to be linear (as one would expect from an unsorted array without an index). If you make sure the vector is sorted, you can also use std::binary_search to achieve the same in logarithmic time.
http://en.cppreference.com/w/cpp/algorithm/find
Check if element is in the list (contains)
Check if element found in array c++
http://en.cppreference.com/w/cpp/algorithm/binary_search
The associative containers provided by the standard library (std::set, std::unordered_set, std::map, ...) provide the member functions find() and count() and contains() (C++20) for this. These will perform better than linear search, i.e., logarithmic or constant time depending on whether you have picked the ordered or the unordered alternative. Which one of these functions to prefer largely depends on what you want to achieve with that info afterwards, but also a bit on personal preference. (Lookup the documentation for details and examples.)
How to check that an element is in a std::set?
How to check if std::map contains a key without doing insert?
https://en.wikipedia.org/wiki/Associative_containers
http://en.cppreference.com/w/cpp/container
If you want to, you can use some template magic to write a wrapper function that picks the correct method for the container at hand, e.g., as presented in this answer.
You can approach this in two ways:
You can use std::find from <algorithm>:
auto it = std::find(container.begin(), container.end(), value);
if (it != container.end())
return it;
or you can iterate through every element in your containers with for ranged loops:
for(const auto& it : container)
{
if(it == value)
return it;
}
Python does different things for in depending on what kind of container it is. In C++, you'd want the same mechanism. Rule of thumb for the standard containers is that if they provide a find(), it's going to be a better algorithm than std::find() (e.g. find() for std::unordered_map is O(1), but std::find() is always O(N)).
So we can write something to do that check ourselves. The most concise would be to take advantage of C++17's if constexpr and use something like Yakk's can_apply:
template <class C, class K>
using find_t = decltype(std::declval<C const&>().find(std::declval<K const&>()));
template <class Container, class Key>
bool in(Container const& c, Key const& key) {
if constexpr (can_apply<find_t, Container, Key>{}) {
// the specialized case
return c.find(key) != c.end();
} else {
// the general case
using std::begin; using std::end;
return std::find(begin(c), end(c), key) != end(c);
}
}
In C++11, we can take advantage of expression SFINAE:
namespace details {
// the specialized case
template <class C, class K>
auto in_impl(C const& c, K const& key, int )
-> decltype(c.find(key), true) {
return c.find(key) != c.end();
}
// the general case
template <class C, class K>
bool in_impl(C const& c, K const& key, ...) {
using std::begin; using std::end;
return std::find(begin(c), end(c), key) != end(c);
}
}
template <class Container, class Key>
bool in(Container const& c, Key const& key) {
return details::in_impl(c, key, 0);
}
Note that in both cases we have the using std::begin; using std::end; two-step in order to handle all the standard containers, raw arrays, and any use-provided/adapted containers.
