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Showing content from https://cplusplus.com/reference/valarray/valarray/apply/ below:

public member function

<valarray>

std::valarray::apply 
valarray apply (T func(T)) const;valarray apply (T func(const T&)) const;

Apply function

Returns a valarray with each of its elements initialized to the result of applying func to its corresponding element in *this.

The valarray returned has the same length as *this.



Parameters
func
Pointer to a function taking an argument of type T (or a constant reference) and returning T.
T is the template argument of valarray (the value type).

Return value A valarray object with the results of applying func to all the elements of *this.

Example 
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// valarray::apply example
#include <iostream>     // std::cout
#include <cstddef>      // std::size_t
#include <valarray>     // std::valarray

int increment (int x) {return ++x;}

int main ()
{
  int init[]={10,20,30,40,50};
  std::valarray<int> foo (init,5);

  std::valarray<int> bar = foo.apply(increment);

  std::cout << "foo contains:";
  for (std::size_t n=0; n<bar.size(); n++)
	  std::cout << ' ' << bar[n];
  std::cout << '\n';

  return 0;
}

Output:


Complexity Depends on library implementation (operations may be parallelized).

Iterator validity No changes: Valid iterators, references and sub-arrays keep their validity.

Data races Both the valarray and its elements are accessed.

Exception safety If any operation performed on the elements throws an exception, it causes undefined behavior.
If the function fails to allocate storage it may throw an exception (such as bad_alloc), although this is not mandated.

See also
valarray::operator=
Assign content (public member function)
valarray operators
Valarray operators (function)

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