Powershell recursion failing? Variable clears when returned

Question

I'm trying to create a list of "sentences" of 1-8 "words", provided a list of words, where I don't care about uniqueness as I can just filter out duplicates later. I decided that a recursive function would be the best method, particularly in case I wanted to change the maximum length at a later time.

When I ran this code, I came back with an empty result. When I debugged and stepped through it, I found that $sents would contain data right before a return statement, then have no data immediately after returned to the $sents variable in the first else statement.

This is the second recursive function I've written for PowerShell and neither worked. The first I ended up breaking down and wrote 4 nested foreach statements. This one would be 8 nested statements (I do not wish to do that) just to build a String array of sentences so I can process them.

Any help would be appreciated. Thank you!

function make-sentences ($ws, $ml, [String[]] $sent = @(), [String[]] $sents = @()) {
    [String[]] $nws = @()
    $sent += $ws[0]

if ($sent.length -ge $ml) {
    $sents += ($sent -join ",")
    return $sents
} elseif ($ws.length -le 1) {
    $sents += ($sent -join ",")
    return $sents
} else {
    for ($i = 1; $i -lt $ws.length; $i++) { $nws += $ws[$i] }
    $sents += make-sentences $nws $ml $sent $sents
}

[String[]] $nws = @()

if ($ws.length -le 1) {
    $sents += ($sent -join ",")
    return $sents
} else {
    $sent = @()
    for ($i = 1; $i -lt $ws.length; $i++) { $nws += $ws[$i] }
    $sents += make-sentences $nws $ml $sent $sents
}
}

$words = ("acfj","acfk","adfk","aefj","aefk","aegi","aehi","afgh")
[String[]] $sentences = make-sentences $words 8

$sentences

It's honestly hard for me to tell what your approach is here. It seems like you should just do this in a while loop. However, a few things stick out. 1 - you can't change inputted non-object variables unless they are reference variables. 2 - this is overly complex for the task at hand :) — JNK
– JNK, Commented Sep 12, 2014 at 20:39

Peter Mortensen · Accepted Answer · 2016-02-08 15:18:44Z

Sorry, I don't spent much time reading your code, but a thing you must know is that in PowerShell when you call a function the default behaviour for the parameters is the following (calling by argument):

function Dumy ($param1)
{
  $param1 = 100
  Write-Host "$param1 in function Dumy"
}


$param1 = 10
Write-Host "$param1 in main function before call to Dumy"
Dumy $param1
Write-Host "$param1 in main function after call to Dumy"

It gives:

10 in main function before call to Dumy
100 in function Dumy
10 in main function after call to Dumy

If you want to modify the value of your parameter, you should use a calling by reference:

function Dumy ([ref]$param1)
{
  $param1.value = 100
  Write-Host "$($param1.value) in function Dumy"
}


$param1 = 10
Write-Host "$param1 in main function before call to Dumy"
Dumy ([ref]$param1)
Write-Host "$param1 in main function after call to Dumy"

It gives:

10 in main function before call to Dumy
100 in function Dumy
100 in main function after call to Dumy

Another solution is to use the variable scope to modify the variable in a function in the calling scope (for me it's like using global variables).

p.campbell · Accepted Answer · 2014-09-12 18:59:28Z

0

I think your $sentences variable is empty, because your function - even if recursive - doesn't have a return statement. You have a return statement for terminal cases only, but you don't ask for a return statement when calling the function recursively. Then, your function returns something inside the call stack, but your main call is not asked to return anything.

Here are two example of recursive functions for factorial, the first one looks like your implementation, and does not return anything. Indeed, the recursive calls are stored in a variable, and then the function doesn't return anything. The second example does return something, because in every case, the function is asked to return a value, even through a recursive call :

function fact-Noresult ([int]$n, [int]$result=1){
  if ($n -eq 0) {
    Write-Host "returning now ! value = $result"
    return $result
  }else {
    $result = $n * (fact-Noresult ($n-1) ($result*$n))        
  }
}

function fact-WithResult ([int]$n, [int]$result=1){
  if ($n -eq 0) {
    return $result
  }else {
    $result=$n*$result
    fact-WithResult ($n-1) $result

  }
}

If you try to call them, you'll see that the first one display the message, and display the good value, but does not return it.

edited Sep 12, 2014 at 18:59

p.campbell

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answered May 26, 2014 at 8:43

Gruntzy

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3 Comments

JNK Over a year ago

It's worth noting that return in Powershell is syntactic sugar - you can just have $result at the end of the function to return it.

Gruntzy Over a year ago

@JNK : I know that returnis not needed to make a function return something. What I meant is that the OP is trying to call a recursive function in a way that does not provide any result. That is also what i tried to show with the two functions above. My two examples are still true if you remove the returnkeywords

JNK Over a year ago

Yep, I wasn't trying to imply you didn't know, I just wanted to make it clear for anyone else reading.

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