This gives you an infix *in* operator:
namespace notstd {
namespace ca_helper {
template<template<class...>class, class, class...>
struct can_apply:std::false_type{};
template<class...>struct voider{using type=void;};
template<class...Ts>using void_t=typename voider<Ts...>::type;
template<template<class...>class Z, class...Ts>
struct can_apply<Z,void_t<Z<Ts...>>, Ts...>:std::true_type{};
}
template<template<class...>class Z, class...Ts>
using can_apply = ca_helper::can_apply<Z,void,Ts...>;
namespace find_helper {
template<class C, class T>
using dot_find_r = decltype(std::declval<C>().find(std::declval<T>()));
template<class C, class T>
using can_dot_find = can_apply< dot_find_r, C, T >;
template<class C, class T>
constexpr std::enable_if_t<can_dot_find<C&, T>{},bool>
find( C&& c, T&& t ) {
using std::end;
return c.find(std::forward<T>(t)) != end(c);
}
template<class C, class T>
constexpr std::enable_if_t<!can_dot_find<C&, T>{},bool>
find( C&& c, T&& t ) {
using std::begin; using std::end;
return std::find(begin(c), end(c), std::forward<T>(t)) != end(c);
}
template<class C, class T>
constexpr bool finder( C&& c, T&& t ) {
return find( std::forward<C>(c), std::forward<T>(t) );
}
}
template<class C, class T>
constexpr bool find( C&& c, T&& t ) {
return find_helper::finder( std::forward<C>(c), std::forward<T>(t) );
}
struct finder_t {
template<class C, class T>
constexpr bool operator()(C&& c, T&& t)const {
return find( std::forward<C>(c), std::forward<T>(t) );
}
constexpr finder_t() {}
};
constexpr finder_t finder{};
namespace named_operator {
template<class D>struct make_operator{make_operator(){}};
template<class T, char, class O> struct half_apply { T&& lhs; };
template<class Lhs, class Op>
half_apply<Lhs, '*', Op> operator*( Lhs&& lhs, make_operator<Op> ) {
return {std::forward<Lhs>(lhs)};
}
template<class Lhs, class Op, class Rhs>
auto operator*( half_apply<Lhs, '*', Op>&& lhs, Rhs&& rhs )
-> decltype( named_invoke( std::forward<Lhs>(lhs.lhs), Op{}, std::forward<Rhs>(rhs) ) )
{
return named_invoke( std::forward<Lhs>(lhs.lhs), Op{}, std::forward<Rhs>(rhs) );
}
}
namespace in_helper {
struct in_t:notstd::named_operator::make_operator<in_t> {};
template<class T, class C>
bool named_invoke( T&& t, in_t, C&& c ) {
return ::notstd::find(std::forward<C>(c), std::forward<T>(t));
}
}
in_helper::in_t in;
}
On a flat container, like a vector array or string, it is O(n).
On an associative sorted container, like a std::map, std::set, it is O(lg(n)).
On an unordered associated container, like std::unordered_set, it is O(1).
Test code:
std::vector<int> v{1,2,3};
if (1 *in* v)
std::cout << "yes\n";
if (7 *in* v)
std::cout << "no\n";
std::map<std::string, std::string, std::less<>> m{
{"hello", "world"}
};
if ("hello" *in* m)
std::cout << "hello world\n";
Live example.
C++14, but mainly for enable_if_t.
So what is going on here?
Well, can_apply is a bit of code that lets me write can_dot_find, which detects (at compile time) if container.find(x) is a valid expression.
This lets me dispatch the searching code to use member-find if it exists. If it doesn't exist, a linear search using std::find is used instead.
Which is a bit of a lie. If you define a free function find(c, t) in the namespace of your container, it will use that rather than either of the above. But that is me being fancy (and it lets you extend 3rd party containers with *in* support).
That ADL (argument dependent lookup) extensibity (the 3rd party extension ability) is why we have three different functions named find, two in a helper namespace and one in notstd. You are intended to call notstd::find.
Next, we want a python-like in, and what is more python like than an infix operator? To do this in C++ you need to wrap your operator name in other operators. I chose *, so we get an infix *in* named operator.
TL;DR
You do using notstd::in; to import the named operator in.
After that, t *in* c first checks if find(t,c) is valid. If not, it checks if c.find(t) is valid. If that fails, it does a linear search of c using std::begin std::end and std::find.
This gives you very good performance on a wide variety of std containers.
The only thing it doesn't support is
if (7 *in* {1,2,3})
as operators (other than =) cannot deduce initializer lists I believe. You could get
if (7 *in* il(1,2,3))
to work.
I guess one might make use of this thread and create a custom version of in function.
The main idea is to use SFINAE (Substitution Failure Is Not An Error) to differentiate associative containers (which have key_type member) from sequence containers (which have no key_type member).
