Is there an stl way to get a list of values from a map?
i.e, I have:
std::map<A,B> myMap;
and I would like a function that will return just the list of values, i.e, std::list<B> (or set for that matter.
Is there a built-in stl way to do this?
A map element is defined as a map::value_type, and the type of it is a pair<A,B>. first is the key and second is the value. You can write a functor to extract second from a value_type, and copy that in to a vector (or a list, or whatever you want.) The best way to do the copying is to use transform, which does just what its name implies: it takes a value of one type and transforms it to a different type of value.
Here's a complete working example:
#include <cstdlib>
#include <map>
#include <string>
#include <algorithm>
#include <iterator>
#include <vector>
#include <iostream>
using namespace std;
typedef map<unsigned, string> MyMap;
MyMap my_map;
struct get_second : public std::unary_function<MyMap::value_type, string>
{
string operator()(const MyMap::value_type& value) const
{
return value.second;
}
};
int main()
{
my_map[1] = "one";
my_map[2] = "two";
my_map[3] = "three";
my_map[4] = "four";
my_map[5] = "five";
// get a vector of values
vector<string> my_vals;
transform(my_map.begin(), my_map.end(), back_inserter(my_vals), get_second() );
// dump the list
copy( my_vals.begin(), my_vals.end(), ostream_iterator<string>(cout, "\n"));
}
EDIT:
If you have a compiler that supports C++0x lambdas, you can eliminate the functor entirely. This is very useful for making code more readable and, arguable, easier to maintain since you don't end up with dozens of little one-off functors floating around in your codebase. Here's how you would change the code above to use a lambda:
transform(my_map.begin(), my_map.end(), back_inserter(my_vals), [](const MyMap::value_type& val){return val.second;} );
There's nothing built in, no. It's simple enough to write your own function, though: Iterate over the map. The iterator will give you a pair<A, B>. Add each second value to the result list.
You can't just "get" such a list because there is no pre-existing list stored anywhere in the guts, but you can build one:
typedef std::map<A,B> myMapType;
myMapType myMap;
std::list<B> valueList;
for (myMapType::const_iterator it=myMap.begin(); it!=myMap.end(); ++it) {
valueList.push_back( it->second );
}
Or if you really like the more STL way:
class GetSecond {
template<typename T1, typename T2>
const T2& operator()( const std::pair<T1,T2>& key_val ) const
{ return key_val.second; }
};
typedef std::map<A,B> myMapType;
myMapType myMap;
std::list<B> valueList;
std::transform(myMap.begin(), myMap.end(), std::back_inserter(valueList),
GetSecond());
One of many "built-in" ways is of course the most obvious one. Just iterate over all pair elements, which are ordered by key (pair::first), and add the value (pair::second) to a new container, which you can construct with the correct capacity to get rid of excess allocations during the iteration and adding.
Just a note: std::list is seldom the container you actually want to be using. Unless, of course, you really, really do need its specific features.
Sure.
std::list<B> list;
std::for_each(myMap.begin(), myMap.end(), [&](const std::pair<const A, B>& ref) {
list.push_back(ref.second);
});
If you don't have a C++0x compiler, first you have my sympathies, and second, you will need to build a quick function object for this purpose.
You can use boost's transform_iterator: http://www.boost.org/doc/libs/1_64_0/libs/iterator/doc/transform_iterator.html
struct GetSecond {
template <typename K, typename T>
const T& operator()(const std::pair<K, T> & p) const { return p.second; }
template <typename K, typename T>
T& operator()(std::pair<K, T> & p) const { return p.second; }
};
template <typename MapType>
auto begin_values(MapType& m) -> decltype(boost::make_transform_iterator(m.begin(), GetSecond())) {
return boost::make_transform_iterator(m.begin(), GetSecond());
}
template <typename MapType>
auto end_values(MapType& m) -> decltype(boost::make_transform_iterator(m.end(), GetSecond())) {
return boost::make_transform_iterator(m.end(), GetSecond());
}
template <typename MapType>
struct MapValues {
MapType & m;
MapValues(MapType & m) : m(m) {}
typedef decltype(begin_values(m)) iterator;
iterator begin() { return begin_values(m); }
iterator end() { return end_values(m); }
};
template <typename MapType>
MapValues<MapType> get_values(MapType & m) {
return MapValues<MapType>(m);
}
int main() {
std::map<int, double> m;
m[0] = 1.0;
m[10] = 2.0;
for (auto& x : get_values(m)) {
std::cout << x << ',';
x += 1;
}
std::cout << std::endl;
const std::map<int, double> mm = m;
for (auto& x : get_values(mm)) {
std::cout << x << ',';
}
std::cout << std::endl;
}
Related
Using the following containers:
std::vector<std::pair<std::string, int>> keyVals = {
{"A", 1}, {"B", 2}, {"C", 3}
};
std::vector<std::string> keys = {
"A", "B", "C"
};
I need to construct a sorted output container
std::set<std::tuple<std::string, int, int>>
using the std::set_intersection between keyVals and keys.
These types use a common field (in my case a string) this is also used for pre-sorting these vectors via the default std::less - however no need in the above example as they are already sorted.
I need help to create a sorted container C that is not just a simple copy of elements from A to the output over the intersection range.
