Generator/Exponential

Generator/Exponential is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

A generator is an executable entity (like a function or procedure) that contains code that yields a sequence of values, one at a time, so that each time you call the generator, the next value in the sequence is provided. Generators are often built on top of coroutines or objects so that the internal state of the object is handled “naturally”. Generators are often used in situations where a sequence is potentially infinite, and where it is possible to construct the next value of the sequence with only minimal state.

See also:

• http://en.wikipedia.org/wiki/Generator_(computer_science)

Task: Write a generator (or generators), in the most natural way in your language, that produces the numbers that are squares ($n^{2}$) but not cubes ($m^{3}$) and use that to print the first 21st to 30th members of that sequence.

Tcl

Works with: Tcl version 8.6

Tcl implements generators in terms of coroutines. If these generators were terminating, they would finish by doing return -code break so as to terminate the calling loop context that is doing the extraction of the values from the generator. <lang tcl>package require Tcl 8.6

proc powers m {

   yield
   for {set n 0} true {incr n} {

yield [expr {$n ** $m}]

   }

} coroutine squares powers 2 coroutine cubes powers 3 coroutine filtered apply {{s1 s2} {

   yield
   set f [$s2]
   set v [$s1]
   while true {

if {$v > $f} { set f [$s2] continue } elseif {$v < $f} { yield $v } set v [$s1]

   }

}} squares cubes

1. Drop 20

for {set i 0} {$i<20} {incr i} {filtered}

1. Take/print 10

for {} {$i<30} {incr i} {

   puts [filtered]

}</lang> Output:

529
576
625
676
784
841
900
961
1024
1089