Related
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 was reading Stroustrup's blog on c++ (http://isocpp.org/blog/2014/12/myths-3) when I found an intersting piece of code:
void do_my_sort(vector<double>& v)
{
sort(v,[](double x, double y) { return x>y; }); // sort v in decreasing order
}
int main()
{
vector<double> vd;
// ... fill vd ...
do_my_sort(v);
// ...
}
Notice that the sort does not use the traditional sort(v.begin(), v.end(), ...) which Stroustrup explains:
I used a container version of sort() to avoid being explicit about the
iterators.
However, I tried the same code on my C++11 compiler but it fails to compile. I also tried the same on a C++14 compiler using ideone but it too fails to compile, saying that there is no matching call to sort.
Why is this?
Also, Stroustrup next mentions:
I could go further and use a C++14 comparison object:
sort(v,greater<>()); // sort v in decreasing order
I have used comparators like great<>() for sort in C++11 also. Why is he stating that this is a C++14 comparison object?
He wrote that himself, it is not standard. Thus you cannot find it in the standard library. You could implement it like this:
template <class Container, class Comp>
void sort (Container& cont, Comp comp) {
using std::begin;
using std::end;
std::sort(begin(cont), end(cont), comp);
}
As Clukester pointed out, there is also boost::sort that offers this functionality.
I have used comparators like great<>() for sort in C++11 also. Why is he stating that this is a C++14 comparison object?
The C++14 comparison functors have the added ability to take forwarding references for its operator() method and deduced return types. The template argument for the Function Objects collection has been changed to have a default argument of type void and using specialization for that type.
template< class T = void >
struct greater
{
constexpr bool operator()(const T &lhs, const T &rhs) const;
};
template<>
struct greater<void>
{
template< class T, class U>
constexpr auto operator()( T&& lhs, U&& rhs ) const
-> decltype(std::forward<T>(lhs) > std::forward<U>(rhs));
};
Perhaps he is using Boost's sort, not the standard sort as one would expect. So it's boost::sort, not std::sort.
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 have a template class template<typename T, typename R>. R is of type vector<T*> or list<T*>.
I want my class to overload [] operator so that in case it is a vector I will use the built in [] operator for efficiency and in case it's a list I will implement it with iterator.
To me it sounds like a job for template specialization so I thought to write something like this:
template<typename T, typename R>
T& tContainer_t<T, R>::operator[]( unsigned i )
{
//TODO with iterators
}
template<>
T& tContainer_t::operator[]<T, std::vector<T*> >( unsigned i )
{
// TODO with built in [] operator
}
This is wrong and the compiler doesn't allow this.
Is there a way to make it work, or should I use typeid() to differ the two objects at runtime and act accordingly ?
The way to do it with templates is to make a static helper function in a class that can be partially specialized. However, what I would do is:
template<typename T, typename R>
T& tContainer_t<T, R>::operator[]( unsigned i )
{
//assuming that the container refernce is name container;
typename R::iterator itr = container.begin();
std::advance(itr, i);
return *itr;
}
std::advance is guaranteed that for a container with random access iterators (such as vector), it is constant time (basically, it does iterator + n), it can be as fast as doing the pointer lookup vector performs. Otherwise, it does iterator++ n times, which will be linear time. The const version will use const_iterator, but is essentially the same.
Doing it this way will let you properly handle different types of containers (not just vector and list), without having to modify the code.
You don't have to overload the operator. The library aleady contains overloaded functions to help you. std::advance will move an iterator, taking advantage of operator+() for random access iterators.
template<typename T, typename R>
T& tContainer_t<T, R>::operator[]( unsigned i )
{
typename R::iterator it = myContainer.begin();
std::advance(it, i);
return *it;
}
Suppose I have a function which looks like this:
template <class In, class In2>
void func(In first, In last, In2 first2);
I would like this function to call another function which accepts a predicate. My initial instinct was to do something like this:
template <class In, class In2>
void func(In first, In last, In2 first2) {
typedef typename std::iterator_traits<In>::value_type T;
other_func(first, last, first2, std::less<T>());
}
But there is a problem, what if In and In2 are iterators to different types? For example, char* vs int*. Depending on which is In and which is In2 the predicate may be truncating values during its comparison. For example, if In is char* then std::less<char> will be called even if In2 is an int*.