Instead I need to be able to construct each output element C - in my case a std::tuple<std::string, int, int> (where the first entry in the tuple is the common link string field and the other 2 int are some global fields.
To illustrate the problem I created a coliru live demo where I commented out the broken code - I don't know how to invoke a custom constructor for each iteration - please help.
#include <vector>
#include <variant>
#include <memory>
#include <iostream>
#include <algorithm>
// Helper to get Lambda with multiple signatures in place
// template deduction guide is a C++17 feature!
template<class... Ts> struct overload : Ts... { using Ts::operator()...; };
// this line is necessary for c++17 - not required for c++20
template<class... Ts> overload(Ts...) -> overload<Ts...>;
template<typename T>
std::ostream& operator<<(std::ostream& os, const std::vector<T>& vec) {
for (auto& el : vec) {
os << el.first << ' ';
}
return os;
}
// if c++17 is not available, you have to write a functor/function
// with both signatures
int main()
{
std::vector<std::pair<std::string, int>> keyVals = {
{"A", 1}, {"B", 2}, {"C", 3}
};
std::vector<std::string> keys = {
"A", "B", "C"
};
// std::set_intersection results can only be of the first iterator type
std::vector<std::pair<std::string, int>> intersection;
// this works
std::set_intersection(
keyVals.begin(), keyVals.end(),
keys.begin(), keys.end(),
std::back_inserter(intersection),
overload {
[](const std::pair<std::string, int>& lhs, const std::string& rhs) {
return lhs.first < rhs;
},
[](const std::string& lhs, const std::pair<std::string, int>& rhs) {
return lhs < rhs.first;
}
}
);
std::cout << intersection << std::endl;
#if 0
// std::set_intersection results can only be of the first iterator type
std::vector<std::tuple<std::string, int, int>> broken_intersection;
// this does not work as no match for 'operator=' *__result = *__first1;
std::set_intersection(
keyVals.begin(), keyVals.end(),
keys.begin(), keys.end(),
std::back_inserter(broken_intersection), // help here - I need to be able to
overload {
[](const std::pair<std::string, int>& lhs, const std::string& rhs) {
return lhs.first < rhs;
},
[](const std::string& lhs, const std::pair<std::string, int>& rhs) {
return lhs < rhs.first;
}
}
);
std::cout << broken_intersection << std::endl;
#endif
}
The problem is, as already noted, the different types of the data.
All input and output containers habe a different type.
And with that, 2 main operations in std::set_difference need to be expressed.
The comparison of the dereferenced input iterators of different type
The assignment of a rvalue from the dereferenced input to the dereferenced output iterator.
Problem 1 can be easily solved with a Functor, for which we will create 2 function call operators with the 2 needed signatures. This will allow for comparison in both directions.
Starting with C++14 you may also use a generic Lambda:
auto cmp = [](auto lhs, auto rhs) { return lhs.mCommonField < rhs.mCommonField; };
But then you need some common fields in both vectors.
The same approach would be possible with a Functor having a templated call operator:
struct Comparator
{
template<typename T, typename U>
bool operator()(T const& lhs, U const& rhs) const {
return lhs.commonField < rhs.commonField;
}
};
Also here you need a common field. And they must have the same name. Maybe this will not fit here.
Or, we could create a wrapper for a Common Field:
struct Common {
std::string const& commonField;
Common(StructA const& sa) : commonField{sa.commonField} {};
Common(StructB const& sb) : commonField{sb.commonField} {};
};
Here the common field must just be of the same type.
Many possibilities
For problem 2 we need a small wrapper. Either for the first input iterator or the output iterator. The wrapper needs to have iterator functionality and a dereferencing operator that returns a Tuple, so that we can assign this Tuple to the resulting vector of Tuples.
So we will create a very small custom iterator, with only the minimumm functions needed by std::set_intersection
Construction
Dereferencing
Pre Incrementing
Comparison for not equal
Within the iterator we will use a simple pointer to a Pair as the real iterator. The constructor can get a pointer to the Pairs in the first vector with the std::vector data function. That is very convenient.
The dereferencing operator does the trick. It will not return a Pair, but construct a Tuple and return that. So, this is the conversion from the Pair to a Tuple.
Putting this altogether may lead to one of many potential solutions:
#include <string>
#include <vector>
#include <iostream>
#include <algorithm>
#include <tuple>
#include <utility>
using Key = std::string;
using Value = int;
using Keys = std::vector<Key>;
using Pair = std::pair<Key, Value>;
using KeyVals = std::vector<Pair>;
using Tuple = std::tuple<Key, Value, Value>;
using Tuples = std::vector<Tuple>;
std::ostream& operator<<(std::ostream& os, const Tuples& t) {
for (auto& el : t) os << std::get<0>(el) << ' ';
return os;
}
struct Iterator {
using iterator_category = std::output_iterator_tag;
using difference_type = std::ptrdiff_t;
using value_type = Tuple;
using pointer = Tuple*;
using reference = Tuple&;
Pair* iter; // The internal iterator. Just a pointer
Iterator(Pair* kv) { iter = kv; }
Tuple operator *() const { return Tuple{ iter->first,0,0}; }
Iterator operator ++() { ++iter; return *this; }
bool operator !=(const Iterator& other) { return iter != other.iter; }
};
struct Comp {
bool operator ()(const Tuple& lhs, const std::string& rhs) { return std::get<0>(lhs) < rhs; }
bool operator ()(const std::string& lhs, const Tuple& rhs) { return lhs < std::get<0>(rhs) ; }
};
int main()
{
KeyVals keyVals = { {"A", 1}, {"B", 2}, {"C", 3} };
Keys keys = { "A", "B", "C" };
// Resulting vector
Tuples tupleResult{};
// Create Iterators to KeyVals
Iterator begin(keyVals.data());
Iterator end(keyVals.data() + keyVals.size());
// Do the intersection
std::set_intersection(begin, end, keys.begin(), keys.end(), std::back_inserter(tupleResult), Comp());
// Show result
std::cout << tupleResult << '\n';
}
Looks a little bit clumsy
On a 2nd thought, you could convert both input vectors to the needed output type using std::transform. But this will cost additional space and time.