Here is a possible implementation:
namespace detail
{
template<typename, typename = void>
struct is_associative : std::false_type {};
template<typename T>
struct is_associative<T,
std::enable_if_t<sizeof(typename T::key_type) != 0>> : std::true_type {};
template<typename C, typename T>
auto in(const C& container, const T& value) ->
std::enable_if_t<is_associative<C>::value, bool>
{
using std::cend;
return container.find(value) != cend(container);
}
template<typename C, typename T>
auto in(const C& container, const T& value) ->
std::enable_if_t<!is_associative<C>::value, bool>
{
using std::cbegin;
using std::cend;
return std::find(cbegin(container), cend(container), value) != cend(container);
}
}
template<typename C, typename T>
auto in(const C& container, const T& value)
{
return detail::in(container, value);
}
Small usage example on WANDBOX.
You can use std::find from <algorithm>, but this works only for datatypes like: std::map and std::vector (etc).
Also note that this will return, iterator to the first element that is found equal to the value you pass, unlike the in operator in Python that returns a bool.
I think one of the nice features of the "in" operator in python is that it can be used with different data types (strings v/s strings, numbers v/s lists, etc).
I am developing a library for using python constructions in C++. It includes "in" and "not_in" operators.
It is based on the same technique used to implement the in operator posted in a previous answer, in which make_operator<in_t> is implemented. However, it is extended for handling more cases:
Searching a string inside a string
Searching an element inside vector and maps
It works by defining several overloads for a function: bool in__(T1 &v1, T2 &v2), in which T1 and T2 consider different possible types of objects. Also, overloads for a function: bool not_in__(T1 &v1, T2 &v2) are defined. Then, the operators "in" and "not_in" call those functions for working.
The implementation is in this repository:
https://github.com/ploncomi/python_like_cpp
I want to write a template function which takes a container of key-value pairs (e.g map<K,V> or vector<pair<K,V>>) and returns a container of container of keys. For example:
template<typename C, typename K, typename V>
vector<vector<K>> partition_keys(const C& input)
{
vector<vector<K>> result;
...
// do something
for (const auto &pair : input)
{
cout << pair.first << "," << pair.second << std::endl;
}
...
return result;
}
Here's how I would like to call it:
// map
map<string, string> product_categories;
partition_keys(product_categories); // doesn't work
partition_keys<map<string, string>, string, string>(product_categories); // have to do this
// vector
vector<pair<string, string>> movie_genres;
partition_keys(movie_genres); // doesn't work
partition_keys<vector<pair<string, string>>, string, string>(movie_genres); // have to do this
But, the compiler cannot deduce the template parameters K and V without explicitly specifying them. I want the function to work with any container having pair of any type; so I want to avoid writing separate template functions for map<K,V>, list<pair<K,V>>, vector<pair<K,V>>, etc.
So, I had to modify the template function signature as follows to make it work the way I want:
template<typename C,
typename K = remove_const_t<C::value_type::first_type>,
typename V = C::value_type::second_type>
vector<vector<K>> partition_keys(const C& input);
Is there a better way to do this? Is it a good practice to deduce types of K and V based on value_type of C? Also, there is a possibility of caller explicitly passing invalid arguments for K and V.
Note also how I had remove constness of key type by calling remove_const_t because for a map, C::value_type::first_type is a const type and the standard doesn't allow creating a collection of a const type.
You are doing the right way, more specifically:
template<typename C,
typename Pair = typename C::value_type,
typename Key = std::remove_const_t<typename Pair::first_type>,
typename Value = typename Pair::first_type
>
vector<vector<Key>> partition_keys(const C& input)
is correct (Demo). However, if you need to use similar type decomposition for different template functions like:
....repeat above templated type decomposition....
vector<vector<Key>> sorted_keys(const C& input);
....repeat above templated type decomposition....
vector<vector<Key>> filtered_keys(const C& input);
It may be too much of typing to do. In that case, you can make a simple trait class to help you with that.
template<typename T>
struct PTraits{
using pair_type = typename T::value_type;
using key_type = std::remove_const_t<typename pair_type::first_type>;
using value_type = typename pair_type::second_type;
};
template<typename T>
using KeyTypper = typename PTraits<T>::key_type;
Then use as...