When ::operator< is given two parameters, the compiler is able to deduce the correct type and the standard type promotion rules apply. However, when selecting a predicate to pass to a function, there is no oportunity to have this happen. Is there some clever way to figure out which version of std::less<> I want to pass based on In and In2?
EDIT:
The following example illustrates the problem:
unsigned int x = 0x80000000;
unsigned char y = 1;
std::cout << std::less<unsigned char>()(x, y) << std::endl;
std::cout << std::less<unsigned int>()(x, y) << std::endl;
will output:
1
0
EDIT:
After thinking about it, what I would really like is to be able to do something like this:
typedef typeof(T1() < T2()) T;
other_func(first, last, first2, std::less<T>());
I suppose I could use gcc's __typeof__ extension..., but I don't love that idea either. Any way to get that net effect in a standard conformant way?
I seemed to remember that there was a traits for this in boost, but I can't find it after a quick search. If you are no more successful than me, you can construct it yourself,
template <typename T1, typename T2>
struct least_common_promotion;
template <>
struct least_common_promotion<short, int>
{
typedef int type;
};
but you'll have to specify quite a few explicit specializations. The type traits library of boost can perhaps help you reduce their number.
Edit: I feel stupid, such kind of things are needed for operation (where the result type depend on the operands types), but not for predicates (where the result type is bool). You can simply write:
template <class T1, T2>
struct unhomogenous_less : public std::binary_function<T1, T2, bool>
{
bool operator()(T1 const& l, T2 const& r) const
{ return l < r; }
};
...
typedef typename std::iterator_traits<In>::value_type value_type_1;
typedef typename std::iterator_traits<In2>::value_type value_type_2;
other_func(first, last, first2, unhomogenous_less<value_type_1, value_type_2>());
If your requirements on the algorithm are such that In's value_type need not be the same as In2's value type, then I would leave the template parameters as you have them; otherwise they should be the same.
Whether they are the same or different it is up to the client of your routine to meet the prerequisites of the algorithm, which you are allowed to specify. For example, here you could require that the value_type of In be the same as the value_type of In2. If that holds true, then, the function should compile and be correct as the client expects.
In such a case, then, you can pass a std::less<T> instance of the value_type of either template type, and you should be fine.
However, if the client violates that precondition (as in the example you provide above where char is not the same as int), then it will be up to the client, not you, to correct the compile-time error.
Make sure your algorithm is well-documented, to say the least :)
Taking SGI's old implementation of std::equal as an example, STL algorithms handle this kind of situation by having two versions of the same algorithm: one that uses the intrinsic < operator which the compiler deduces at compile time, and one that takes a user-defined binary predicate so the user can use any types they'd prefer:
template <class _InputIter1, class _InputIter2>
inline bool equal(_InputIter1 __first1, _InputIter1 __last1,
_InputIter2 __first2) {
__STL_REQUIRES(_InputIter1, _InputIterator);
__STL_REQUIRES(_InputIter2, _InputIterator);
__STL_REQUIRES(typename iterator_traits<_InputIter1>::value_type,
_EqualityComparable);
__STL_REQUIRES(typename iterator_traits<_InputIter2>::value_type,
_EqualityComparable);
for ( ; __first1 != __last1; ++__first1, ++__first2)
if (*__first1 != *__first2)
return false;
return true;
}
template <class _InputIter1, class _InputIter2, class _BinaryPredicate>
inline bool equal(_InputIter1 __first1, _InputIter1 __last1,
_InputIter2 __first2, _BinaryPredicate __binary_pred) {
__STL_REQUIRES(_InputIter1, _InputIterator);
__STL_REQUIRES(_InputIter2, _InputIterator);
for ( ; __first1 != __last1; ++__first1, ++__first2)
if (!__binary_pred(*__first1, *__first2))
return false;
return true;
}
(Note: Old SGI STL code taken from here.)