Applying

1.5.3.3 Applying


ufunc for_each(i,i,ufunc)	Apply a function to every item in a range and return the function.	ufunc may not return value.

void fun(int n){
    cout<<"␣"<<n;
} ____________________________________________________________________________________//all function definition end no semicolon

struct Sum{
    Sum(): sumEven{0} { }  _____________________________________________//no semicolon here
    void operator()(int n) { if(n%2 ==0) ; sumEven += n; }
    int sumEven;
}; ____________________________________________________________// type definition need semicolon

vector<int> nums{3, 4, 2, 8, 15, 267};
for_each(nums.begin(), nums.end(), fun); ________________________// no () here
Sum s = for_each(nums.begin(), nums.end(), Sum()); ________________________//have() here
for_each(nums.begin(), nums.end(), [sumEven](int n){
                                               if(n%2==0) sumEven+=n;
                                               });

for_each need a functor, So when to use for_each eaual another question, when use functor.
1. For fun example, a better way is for(auto e: nums){cout�e�" ";}. Here just demonstrate that you can input fun.
2. For struct Sum just use once, lambda function is better, because it is cleaner.
3. If functor need to be 1)customized state and 2)reuse many time, then functor is better.
  struct GreatThanX{
      GreatThanX(int x): cutoff{x} { }  _____________________________________________//no semicolon here
      bool operator()(int n) { if(n>x) ; return true; return false; }
      int cutoff;
  };
  
  vector<int> nums{3, 4, 2, 8, 15, 267};
  find_if(nums.begin(), nums.end(),GreatThanX(3));
  copy_if(nums.begin(), nums.end(),GreatThanX(7));
  ..........
4. Only functor can be use a argument input to map or set container set. See below examples.
  struct lex_compare {
      bool operator() (const int64_t& lhs, const int64_t& rhs) const{
          stringstream s1,s2;        s1 << lhs;  s2 << rhs;
          return s1.str() < s2.str();
      }
  };
  set<int64_t, lex_compare> s;
5. Lambdas aren’t useful for more complex scenarios because they weren’t made for them. They provide a short and concise way of creating simple function objects for correspondingly simple situations.
6. with function, bind and name lambda, You can reach above requirement. but it’s not as good as functor
  auto f = [](int x, int y){if(y>x) return true; return false;} ;
  auto f1 = bind(f, 3, placeholders::_1) // GreatThanX(3);
  if(f1(8)) cout<<"8>3"<<endl;

Transforming

o transform(i1,i1end,o,ufunc)

o transform(i1,i1end,i2, o, bfunc)

Transform one range of values into another.

Ret of ufunc or bfunc writed to o.

If output is associate container, you need use inserter( ) function to get inserter iterator.
std::transform(a.begin(), a.end(),
std::inserter(set, set.begin()), modify);

std::transform(a.begin(), a.end(), b.begin(),
a.begin(), plus<int>() );
// 1) you can make in place modification.
// 2) plus minus, multiplies, divides, modulus, negate, equal_to
// are often used in transform algorithms.

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