Anyway, please see the next potential solution:
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <iterator>
#include <utility>
#include <tuple>
using Key = std::string;
using Value = int;
using Keys = std::vector<Key>;
using Pair = std::pair<Key, Value>;
using KeyVals = std::vector<Pair>;
using Tuple = std::tuple<Key, Value, Value>;
using Tuples = std::vector<Tuple>;
std::ostream& operator<<(std::ostream& os, const Tuples& t) {
for (auto& el : t) os << std::get<0>(el) << ' ';
return os;
}
int main()
{
KeyVals keyVals = { {"A", 1}, {"B", 2}, {"C", 3} };
Keys keys = { "A", "B", "C" };
// Convert to same type
Tuples tupleKeyVals{}, tupleKeys{};
std::transform(keyVals.begin(), keyVals.end(), std::back_inserter(tupleKeyVals), [](const Pair& p) { return Tuple{ p.first, p.second, 0 }; });
std::transform(keys.begin(), keys.end(), std::back_inserter(tupleKeys), [](const std::string& s) { return Tuple{ s, 0, 0 }; });
// Resulting vector
Tuples tupleResult{};
// Build intersection based on first tuple element
std::set_intersection(tupleKeyVals.begin(), tupleKeyVals.end(), tupleKeys.begin(), tupleKeys.end(), std::back_inserter(tupleResult),
[](const Tuple& t1, const Tuple& t2) { return std::get<0>(t1) < std::get<0>(t2); });
std::cout << tupleResult << '\n';
}
This looks a little bit cleaner. But disadvantage is much more memory consumption and longer operation time.
Last but not least, we could implement an own set_intersetcion function, which is quite simple.
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <iterator>
#include <utility>
#include <tuple>
using Key = std::string;
using Value = int;
using Keys = std::vector<Key>;
using Pair = std::pair<Key, Value>;
using KeyVals = std::vector<Pair>;
using Tuple = std::tuple<Key, Value, Value>;
using Tuples = std::vector<Tuple>;
std::ostream& operator<<(std::ostream& os, const Tuples& t) {
for (auto& el : t) os << std::get<0>(el) << ' ';
return os;
}
// Specialized intersection function
Tuples set_intersection(KeyVals& kv, Keys& k) {
Tuples result{};
KeyVals::iterator iterKeyVal = kv.begin();
Keys::iterator iterKey = k.begin();
while (iterKeyVal != kv.end() and iterKey != k.end()) {
if (iterKeyVal->first < *iterKey)
++iterKeyVal;
else {
if (not(*iterKey < iterKeyVal->first))
result.emplace_back(Tuple{ (iterKeyVal++)->first ,0,0 });
++iterKey;
}
}
return result;
}
int main()
{
// Input
KeyVals keyVals = { {"A", 1}, {"B", 2}, {"C", 3} };
Keys keys = { "A", "B", "C" };
// Resulting vector
Tuples tupleResult = set_intersection(keyVals, keys);
// Show result
std::cout << tupleResult << '\n';
}
But in the end you need to decide based on other requirements or constraints.
i want to initialize a const std::vector<int> member variable in the initializer list of a constructor, given a std::vector<std::tuple<int, float>> constructor argument. The vector should contain all the first tuple items.
Is there a one-liner that extracts an std::vector<int> containing all the first tuple entries from an std::vector<std::tuple<int, float>> ?
With C++20 ranges:
struct X
{
const std::vector<int> v;
template <std::ranges::range R>
requires std::convertible_to<std::ranges::range_value_t<R>, int>
X(R r)
: v{r.begin(), r.end()}
{}
X(const std::vector<std::tuple<int, float>>& vt)
: X{vt | std::ranges::views::elements<0>}
{}
};
With ranges-v3:
struct X
{
const std::vector<int> v;
X(const std::vector<std::tuple<int, float>>& vt)
: v{vt | ranges::views::transform([] (auto t) {
return std::get<0>(t); })
| ranges::to<std::vector>()}
{}
};
And a Frankenstein's monster:
#include <ranges>
#include <range/v3/range/conversion.hpp>
struct X
{
const std::vector<int> v;
X(const std::vector<std::tuple<int, float>>& vt)
: v{vt | std::ranges::views::elements<0>
| ranges::to<std::vector>()}
{}
};
Not a one-liner to setup, but certainly one to use - you can write a function to do the conversion, and then the constructor can call that function when initializing the vector member, eg:
std::vector<int> convertVec(const std::vector<std::tuple<int, float>> &arg)
{
std::vector<int> vec;
vec.reserve(arg.size());
std::transform(arg.begin(), arg.end(), std::back_inserter(vec),
[](const std::tuple<int, float> &t){ return std::get<int>(t); }
);
return vec;
}
struct Test
{
const std::vector<int> vec;
Test(const std::vector<std::tuple<int, float>> &arg)
: vec(convertVec(arg))
{
}
};
Adding to #bolov's answer, let's talk about what you might have liked to do, but can't in a one-liner.