template<typename C, typename Key = KeyTypper<C>>
vector<vector<Key>> partition_keys(const C& input);
template<typename C, typename Key = KeyTypper<C>>
vector<vector<Key>> sorted_keys(const C& input);
template<typename C, typename Key = KeyTypper<C>>
vector<vector<Key>> filtered_keys(const C& input);
Demo
It is fine. If you don't like the clutter in template parameters, you can put the key type directly in the return type, possibly in trailing form:
template <typename C>
auto partition_keys(const C& input)
-> vector<vector<remove_const_t<typename C::value_type::first_type>>>;
or rely on return type deduction for normal functions and omit the return type entirely:
template <typename C>
auto partition_keys(const C& input)
{
vector<vector<remove_const_t<typename C::value_type::first_type>>> result;
//...
return result;
}
First of, I'm using C++11 (and my topic sucks).
What I'm trying to do is write a generic template function that implements something usually called sort_by in other programming languages. It involves calculating an arbitrary criterion for each member of a range exactly once and then sorting that range according to those criteria. Such a criterion doesn't have to be a POD, all it has to be is less-than-comparable. For things for which std::less doesn't work the caller should be able to provide her own comparison functor.
I've successfully written said function which uses the following signature:
template< typename Tcriterion
, typename Titer
, typename Tcompare = std::less<Tcriterion>
>
void
sort_by(Titer first, Titer last,
std::function<Tcriterion(typename std::iterator_traits<Titer>::value_type const &)> criterion_maker,
Tcompare comparator = Tcompare()) {
}
It can be used e.g. like this:
struct S { int a; std::string b; double c; };
std::vector<S> s_vec{
{ 42, "hello", 0.5 },
{ 42, "moo!", 1.2 },
{ 23, "fubar", 0.2 },
};
sort_by1< std::pair<int, double> >(
s_vec.begin(), s_vec.end(),
[](S const &one_s) { return std::make_pair(one_s.a, one_s.c); }
);
What I don't like about this approach is that I have to provide the Tcriterion argument myself because the compiler cannot deduce that type from the lambda expression. Therefore this does not work:
sort_by1(s_vec.begin(), s_vec.end(), [](S const &one_s) { return std::make_pair(one_s.a, one_s.c); });
clang 3.1 and gcc 4.7.1 both bark on this (gcc 4.7.1 even barks on the code above, so I guess I'm really doing something wrong here).
However, if I assign the lambda to a std::function first then at least clang 3.1 can deduce the argument, meaning this works:
typedef std::pair<int, double> criterion_type;
std::function<criterion_type(S const &)> criterion_maker = [](S const &one_s) {
return std::make_pair(one_s.a, one_s.c);
};
sort_by1(s_vec.begin(), s_vec.end(), criterion_maker);
So my questions are: How do I have to change my function signature so that I don't need to specify that one argument? And (probably related) how would I fix my example to have it working with gcc?
Don't use std::function in tandem with template argument deduction. In fact, there's very likely no reason to use std::function in a function or function template argument list. More often than not, you should not use std::function; it is a very specialized tool that is very good at solving one particular problem. The rest of the time, you can dispense with it altogether.
In your case you don't need template argument deduction if you use a polymorphic functor to order things:
struct less {
template<typename T, typename U>
auto operator()(T&& t, U&& u) const
-> decltype( std::declval<T>() < std::declval<U>() )
{ return std::forward<T>(t) < std::forward<U>(u); }
// operator< is not appropriate for pointers however
// the Standard defines a 'composite pointer type' that
// would be very helpful here, left as an exercise to implement
template<typename T, typename U>
bool operator()(T* t, U* u) const
{ return std::less<typename std::common_type<T*, U*>::type> {}(t, u); }
};
You can then declare:
template<typename Iter, typename Criterion, typename Comparator = less>
void sort_by(Iter first, Iter last, Criterion crit, Comparator comp = less {});
and comp(*ita, *itb) will do the right thing, as well as comp(crit(*ita), crit(*itb)) or anything else as long as it makes sense.
