selectKeys -- select a part of a hash table by keys

• Usage:

  selectKeys(x, f)

  selectKeys(n, x, f)

• Inputs:
  • n, an integer,
  • x, a hash table, must be immutable;
  • f, a function,
• Outputs:
  • a hash table, containing all (or n, if it is given) key-value pairs (k,v) from x for which f k evaluates to true.

i1 : x = hashTable{(1, a), (2, b), (3, c), (4, d), (5, e)}

o1 = HashTable{1 => a}
               2 => b
               3 => c
               4 => d
               5 => e

o1 : HashTable

i2 : selectKeys(x, odd)

o2 = HashTable{1 => a}
               3 => c
               5 => e

o2 : HashTable

i3 : selectKeys(2, x, odd)

o3 = HashTable{1 => a}
               3 => c

o3 : HashTable