There's the to<>() function from ranges-v3 from #bolov 's answer - it materializes a range into an actual container (a range is lazily-evaluated, and when you create it you don't actually iterate over the elements). There is no reason it shouldn't be in the standard library, if you ask me - maybe they'll add it 2023?
You may be wondering - why do I need that to()? Can't I just initialize a vector by a range? And the answer is - sort of, sometimes, maybe. Consider this program:
#include <vector>
#include <tuple>
#include <ranges>
void foo()
{
using pair_type = std::tuple<int, double>;
std::vector<pair_type> tvec {
{12, 3.4}, {56, 7.8}, { 90, 91.2}
};
auto tv = std::ranges::transform_view(
tvec,
[](const pair_type& p) { return std::get<0>(p);}
);
std::vector<int> vec1 { tv.begin(), tv.end() };
std::vector<std::tuple<int>> vec2 {
std::ranges::transform_view{
tvec,
[](const pair_type& p) { return std::get<0>(p);}
}.begin(),
std::ranges::transform_view{
tvec,
[](const pair_type& p) { return std::get<0>(p);}
}.end()
};
}
The vec1 statement will compile just fine, but the vec2 statement will fail (GodBolt.org). This is surprising (to me anyway)!
You may also be wondering why you even need to go through ranges at all. Why isn't there a...
template <typename Container, typename UnaryOperation>
auto transform(const Container& container, UnaryOperation op);
which constructs the transformed container as its return value? That would allow you to write:
std::vector<int> vec3 {
transform(
tvec,
[](const pair_type& p) { return std::get<0>(p); }
)
}
... and Bob's your uncle. Well, we just don't have functions in the C++ standard library which take containers. It's either iterator pairs, which is the "classic" pre-C++20 standard library, or ranges.
Now, the way I've declared the transform() function, it is actually tricky/impossible to implement generally, since you don't have a way of converting the type of a container to the same type of container but with a different element. So, instead, let's write something a little easier:
template <
typename T,
template <typename> class Container,
typename UnaryOperation
>
auto transform(const Container<T>& container, UnaryOperation op)
{
auto tv = std::ranges::transform_view(container, op);
using op_result = decltype(op(*container.begin()));
return Container<op_result> { tv.begin(), tv.end() };
}
and this works (GodBolt.org).
Yes, there is a one-liner if you use this library in Github.
The code goes like this.
#include <iostream>
#include <tuple>
#include <vector>
#include <LazyExpression/LazyExpression.h>
using std::vector;
using std::tuple;
using std::ref;
using std::cout;
using LazyExpression::Expression;
int main()
{
// Define test data
vector<tuple<int, float>> vecTuple { {1,1.1}, {2,2.2}, {3, 3.3} };
// Initialize int vector with the int part of the tuple (the one-liner :)
vector<int> vecInt = Expression([](const tuple<int, float>& t) { return std::get<int>(t); }, ref(vecTuple))();
// print the results
for (int i : vecInt)
cout << i << ' ';
cout << "\n";
}
// Output: 1 2 3
I have trouble describing my problem so I'll give an example:
I have a class description that has a couple of variables in it, for example:
class A{
float a, b, c, d;
}
Now, I maintain a vector<A> that contains many of these classes. What I need to do very very often is to find the object inside this vector that satisfies that one of it's parameters is maximal w.r.t to the others. i.e code looks something like:
int maxi=-1;
float maxa=-1000;
for(int i=0;i<vec.size();i++){
res= vec[i].a;
if(res > maxa) {
maxa= res;
maxi=i;
}
}
return vec[maxi];
However, sometimes I need to find class with maximal a, sometimes with maximal b, sometimes the class with maximal 0.8*a + 0.2*b, sometimes I want a maximal a*VAR + b, where VAR is some variable that is assigned in front, etc. In other words, I need to evaluate an expression for every class, and take the max. I find myself copy-pasting this everywhere, and only changing the single line that defines res.
Is there some nice way to avoid this insanity in C++? What's the neatest way to handle this?
Thank you!
I know this thread is old, but i find it quite useful to implement a powerful argmax function in C++.
However, as far as i can see, all the given examples above rely on std::max_element, which does comparison between the elements (either using a functor or by calling the operator<). this can be slow, if the calculation for each element is expensive. It works well for sorting numbers and handling simple classes, but what if the functor is much more complex? Maybe calculating a heuristic value of a chess position or something else that generate a huge tree etc.
A real argmax, as the thread starter mentioned, would only calculate its arg once, then save it to be compared with the others.
