Linq for list comprehension in C++
Linq for list comprehension in C++, provides an implementation of linq for C++. Currently it only supports C++ ranges, but it does support both the extension and query methods for linq. Here's an example:
struct student_t
{
std::string last_name;
std::vector<int> scores;
};
std::vector<student_t> students =
{
{"Omelchenko", {97, 72, 81, 60}},
{"O'Donnell", {75, 84, 91, 39}},
{"Mortensen", {88, 94, 65, 85}},
{"Garcia", {97, 89, 85, 82}},
{"Beebe", {35, 72, 91, 70}}
};
auto scores = LINQ(from(student, students)
from(score, student.scores)
where(score > 90)
select(std::make_pair(student.last_name, score)));
for (auto x : scores)
{
printf("%s score: %i\n", x.first.c_str(), x.second);
}
The C++ code above will outputs this(yes that is C++ code above):
Omelchenko score: 97
O'Donnell score: 91
Mortensen score: 94
Garcia score: 97
Beebe score: 91
Extensions are implemented using the | pipe operator. This allows them to be applied to any range, without requiring inherting from some special base class. So extensions can work like this:
vector<int> numbers = { 1, 2, 3, 4, 5, 6, 7, 8 };
auto r = numbers
| linq::where([](int x) { return x > 2; })
| linq::select([](int x) { return x * x; });
The extension methods supported are:
The library also provides a range_extension class, that can be used to write your own extensions, as well. First just define the function as a function object class, like this:
struct contains_t
{
template<class Range, class T>
bool operator()(Range && r, T && x) const
{ return (r | linq::find(x)) != boost::end(r); };
};
Then initialize the extension using static initialization:
range_extension<contains_t> contains = {};
Then the extension can be used like this:
if (numbers | contains(5)) printf("We have a 5");
All linq queries must start with a from statement. This specifies the variable name to be used for the lambdas and the container that the queries will be applied to. Also, multiple from statements can be used.
struct student
{
std::string name;
std::vector<int> grades;
student() {}
template<class Range>
student(std::string name, Range&& r)
: name(name), grades(boost::begin(r), boost::end(r))
{}
};
std::vector<student> students =
{
student("Bob", { 90, 100, 75 })
student("Tom", { 92, 81, 70 })
student("Terry", { 105, 98, 94 })
};
// { 90, 100, 75, 92, 81, 70, 105, 98, 94 }
auto q = LINQ(from(s, students) from(g, s.grades) select(g));
The where clause returns the element that matches the predicate. It is optional but must come after a from clause and should be before a select clause if there is one.
vector<int> v = { 1, 3, 4, 5 };
// { 1, 3, 5 }
auto q = LINQ(from(i, v) where(i % 2));
The select clause applies a transformation to the elements. It is optional also, but should be the very last clause.
std::vector<int> v = { 1, 2, 4 };
// { 3, 6, 24 }
auto q = LINQ(from(x, v) select(x * 3));
The orderby clause lets you specify elements to order the range by. Also ascending or descending can be preceded by the selector in order to specify the direction of ordering. The default is ascending.
struct person
{
std::string name;
int age;
person() {}
person(std::string name, int age)
: name(name), age(age)
{}
};
std::vector<person> people =
{
person("Tom", 25),
person("Bob", 22),
person("Terry", 37),
person("Jerry", 22)
}
// { "Jerry", "Bob", "Tom", "Terry" }
auto q = LINQ(from(p, people) orderby(p.age, descending p.name) select(p.name));
The group clause groups the elements of a sequence. The first parameter is the key selector, and the second one is the value selector.
struct person
{
std::string name;
int age;
person() {}
person(std::string name, int age)
: name(name), age(age)
{}
};
std::vector<person> people =
{
person("Tom", 25),
person("Bob", 22),
person("Terry", 37),
person("Jerry", 22)
}
auto q = LINQ(from(p, people) group(p.age, p.name));
Just copy the linq.h and linq folder to the include directory for your compiler. Then just include the linq.h header, to include all of linq.
The linq/extensions.h header will just include whats needed for the extension methods, without the query syntax, and the linq/query.h will include the query syntax as well.
For full support it requires clang or gcc, and boost. There is partial support for visual studio. Visual studio doesn't support the default_if_empty, group_by, group_join, join, order_by, select_many, and then_by extensions, and it doesn't support orderby, group, and nested from clauses. Perhaps some visual studio wizards could help find workarounds for msvc bugs.
Multiple from statements are implemented using nested lambdas. However, because of a bug in MSVC 2010, nested lambas don't work(This should be fixed in MSVC 2012, but I haven't tested it). If there is a way to implement transparent identifiers in C++, nested lambdas could be avoided.
Also, the let, and join clauses aren't supported yet. The into clause can't be supported without the help of polymorhpic lambdas.