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Idiom #280 Filter map

Remove all the elements from the map m that don't satisfy the predicate p.
Keep all the elements that do satisfy p.

Explain if the filtering happens in-place, i.e. if m is reused or if a new map is created.

  real, dimension(:), allocatable :: a
  a = pack(a,p(a))
  elemental logical function p(x)
    real, intent(in) :: x
    p = x > 0.7
  end function p

for k, v := range m {
	if !p(v) {
		delete(m, k)
import "golang.org/x/exp/maps"
maps.DeleteFunc(m, func(k K, v V) bool {
	return !p(v)
uses classes;
for i := m.count-1 downto 0 do
  if not p(m.items[i]) then m.delete(i);
for k in list(m):
    if p(m[k]): m.pop(k)
m = {k:v for k, v in m.items() if p(v)}
m.select{|k,v| p(v) }
m.retain(|_, &mut v| p(v));

New implementation...
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