EDIT: Ok i got annoyed and had too much free time, so i created one < C++11 and one C++11 version with r-value references, first the C++11 version:
#include <iostream>
#include <algorithm>
#include <iterator>
#include <vector>
template<typename IteratorT, typename HeuristicFunctorT>
IteratorT argmax(IteratorT && it, const IteratorT & end, const HeuristicFunctorT & functor) {
IteratorT best(it++);
typename HeuristicFunctorT::result_type best_value(functor(*best));
for(; it != end; ++it) {
typename HeuristicFunctorT::result_type value(functor(*it));
if (value > best_value) {
best_value = value;
best = it;
}
}
return best;
}
template<typename IteratorT, typename HeuristicFunctorT>
inline IteratorT argmax(const IteratorT & begin, const IteratorT & end, const HeuristicFunctorT & functor) {
return argmax(IteratorT(begin), end, functor);
}
class IntPairFunctor : public std::unary_function< std::pair<int, int>, int > {
public:
int operator() (const std::pair<int, int> & v) const {
return v.first + v.second;
}
};
std::pair<int, int> rand_pair() {
return std::make_pair(rand(), rand());
}
int main(int argc, const char **argv) {
srand(time(NULL));
std::vector< std::pair<int, int> > ints;
std::generate_n(std::back_insert_iterator< std::vector< std::pair<int, int> > >(ints), 1000, rand_pair);
std::vector< std::pair<int, int> >::iterator m (argmax(ints.begin(), ints.end(), IntPairFunctor()));
std::cout << std::endl << "argmax: " << *m << std::endl;
}
The non C++11 version is much simpler, only the template:
template<typename IteratorT, typename HeuristicFunctorT>
IteratorT argmax(IteratorT it, const IteratorT & end, const HeuristicFunctorT & functor) {
IteratorT best(it++);
typename HeuristicFunctorT::result_type best_value(functor(*best));
for(; it != end; ++it) {
typename HeuristicFunctorT::result_type value(functor(*it));
if (value > best_value) {
best_value = value;
best = it;
}
}
return best;
}
Note that neither version requires any template arguments, the only requirement is that the heuristic implements the unary_function class
template <typename F>
struct CompareBy
{
bool operator()(const typename F::argument_type& x,
const typename F::argument_type& y)
{ return f(x) < f(y); }
CompareBy(const F& f) : f(f) {}
private:
F f;
};
template <typename T, typename U>
struct Member : std::unary_function<U, T>
{
Member(T U::*ptr) : ptr(ptr) {}
const T& operator()(const U& x) { return x.*ptr; }
private:
T U::*ptr;
};
template <typename F>
CompareBy<F> by(const F& f) { return CompareBy<F>(f); }
template <typename T, typename U>
Member<T, U> mem_ptr(T U::*ptr) { return Member<T, U>(ptr); }
You need to include <functional> for this to work. Now use, from header <algorithm>
std::max_element(v.begin(), v.end(), by(mem_ptr(&A::a)));
or
double combination(A x) { return 0.2 * x.a + 0.8 * x.b; }
and
std::max_element(v.begin(), v.end(), by(std::fun_ptr(combination)));
or even
struct combination : std::unary_function<A, double>
{
combination(double x, double y) : x(x), y(y) {}
double operator()(const A& u) { return x * u.a + y * u.b; }
private:
double x, y;
};
with
std::max_element(v.begin(), v.end(), by(combination(0.2, 0.8)));
to compare by a member or by linear combinations of a and b members. I split the comparer in two because the mem_ptr thing is damn useful and worth being reused. The return value of std::max_element is an iterator to the maximum value. You can dereference it to get the max element, or you can use std::distance(v.begin(), i) to find the corresponding index (include <iterator> first).
See http://codepad.org/XQTx0vql for the complete code.
This is what functors and STL are made for:
// A class whose objects perform custom comparisons
class my_comparator
{
public:
explicit my_comparator(float c1, float c2) : c1(c1), c2(c2) {}
// std::max_element calls this on pairs of elements
bool operator() (const A &x, const A &y) const
{
return (x.a*c1 + x.b*c2) < (y.a*c1 + y.b*c2);
}
private:
const float c1, c2;
};
// Returns the "max" element in vec
*std::max_element(vec.begin(), vec.end(), my_comparator(0.8,0.2));
Is the expression always linear? You could pass in an array of four coefficients. If you need to support arbitrary expressions, you'll need a functor, but if it's just an affine combination of the four fields then there's no need for all that complexity.
You can use the std::max_element algorithm with a custom comparator.
It's easy to write the comparator if your compiler supports lambda expressions.
If it doesn't, you can write a custom comparator functor. For the simple case of just comparing a single member, you can write a generic "member comparator" function object, which would look something like this:
template <typename MemberPointer>
struct member_comparator
{
MemberPointer p_;
member_comparator(MemberPointer p) : p_(p) { }
template <typename T>
bool operator()(const T& lhs, const T& rhs) const
{
return lhs.*p_ < rhs.*p_;
}
};
template <typename MemberPointer>
member_comparator<MemberPointer> make_member_comparator(MemberPointer p)
{
return member_comparator<MemberPointer>(p);
}
used as:
// returns an iterator to the element that has the maximum 'd' member:
std::max_element(v.begin(), v.end(), make_member_comparator(&A::d));
You could use the std::max_element STL algorithm providing a custom comparison predicate each time.