How about something like this:
template< typename Titer
, typename Tmaker
, typename Tcompare
>
void
sort_by(Titer first, Titer last,
Tmaker criterion_maker,
Tcompare comparator)
{
typedef decltype(criterion_maker(*first)) Tcriterion;
/*
Now that you know the actual type of your criterion,
you can do the real work here
*/
}
The problem is that you can obviously not use a default for the comparator with this, but you can easily overcome that by providing an overload that doesn't take a comparator and fills in std::less internally.
To do it like you originally suggested, the compiler would have to be able to "invert" the template instantiation process. I.e. for a given std::function<> instantiation, what parameter do I have to supply as the result to get it. This "looks" easy, but it is not!
You can use also something like this.
template< typename Titer
, typename Tmaker
, typename TCriterion = typename
std::result_of
<
Tmaker
(
decltype(*std::declval<Titer>())
)
>::type
, typename Tcompare = std::less<TCriterion>
>
void
sort_by(Titer first, Titer last,
Tmaker criterion_maker, Tcompare comparator = Tcompare())
{
}
http://liveworkspace.org/code/0aacc8906ab4102ac62ef0e45a37707d
I want a generic zipWith function in C++ of variable arity. I have two problems. The first is that I cannot determine the type of the function pointer passed to zipWith. It must be of the same arity as the number of vectors passed to zipWith and it must accept references to the vectors' element types respectively. The second is that I have no idea how to walk these vectors in parallel to build an argument list, call func(), and bail once the shortest vector is exhausted.
template <typename R, typename T, typename... Vargs>
std::vector<R> zipWith (R func(???<what goes here>), std::vector<T> first, Vargs rest) {
???
}
I had a long answer, then I changed my mind in a way that made the solution much shorter. But I'm going to show my thought process and give you both answers!
My first step is to determine the proper signature. I don't understand all of it, but you can treat a parameter pack as a comma-separated list of the actual items with the text-dump hidden. You can extend the list on either side by more comma-separated items! So directly applying that:
template <typename R, typename T, typename... Vargs>
std::vector<R> zipWith (R func(T,Vargs...), std::vector<T> first, Vargs rest) {
???
}
You have to put a "..." after a parameter pack for an expression section to see the expanded list. You have to put one in the regular parameter portion, too:
template <typename R, typename T, typename... Vargs>
std::vector<R> zipWith (R func(T,Vargs...), std::vector<T> first, Vargs... rest) {
???
}
You said that your function parameters are a bunch of vectors. Here, you're hoping that each of Vargs is really a std::vector. Type transformations can be applied to a parameter pack, so why don't we ensure that you have vectors:
template <typename R, typename T, typename... Vargs>
std::vector<R> zipWith (R func(T,Vargs...), std::vector<T> first, std::vector<Vargs> ...rest) {
???
}
Vectors can be huge objects, so let's use const l-value references. Also, we could use std::function so we can use lambda or std::bind expressions:
template <typename R, typename T, typename... Vargs>
std::vector<R> zipWith (std::function<R(T, Vargs...)> func, std::vector<T> const &first, std::vector<Vargs> const &...rest) {
???
}
(I ran into problems here from using std::pow for testing. My compiler wouldn't accept a classic function pointer being converted into a std::function object. So I had to wrap it in a lambda. Maybe I should ask here about that....)
At this point, I reloaded the page and saw one response (by pmr). I don't really understand this zipping, folding, exploding, whatever stuff, so I thought his/her solution was too complicated. So I thought about a more direct solution:
template < typename R, typename T, typename ...MoreTs >
std::vector<R>
zip_with( std::function<R(T,MoreTs...)> func,
const std::vector<T>& first, const std::vector<MoreTs>& ...rest )
{
auto const tuples = rearrange_vectors( first, rest... );
std::vector<R> result;
result.reserve( tuples.size() );
for ( auto const &x : tuples )
result.push_back( evaluate(x, func) );
return result;
}
I would create a vector of tuples, where each tuple was made from plucking corresponding elements from each vector. Then I would create a vector of
evaluation results from passing a tuple and func each time.
