Logo

Programming-Idioms

This language bar is your friend. Select your favorite languages!
  • Perl

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.

$p = sub { $_[0] };

while ( ($k,$v) = each %m ) {
    $f{$k} = $v if $p->($v);
}

%m = %f;

$p is an anonymous function that (in this example) simply returns the argument it is passed, which is then taken for its boolean value. It could, of course, be much more complex.

The while loop extracts the keys and values from the map (hash, in perl lingo) and creates a new hash %f whenever the value satisfies the predicate function. (Deleting keys during iteration is unpredictable so we use a second hash). When done, we assign the new hash to the old one.
$p = sub { $_[0] };

foreach $k (keys %m) {
    delete $m{$k} if not $p->( $m{$k} );            
}

$p is an anonymous function that simply returns the value of its argument, which is expected to be interpreted as a boolean. It could be more complex.

The foreach loop iterates over the keys of the map (or hash, in perl parlance) and the code block deletes entries that return false if the predicate function returns false given the entry value.
for (auto it = m.begin(); it != m.end();) {
  if (!p(it->second)) {
    it = m.erase(it);
  } else {
    ++it;
  }
}

Filters in place.
New implementation...
< > ⌨
programming-idioms.org 
$p = sub { $_[0] };

while ( ($k,$v) = each %m ) {
    $f{$k} = $v if $p->($v);
}

%m = %f;
$p = sub { $_[0] };

foreach $k (keys %m) {
    delete $m{$k} if not $p->( $m{$k} );            
}

for (auto it = m.begin(); it != m.end();) {
  if (!p(it->second)) {
    it = m.erase(it);
  } else {
    ++it;
  }
}
m.Where(p);
  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


maps.DeleteFunc(m, func(k K, v V) bool {
	return !p(v)
})
for k, v := range m {
	if !p(v) {
		delete(m, k)
	}
}
Iterator<?> i = m.entrySet().iterator();
while (i.hasNext()) 
    if (!p.test(i.next())) i.remove();
m.entrySet().removeIf(e -> !p.test(e));
for i := m.count-1 downto 0 do
  if not p(m.items[i]) then m.delete(i);
m = dict(filter(p, m.items()))
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));