Functional Programming in C++

README.md

#Compilng You need g++ 4.9 to compile this code. Follow these steps to install g++-4.9

After installing run the following command to compile

/usr/bin/g++-4.9 -std=c++11 lambda.cpp

#Running

./a.out

lambda.cpp

#include <iostream>
#include <vector>
#include <algorithm>

/**
  Example showing the function programming ideas in
  C++ 11. Features used here are part of standard language.  
**/

template <typename Collection,typename unop>
void for_each(Collection col, unop op){
  std::for_each(col.begin(),col.end(),op);
}

template <typename Collection,typename unop>
  Collection map(Collection col,unop op) {
  std::transform(col.begin(),col.end(),col.begin(),op);
  return col;
}

template <typename Collection,typename binop>
Collection zip(Collection fc,Collection sc,binop op) {
   std::transform(fc.begin(),fc.end(),sc.begin(),fc.begin(),op);
   return fc;
}

template <typename Collection,typename Condition>
bool exists(Collection col,Condition con) {
   auto exist = std::find_if(col.begin(),col.end(),con);
   return exist != col.end();
}


template <typename Collection,typename Predicate>
Collection filterNot(Collection col,Predicate predicate ) {   
    auto returnIterator = std::remove_if(col.begin(),col.end(),predicate);
    col.erase(returnIterator,std::end(col));    
    return col;
}

template <typename Collection,typename Predicate>
Collection filter(Collection col,Predicate predicate) {
 auto fnCol = filterNot(col,[predicate](typename Collection::value_type i) { return !predicate(i);});
 return fnCol; 
}

int main()
{
  //helper println
  auto println = [](const char  *message){ std::cout << message << std::endl;};
  
  auto lambda_echo = [](int i ) { std::cout << i << std::endl; };  
  std::vector<int> col{20,24,37,42,23,45,37};
 
  //call foreach to print the vector
  println("running foreach");
  for_each(col,lambda_echo);

  // add one to each element
  println("running map operation");
  auto addOne = [] (int i) { return i+1;};
  auto returnCol = map(col,addOne);
  for_each(returnCol,lambda_echo);
 
  //zip operation
  println("running zip operation");
  auto zipAdd = [] (int a, int b){ return a+b;};
  std::vector<int> secondCol{40,22,22,24,23,45,34};
  auto zipCol = zip(col,secondCol,zipAdd);
  for_each(zipCol,lambda_echo);

  println("runnig exists");
  //prints 1 if true 0 if false
  std::cout << "value 20 exist= " << exists(col, [] (int value){ return value==20;}) << std::endl;
  std::cout << "value 43 exist= " << exists(col, [] (int value){ return value==43;}) << std::endl;

  println("running filterNot");
  auto filterCol = filterNot(col,[](int value){ return value==23;});
  for_each(filterCol,lambda_echo);

  println("running filter");
  auto filteredCol = filter(col,[](int value){ return value > 30;});
  for_each(filteredCol,lambda_echo);  
}

@stokes3452 Thank you. updated it.

Is there a way to generalize your map combinator so that the output collection does not necessarily have the same type as the input? For example, say I want to take a vector of ints and get a vector of vectors of ints. Perhaps I have misunderstood your code or my compiler errors?

Here is my attempt (which generates compiler errors I don't know how to fix):

template <typename Collection, typename function> 
auto map(function f, Collection c)  {
  auto result;
  std::transform(c.begin(),c.end(),result.begin(),f);
  return result;
}

A more stl-ish approach of map with a the additional ability to define a different target container would be

template<typename RET_CONT, typename IT, typename OPERATION>
RET_CONT map(IT begin, IT end, OPERATION op)
{
   RET_CONT ret;
   std::transform(begin, end, back_inserter(ret), op);
   return ret;
}

its usage would then be

std::vector<int> in = { 0, 1, 2, 3 }:

typedef std::list<int> target_cont_type;
target_cont_type out = map<target_cont_type>(in.begin(), in.end(), [] (auto i) { return i * 10; });