With C++0x you can even use a lambda function for it for maximum conciseness:
auto maxElement=*std::max_element(vector.begin(), vector.end(), [](const A& Left, const A& Right) {
return (0.8*Left.a + 0.2*Left.b)<(0.8*Right.a + 0.2*Right.b);
});
Sample of using max_element/min_element with custom functor
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
struct A{
float a, b, c, d;
};
struct CompareA {
bool operator()(A const & Left, A const & Right) const {
return Left.a < Right.a;
}
};
int main() {
vector<A> vec;
vec.resize(3);
vec[0].a = 1;
vec[1].a = 2;
vec[2].a = 1.5;
vector<A>::iterator it = std::max_element(vec.begin(), vec.end(), CompareA());
cout << "Largest A: " << it->a << endl;
it = std::min_element(vec.begin(), vec.end(), CompareA());
cout << "Smallest A: " << it->a << endl;
}
Is there a way to specify the default value std::map's operator[] returns when an key does not exist?
While this does not exactly answer the question, I have circumvented the problem with code like this:
struct IntDefaultedToMinusOne
{
int i = -1;
};
std::map<std::string, IntDefaultedToMinusOne > mymap;
No, there isn't. The simplest solution is to write your own free template function to do this. Something like:
#include <string>
#include <map>
using namespace std;
template <typename K, typename V>
V GetWithDef(const std::map <K,V> & m, const K & key, const V & defval ) {
typename std::map<K,V>::const_iterator it = m.find( key );
if ( it == m.end() ) {
return defval;
}
else {
return it->second;
}
}
int main() {
map <string,int> x;
...
int i = GetWithDef( x, string("foo"), 42 );
}
C++11 Update
Purpose: Account for generic associative containers, as well as optional comparator and allocator parameters.
template <template<class,class,class...> class C, typename K, typename V, typename... Args>
V GetWithDef(const C<K,V,Args...>& m, K const& key, const V & defval)
{
typename C<K,V,Args...>::const_iterator it = m.find( key );
if (it == m.end())
return defval;
return it->second;
}
C++17 provides try_emplace which does exactly this. It takes a key and an argument list for the value constructor and returns a pair: an iterator and a bool.: http://en.cppreference.com/w/cpp/container/map/try_emplace
The C++ standard (23.3.1.2) specifies that the newly inserted value is default constructed, so map itself doesn't provide a way of doing it. Your choices are:
Give the value type a default constructor that initialises it to the value you want, or
Wrap the map in your own class that provides a default value and implements operator[] to insert that default.
The value is initialized using the default constructor, as the other answers say. However, it is useful to add that in case of simple types (integral types such as int, float, pointer or POD (plan old data) types), the values are zero-initialized (or zeroed by value-initialization (which is effectively the same thing), depending on which version of C++ is used).
Anyway, the bottomline is, that maps with simple types will zero-initialize the new items automatically. So in some cases, there is no need to worry about explicitly specifying the default initial value.
std::map<int, char*> map;
typedef char *P;
char *p = map[123],
*p1 = P(); // map uses the same construct inside, causes zero-initialization
assert(!p && !p1); // both will be 0
See Do the parentheses after the type name make a difference with new? for more details on the matter.
There is no way to specify the default value - it is always value constructed by the default (zero parameter constructor).
In fact operator[] probably does more than you expect as if a value does not exist for the given key in the map it will insert a new one with the value from the default constructor.
template<typename T, T X>
struct Default {
Default () : val(T(X)) {}
Default (T const & val) : val(val) {}
operator T & () { return val; }
operator T const & () const { return val; }
T val;
};
<...>
std::map<KeyType, Default<ValueType, DefaultValue> > mapping;
More General Version, Support C++98/03 and More Containers
Works with generic associative containers, the only template parameter is the container type itself.
Supported containers: std::map, std::multimap, std::unordered_map, std::unordered_multimap, wxHashMap, QMap, QMultiMap, QHash, QMultiHash, etc.
template<typename MAP>
const typename MAP::mapped_type& get_with_default(const MAP& m,
const typename MAP::key_type& key,
const typename MAP::mapped_type& defval)
{
typename MAP::const_iterator it = m.find(key);
if (it == m.end())
return defval;
return it->second;
}
Usage:
std::map<int, std::string> t;
t[1] = "one";
string s = get_with_default(t, 2, "unknown");
Here is a similar implementation by using a wrapper class, which is more similar to the method get() of dict type in Python: https://github.com/hltj/wxMEdit/blob/master/src/xm/xm_utils.hpp
template<typename MAP>
struct map_wrapper
{
typedef typename MAP::key_type K;
typedef typename MAP::mapped_type V;
typedef typename MAP::const_iterator CIT;
map_wrapper(const MAP& m) :m_map(m) {}
const V& get(const K& key, const V& default_val) const
{
CIT it = m_map.find(key);
if (it == m_map.end())
return default_val;
return it->second;
}
private:
const MAP& m_map;
};
template<typename MAP>
map_wrapper<MAP> wrap_map(const MAP& m)
{
return map_wrapper<MAP>(m);
}
Usage:
std::map<int, std::string> t;
t[1] = "one";
string s = wrap_map(t).get(2, "unknown");
One workaround is to use map::at() instead of [].