The rearrange_vectors has to make table of values in advance (default-constructed) and fill out each entry a sub-object at a time:
template < typename T, typename ...MoreTs >
std::vector<std::tuple<T, MoreTs...>>
rearrange_vectors( const std::vector<T>& first,
const std::vector<MoreTs>& ...rest )
{
decltype(rearrange_vectors(first, rest...))
result( first.size() );
fill_vector_perpendicularly<0>( result, first, rest... );
return result;
}
The first part of the first line lets the function access its own return type without copy-and-paste. The only caveat is that r-value reference parameters must be surrounded by std::forward (or move) so a l-value overload of the recursive call doesn't get chosen by mistake. The function that mutates part of each tuple element has to explicitly take the current index. The index moves up by one during parameter pack peeling:
template < std::size_t, typename ...U >
void fill_vector_perpendicularly( std::vector<std::tuple<U...>>& )
{ }
template < std::size_t I, class Seq, class ...MoreSeqs, typename ...U >
void fill_vector_perpendicularly( std::vector<std::tuple<U...>>&
table, const Seq& first, const MoreSeqs& ...rest )
{
auto t = table.begin();
auto const te = table.end();
for ( auto f = first.begin(), fe = first.end(); (te != t) && (fe
!= f) ; ++t, ++f )
std::get<I>( *t ) = *f;
table.erase( t, te );
fill_vector_perpendicularly<I + 1u>( table, rest... );
}
The table is as long as the shortest input vector, so we have to trim the table whenever the current input vector ends first. (I wish I could mark fe as const within the for block.) I originally had first and rest as std::vector, but I realized I could abstract that out; all I need are types that match the standard (sequence) containers in iteration interface. But now I'm stumped on evaluate:
template < typename R, typename T, typename ...MoreTs >
R evaluate( const std::tuple<T, MoreTs...>& x,
std::function<R(T,MoreTs...)> func )
{
//???
}
I can do individual cases:
template < typename R >
R evaluate( const std::tuple<>& x, std::function<R()> func )
{ return func(); }
template < typename R, typename T >
R evaluate( const std::tuple<T>& x, std::function<R(T)> func )
{ return func( std::get<0>(x) ); }
but I can't generalize it for a recursive case. IIUC, std::tuple doesn't support peeling off the tail (and/or head) as a sub-tuple. Nor does std::bind support currying arguments into a function in piecemeal, and its placeholder system isn't compatible with arbitrary-length parameter packs. I wish I could just list each parameter like I could if I had access to the original input vectors....
...Wait, why don't I do just that?!...
