Question

I understand everything that is happening in the following code except for the while output section:

1 def doUntilFalse firstInput, someProc
2  input  = firstInput
3  output = firstInput
4  
5  while output
6    input  = output
7    output = someProc.call input
8  end
9  
10  input
11 end
12 
13 buildArrayOfSquares = Proc.new do |array|
14  lastNumber = array.last
15  if lastNumber <= 0
16    false
17  else
18    array.pop                         #  Take off the last number...
19    array.push lastNumber*lastNumber  #  ...and replace it with its square...
20    array.push lastNumber-1           #  ...followed by the next smaller number.
21  end
22 end
23
24 alwaysFalse = Proc.new do |justIgnoreMe|
25  false
26 end
27
28 puts doUntilFalse([5], buildArrayOfSquares).inspect

I understand while for the most part but for some reason I can’t see the forest through the trees in this code. Could someone explain what is happening in the while output part between lines 5 and 8. I have no doubt it is extremely simple but I’ve hit a wall with it. Any help much appreciated.

Answer 1

output will become the return value of the procedure someProc, which is in turn passed as a parameter in line #28, as buildArrayOfSquares. This, in turn, will return false in a certain case; when this happens, the while loop will terminate.

In detail, firstInput is [5], which becomes the first input. We call buildArrayOfSquares with [5]. Since 5 is not <= 0, we take 5 out, put in 25 and 4.

Next iteration of while, output is [25, 4]. We continue. Back into buildArrayOfSquares. Take 4 off the end; push in 16, then 3. output is now [25, 16, 3].

Next time, output is [25, 16, 9, 2]. Then [25, 16, 9, 4, 1]. Then [25, 16, 9, 4, 1, 0].

And the next time, 0 <= 0, and buildArrayOfSquares returns false into output. The loop terminates. input is still [25, 16, 9, 4, 1, 0], which is what we presumably wanted.