If a key does not exist, at throws an exception.
Even nicer, this also works for vectors, and is thus suited for generic programming where you may swap the map with a vector.
Using a custom value for unregistered key may be dangerous since that custom value (like -1) may be processed further down in the code. With exceptions, it's easier to spot bugs.
Expanding on the answer https://stackoverflow.com/a/2333816/272642, this template function uses std::map's key_type and mapped_type typedefs to deduce the type of key and def.
This doesn't work with containers without these typedefs.
template <typename C>
typename C::mapped_type getWithDefault(const C& m, const typename C::key_type& key, const typename C::mapped_type& def) {
typename C::const_iterator it = m.find(key);
if (it == m.end())
return def;
return it->second;
}
This allows you to use
std::map<std::string, int*> m;
int* v = getWithDefault(m, "a", NULL);
without needing to cast the arguments like std::string("a"), (int*) NULL.
Pre-C++17, use std::map::insert(), for newer versions use try_emplace(). It may be counter-intuitive, but these functions effectively have the behaviour of operator[] with custom default values.
Realizing that I'm quite late to this party, but if you're interested in the behaviour of operator[] with custom defaults (that is: find the element with the given key, if it isn't present insert a chosen default value and return a reference to either the newly inserted value or the existing value), there is already a function available to you pre C++17: std::map::insert(). insert will not actually insert if the key already exists, but instead return an iterator to the existing value.
Say, you wanted a map of string-to-int and insert a default value of 42 if the key wasn't present yet:
std::map<std::string, int> answers;
int count_answers( const std::string &question)
{
auto &value = answers.insert( {question, 42}).first->second;
return value++;
}
int main() {
std::cout << count_answers( "Life, the universe and everything") << '\n';
std::cout << count_answers( "Life, the universe and everything") << '\n';
std::cout << count_answers( "Life, the universe and everything") << '\n';
return 0;
}
which should output 42, 43 and 44.
If the cost of constructing the map value is high (if either copying/moving the key or the value type is expensive), this comes at a significant performance penalty, which would be circumvented with C++17's try_emplace().
If you have access to C++17, my solution is as follows:
std::map<std::string, std::optional<int>> myNullables;
std::cout << myNullables["empty-key"].value_or(-1) << std::endl;
This allows you to specify a 'default value' at each use of the map. This may not necessarily be what you want or need, but I'll post it here for the sake of completeness. This solution lends itself well to a functional paradigm, as maps (and dictionaries) are often used with such a style anyway:
Map<String, int> myNullables;
print(myNullables["empty-key"] ?? -1);
Maybe you can give a custom allocator who allocate with a default value you want.
template < class Key, class T, class Compare = less<Key>,
class Allocator = allocator<pair<const Key,T> > > class map;
With C++20 it is simple to write such getter:
constexpr auto &getOrDefault(const auto &map, const auto &key, const auto &defaultValue)
{
const auto itr = map.find(key);
return itr == map.cend() ? defaultValue : itr->second;
}
Here is a correct approach that will conditionally return a reference if the caller passes in an lvalue reference to the mapped type.
template <typename Map, typename DefVal>
using get_default_return_t = std::conditional_t<std::is_same_v<std::decay_t<DefVal>,
typename Map::mapped_type> && std::is_lvalue_reference_v<DefVal>,
const typename Map::mapped_type&, typename Map::mapped_type>;
template <typename Map, typename Key, typename DefVal>
get_default_return_t<Map, DefVal> get_default(const Map& map, const Key& key, DefVal&& defval)
{
auto i = map.find(key);
return i != map.end() ? i->second : defval;
}
int main()
{
std::map<std::string, std::string> map;
const char cstr[] = "world";
std::string str = "world";
auto& ref = get_default(map, "hello", str);
auto& ref2 = get_default(map, "hello", std::string{"world"}); // fails to compile
auto& ref3 = get_default(map, "hello", cstr); // fails to compile
return 0;
}
If you would like to keep using operator[] just like when you don't have to specify a default value other than what comes out from T() (where T is the value type), you can inherit T and specify a different default value in the constructor:
#include <iostream>
#include <map>
#include <string>
int main() {
class string_with_my_default : public std::string {
public:
string_with_my_default() : std::string("my default") {}
};
std::map<std::string, string_with_my_default> m;
std::cout << m["first-key"] << std::endl;
}
However, if T is a primitive type, try this:
#include <iostream>
#include <map>
#include <string>
template <int default_val>
class int_with_my_default {
private:
int val = default_val;
public:
operator int &() { return val; }
int* operator &() { return &val; }
};
int main() {
std::map<std::string, int_with_my_default<1> > m;
std::cout << m["first-key"] << std::endl;
++ m["second-key"];
std::cout << m["second-key"] << std::endl;
}
See also C++ Class wrapper around fundamental types
How can I use BOOST_FOREACH efficiently (number-of-character/readability-wise) with a boost::ptr_map?