...Well, I never heard of it. I've seen transferring a template parameter pack to the function parameters; I just showed it in zipWith. Can I do it from the function parameter list to the function's internals? (As I'm writing, I now remember seeing it in the member-initialization part of class constructors, for non-static members that are arrays or class types.) Only one way to find out:
template < typename R, typename T, typename ...MoreTs >
std::vector<R>
zip_with( std::function<R(T,MoreTs...)> func, const std::vector<T>&
first, const std::vector<MoreTs>& ...rest )
{
auto const s = minimum_common_size( first, rest... );
decltype(zip_with(func,first,rest...)) result;
result.reserve( s );
for ( std::size_t i = 0 ; i < s ; ++i )
result.push_back( func(first[i], rest[i]...) );
return result;
}
where I'm forced to compute the total number of calls beforehand:
inline std::size_t minimum_common_size() { return 0u; }
template < class SizedSequence >
std::size_t minimum_common_size( const SizedSequence& first )
{ return first.size(); }
template < class Seq, class ...MoreSeqs >
std::size_t
minimum_common_size( const Seq& first, const MoreSeqs& ...rest )
{ return std::min( first.size(), minimum_common_size(rest...) ); }
and sure enough, it worked! Of course, this meant that I over-thought the problem just as bad as the other respondent (in a different way). It also means that I unnecessarily bored you with most of this post. As I wrapped this up, I realized that the replacement of std::vector with generic sequence-container types can be applied in zip_width. And I realized that I could reduce the mandatory one vector to no mandatory vectors:
template < typename R, typename ...T, class ...SizedSequences >
std::vector<R>
zip_with( R func(T...) /*std::function<R(T...)> func*/,
SizedSequences const& ...containers )
{
static_assert( sizeof...(T) == sizeof...(SizedSequences),
"The input and processing lengths don't match." );
auto const s = minimum_common_size( containers... );
decltype( zip_with(func, containers...) ) result;
result.reserve( s );
for ( std::size_t i = 0 ; i < s ; ++i )
result.push_back( func(containers[i]...) );
return result;
}
I added the static_assert as I copied the code here, since I forgot to make sure that the func's argument count and the number of input vectors agree. Now I realize that I can fix the dueling function-pointer vs. std::function object by abstracting both away:
template < typename R, typename Func, class ...SizedSequences >
std::vector<R>
zip_with( Func&& func, SizedSequences&& ...containers )
{
auto const s = minimum_common_size( containers... );
decltype( zip_with<R>(std::forward<Func>(func),
std::forward<SizedSequences>(containers)...) ) result;
result.reserve( s );
for ( std::size_t i = 0 ; i < s ; ++i )
result.push_back( func(containers[i]...) );
return result;
}
Marking a function parameter with an r-value reference is the universal passing method. It handles all kinds of references and const/volatile (cv) qualifications. That's why I switched containers to it. The func could have any structure; it can even be a class object with multiple versions of operator (). Since I'm using r-values for the containers, they'll use the best cv-qualification for element dereferencing, and the function can use that for overload resolution. The recursive "call" to internally determine the result type needs to use std::forward to prevent any "downgrades" to l-value references. It also reveals a flaw in this iteration: I must provide the return type.
I'll fix that, but first I want to explain the STL way. You do not pre-determine a specific container type and return that to the user. You ask for a special object, an output-iterator, that you send the results to. The iterator could be connected to a container, of which the standard provides several varieties. It could be connected to an output stream instead, directly printing the results! The iterator method also relieves me from directly worrying about memory concerns.
#include <algorithm>
#include <cstddef>
#include <iterator>
#include <utility>
#include <vector>
inline std::size_t minimum_common_size() { return 0u; }
template < class SizedSequence >
std::size_t minimum_common_size( const SizedSequence& first )
{ return first.size(); }
template < class Seq, class ...MoreSeqs >
std::size_t minimum_common_size( const Seq& first,
const MoreSeqs& ...rest )
{
return std::min<std::size_t>( first.size(),
minimum_common_size(rest...) );
}
template < typename OutIter, typename Func, class ...SizedSequences >
OutIter
zip_with( OutIter o, Func&& func, SizedSequences&& ...containers )
{
auto const s = minimum_common_size( containers... );
for ( std::size_t i = 0 ; i < s ; ++i )
*o++ = func( containers[i]... );
return o;
}
template < typename Func, class ...SizedSequences >
auto zipWith( Func&& func, SizedSequences&& ...containers )
-> std::vector<decltype( func(containers.front()...) )>
{
using std::forward;
decltype( zipWith(forward<Func>( func ), forward<SizedSequences>(
containers )...) ) result;
#if 1
// `std::vector` is the only standard container with the `reserve`
// member function. Using it saves time when doing multiple small
// inserts, since you'll do reallocation at most (hopefully) once.
