functional programming in c++

16 May 2014

I’m always thinking c++ is very boring, but it can be fun

Iterator Collection

  std::vector<int> v1;
  v1.push_back(111);
  v1.push_back(222);
  v1.push_back(333);

create a vector

1 simplest way to iterator a vector

for (auto i = v1.begin(); i < v1.end(); i++)
{
    std::cout << *i << std::endl;
}

auto is new syntax in c++ 11

2 use for_each

#include <algorithm>
std::for_each(v1.begin(),v1.end(),[](int i){ std::cout << i << std::endl; });

[](int i){ std::cout << i << std::endl; } is lambda in c++, and it can be replaced by function

return a value from lambda

std::transform(v1.begin(),v2.end(),std::back_inserter(v2),[](int n)->int { return n * n * n ;});

here I create a new vector v2, and want to transform the item from v1 to v2, use std::transform from <algorithm>, the last parameter it accept is a lambda, that return int ->int { return n * n * n ;}

capture

[x,y](int n)->int { return x * y * n ;}

[&x,&y](int n)->int { return x * y * n ;}  // can also pass by reference

[=] //copy "everything"

[&] //copy everything my reference

this will copy x and y from local scope into lambda, then when lambda execute, it can get the copy of x and y

std::for_each(v1.begin(),v1.end(),[&x,y](int i){ 
    std::cout << i << " " << x << " "<< y << std::endl; 
    x = 9;
});

populating a vector

	//use for loop
	for (int i =0; i <5; i++)
	{
		v.push_bach(i);
	}

	// use algorithm
    vector<int> v;
    int i=0;
    std::generate_n(std::back_inserter(v),5,[&i]()->int{ return i++; });

counting the number of 3’s

use algorithm cpp int count3 = std::count(v.begin(),v.end(),3); std::cout << "the count3 of v is " << count3 << std::endl; use for loop

	
	int count = 0;
	for(unsigned int i = 0; i<v.size(); i++)
	{
		if(v[i] == 3)
			count++;
	}

use iterator

	int count = 0;
	for(auto it = begin(v); it != end(v); it++)
	{
		if(*it == 3)
		{
			count++;
		}
	}

remove element from vector

** the result of remove_if point to the end of the vector, for preformance reason, remove_if didn’t erase element, it will push the “removed” element back to end**

	  vector<int> v;
      int i=0;
      std::generate_n(std::back_inserter(v),5,[&i]()->int{ return i++; });
      auto vcopy = v;
	  auto endv = std::remove_if(begin(vcopy),end(vcopy),[=](int elem)->bool { return (elem == 2 || elem == 3);});
      vcopy.erase(endv, end(vcopy));

vcopy.erase(endv, end(vcopy)); means remove from the new end to original end

function pointer

typedef int (*CHANGER)(int i);

define a function pointer CHANGER, take integer as parameter and return integer type

int doubler(int i)
{
	return i * 2;
}

int tripler(int i)
{
	return i * 3;
}

CHANGER = doubler;

CHANGER(2);  ==> 4

Pointer to Member Function

typedef int (Utility::* UF)();

UF action;
action = &Utility::triplex;

call:
Utility u(i);
UF action;
if(i > 3)
{
	action = &Utility::triplex;
}
else
{
	action = &Utility::doublex;
}
return (u.*action)();  //because it is pointer, need to be deferenced

void*

can be cast to any kind of pointer

void SomeMethod(void* something)
{
	int* s = (int*) something;
	cout << *s << endl;
}

cpp 1