Kristo demonstrated in his answer that it is possible to use BOOST_FOREACH with a ptr_map, but it does not really save me any typing (or makes my code really more readable) than iterating over the ptr_map with an iterator:
typedef boost::ptr_container_detail::ref_pair<int, int* const> IntPair;
BOOST_FOREACH(IntPair p, mymap) {
int i = p.first;
}
// vs.
boost::ptr_map<int, T>::iterator it;
for (it = mymap.begin(); it != mymap.end(); ++it) {
// doSomething()
}
The following code is somewhere along the lines what I wish for. It follows the standard way on how to use BOOST_FOREACH with a std::map. Unfortunately this does not compile:
boost::ptr_map<int, T> mymap;
// insert something into mymap
// ...
typedef pair<int, T> IntTpair;
BOOST_FOREACH (IntTpair &p, mymap) {
int i = p.first;
}
As STL style containers, the pointer containers have a value_type typedef that you can use:
#include <boost/ptr_container/ptr_map.hpp>
#include <boost/foreach.hpp>
int main()
{
typedef boost::ptr_map<int, int> int_map;
int_map mymap;
BOOST_FOREACH(int_map::value_type p, mymap)
{
}
}
I find that using a typedef for the container makes the code a lot easier to write.
Also, you should try to avoid using the contents of detail namespaces in boost, it's a boost convention that they contain implementation details.
I just ran into the same problem today. Unfortunately, Daniel's suggestion will not work with a constant reference to a map. In my case, the ptr_map was a member of a class, and I wanted to loop through it in a const member function. Borrowing Daniel's example, this is what I had to do in my case:
#include "boost/ptr_container/ptr_map.hpp"
#include "boost/foreach.hpp"
int main()
{
typedef boost::ptr_map<int, int> int_map;
int_map mymap;
const int_map& mymap_const_ref(mymap);
BOOST_FOREACH(int_map::const_iterator::value_type p, mymap_const_ref)
{
}
}
It seems that int_map::const_iterator::value_type is equivalent to boost::ptr_container_detail::ref_pair<int, const int* const>.
Save yourself the typing and improve readability by using tuples:
boost::ptr_map<int, T> mymap;
int key;
T * value;
BOOST_FOREACH(boost::tie(key, value), mymap)
{
...
}
This example code compiled for me with g++ 4.1.2:
#include "boost/ptr_container/ptr_map.hpp"
#include "boost/foreach.hpp"
int main()
{
boost::ptr_map<int, int> mymap;
typedef boost::ptr_container_detail::ref_pair<int, int* const> IntPair;
BOOST_FOREACH(IntPair p, mymap)
{
int i = p.first;
}
return 0;
}
I use this homebrew template which adds an iteration type which can be handled by BOOST_FOREACH
namspace homebrew
{
template
<
class Key,
class T,
class Compare = std::less<Key>,
class CloneAllocator = boost::heap_clone_allocator,
class Allocator = std::allocator< std::pair<const Key,void*> >
>
class ptr_map :
public boost::ptr_map<Key,T,Compare,CloneAllocator,Allocator>
{
public:
typedef boost::ptr_container_detail::ref_pair<Key,const T* const> const_ref;
typedef boost::ptr_container_detail::ref_pair<Key,T* const> ref;
};
}
Let's assume that foo and bar are two of your favorite types ;)
typedef homebrew::ptr_map<foo,bar> Map;
int f( const Map& m )
{
BOOST_FOREACH(Map::const_ref v, m)
{
v.first; // foo
v.second; // const bar* const
}
}
or
int f( Map& m )
{
BOOST_FOREACH(Map::ref v, m)
{
v.first; // foo
v.second; // bar* const
}
}
Which one you have to use doesn't seem to depend on the way you use it in the loop (const or non-const) but on the map's constness!! So the following will end up in an error...
int f( Map& m )
{
BOOST_FOREACH(Map::const_ref v, m) // can't use const_ref because m isn't const
{
...
}
}
Weird! isn't it?
The greatest thing to me is that of all this solutions which were suggested here, this is the first one which is handled correctly by the Eclipse CDT syntax coloring (when using the 'Code/Problem' syntax coloring attribute).
It should compile without the reference:
BOOST_FOREACH (IntTpair p, mymap)
I think the problem is that maps do not actually store objects as pairs, but as a tree structure with the first element as the key, so BOOST_FOREACH can't get a reference to a pair but it can create a temporary copy of one.
using ::value_type won't let you const-iterate through the container. I use iterator reference types
typedef boost::ptr_map< myKey, myPtrType > MyMap;
MyMap m;
BOOST_FOREACH( MyMap::iterator::reference it, m )
do_something( it.second );
BOOST_FOREACH( MyMap::const_iterator::reference it, m )
do_something_const( it.second );
In the end, I went for declaring the iteration variable before the loop.
std::pair<std::string, TrailStep*> step;
BOOST_FOREACH(step, m_steps)
{
SAFE_DELETE(step.second);
}
But indeed, there ought to be a simpler way. (Use D instead?)
You might try this uber-cool way to iterate over maps, ptr or otherwise:
https://svn.boost.org/trac/boost/attachment/ticket/3469/foreach_field.hpp
// no typedef needed
BOOST_FOREACH_FIELD((int i)(const T& t), mymap)
// do something with i or t instead of first/second
I'm not sure it will work with a template parameter, but maybe you used that just for abstraction.