// The cost is that `s` is already computed within `zip_with`, but
// we can't get at it. (Remember that most container types
// wouldn't need it.) Change the preprocessor flag to change the
// trade-off.
result.reserve( minimum_common_size(containers...) );
#endif
zip_with( std::back_inserter(result), forward<Func>(func),
forward<SizedSequences>(containers)... );
return result;
}
I copied minimum_common_size here, but explicitly mentioned the result type for the least-base case, proofing against different container types using different size types.
Functions taking an output-iterator usually return iterator after all the iterators are done. This lets you start a new output run (even with a different output function) where you left off. It's not critical for the standard output iterators, since they're all pseudo-iterators. It is important when using a forward-iterator (or above) as an output iterator since they do track position. (Using a forward iterator as an output one is safe as long as the maximum number of transfers doesn't exceed the remaining iteration space.) Some functions put the output iterator at the end of the parameter list, others at the beginning; zip_width must use the latter since parameter packs have to go at the end.
Moving to a suffix return type in zipWith makes every part of the function's signature fair game when computing the return type expression. It also lets me know right away if the computation can't be done due to incompatibilities at compile-time. The std::back_inserter function returns a special output-iterator to the vector that adds elements via the push_back member function.
Here is what I cobbled together:
#include <iostream>
#include <vector>
#include <utility>
template<typename F, typename T, typename Arg>
auto fold(F f, T&& t, Arg&& a)
-> decltype(f(std::forward<T>(t), std::forward<Arg>(a)))
{ return f(std::forward<T>(t), std::forward<Arg>(a)); }
template<typename F, typename T, typename Head, typename... Args>
auto fold(F f, T&& init, Head&& h, Args&&... args)
-> decltype(f(std::forward<T>(init), std::forward<Head>(h)))
{
return fold(f, f(std::forward<T>(init), std::forward<Head>(h)),
std::forward<Args>(args)...);
}
// hack in a fold for void functions
struct ignore {};
// cannot be a lambda, needs to be polymorphic on the iterator type
struct end_or {
template<typename InputIterator>
bool operator()(bool in, const std::pair<InputIterator, InputIterator>& p)
{ return in || p.first == p.second; }
};
// same same but different
struct inc {
template<typename InputIterator>
ignore operator()(ignore, std::pair<InputIterator, InputIterator>& p)
{ p.first++; return ignore(); }
};
template<typename Fun, typename OutputIterator,
typename... InputIterators>
void zipWith(Fun f, OutputIterator out,
std::pair<InputIterators, InputIterators>... inputs) {
if(fold(end_or(), false, inputs...)) return;
while(!fold(end_or(), false, inputs...)) {
*out++ = f( *(inputs.first)... );
fold(inc(), ignore(), inputs...);
}
}
template<typename Fun, typename OutputIterator,
typename InputIterator, typename... Rest>
void transformV(Fun f, OutputIterator out, InputIterator begin, InputIterator end,
Rest... rest)
{
if(begin == end) return ;
while(begin != end) {
*out++ = f(*begin, *(rest)... );
fold(inc2(), ignore(), begin, rest...);
}
}
struct ternary_plus {
template<typename T, typename U, typename V>
auto operator()(const T& t, const U& u, const V& v)
-> decltype( t + u + v) // common type?
{ return t + u + v; }
};
int main()
{
using namespace std;
vector<int> a = {1, 2, 3}, b = {1, 2}, c = {1, 2, 3};
vector<int> out;
zipWith(ternary_plus(), back_inserter(out)
, make_pair(begin(a), end(a))
, make_pair(begin(b), end(b))
, make_pair(begin(c), end(c)));
transformV(ternary_plus(), back_inserter(out),
begin(a), end(a), begin(b), begin(c));
for(auto x : out) {
std::cout << x << std::endl;
}
return 0;
}
This is a slightly improved variant over previous versions. As every
good program should, it starts by defining a left-fold.
It still does not solve the problem of iterators packed in pairs.
In stdlib terms this function would be called transform and would
require that only the length of one sequence is specified and the
others be at least as long. I called it transformV here to avoid
name clashes.