# Recursive method not working

**URL:** <https://www.ruby-forum.com/t/recursive-method-not-working/92362>\
**Category:** Ruby\
**Created:** [April 18, 2007, 6:05pm UTC](https://www.ruby-forum.com/t/recursive-method-not-working/92362 "2007-04-18T18:05:34Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Zouplaz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/a4c791/32.png) [@Zouplaz](https://www.ruby-forum.com/u/Zouplaz)\
**Post date:** [April 18, 2007, 6:05pm UTC](https://www.ruby-forum.com/t/recursive-method-not-working/92362/1 "2007-04-18T18:05:34Z")

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Hi, I’m trying to display a hierarchical tree but there’s something  
wrong with the method below.  
The result var is ‘lost’ between the calls - I mean, each “#{result}”  
contains the right value at the end of the method (I checked that), but  
that value is lost when returning to the upper level.

I don’t see why…

Thanks for your help !

def affiche\_arbre(rubriques)  
for rubrique in rubriques  
result = ‘  
’  
0.step(rubrique.level) { result += ’&nbsp;‘}  
result += rubrique.libelle + ’ ’ + link\_to(’+’, :action =\> ‘new’,  
:parent\_id =\> rubrique) + ’ ’  
unless rubrique.parent\_id == nil  
result += link\_to(‘E’, :action =\> ‘edit’, :id =\> rubrique) + ’  
’ + link\_to(‘D’, :action =\> ‘delete’, :id =\> rubrique )  
end  
if rubrique.children.size \> 0  
result += affiche\_arbre(rubrique.children)  
end  
end  
“#{result}”  
end

---

<div class="post-metadata">

**Author:** ![Zouplaz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/a4c791/32.png) [@Zouplaz](https://www.ruby-forum.com/u/Zouplaz)\
**Post date:** [April 18, 2007, 6:16pm UTC](https://www.ruby-forum.com/t/recursive-method-not-working/92362/2 "2007-04-18T18:16:35Z")

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On 18.04.2007 18:01, Zouplaz wrote:

> def affiche\_arbre(rubriques)  
> I’d try to insert

result=""

here.

> ```
> result += affiche_arbre(rubrique.children)
> end
> end
> "#{result}"
> 
> ```

Why do you do that? You could simply return result.

> end

A general note: using \<\< instead of += is much more efficient. You can  
get even better by passing the result like this:

def meth(node, result="")  
…  
result \<\< “START”  
…

# recursion

meth(another\_node, result)  
…  
result \<\< “END”  
…  
result  
end

Kind regards

robert

---

<div class="post-metadata">

**Author:** ![Zouplaz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/a4c791/32.png) [@Zouplaz](https://www.ruby-forum.com/u/Zouplaz)\
**Post date:** [April 18, 2007, 6:46pm UTC](https://www.ruby-forum.com/t/recursive-method-not-working/92362/3 "2007-04-18T18:46:36Z")

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On 4/18/07, Robert K. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > ```
> > 0.step(rubrique.level) { result += '&nbsp;'}
> > "#{result}"
> > 
> > ```
> 
> result \<\< “START”  
> Kind regards
> 
> ```
> robert
> 
> ```

More remarks:

you should change

> > ```
> > result = '<br/>'
> > 
> > ```

to  
result \<\< ‘  
’  
as well

now some enhacements:

> > ```
> > for rubrique in rubriques
> > 
> > ```

rubriques.each do |rubrique|  
– it’s a question of style/taste otherwise equal

> > ```
> > unless rubrique.parent_id == nil
> > 
> > ```

unless rubrique.parent\_id.nil?

> > ```
> > if rubrique.children.size > 0
> > 
> > ```

unless rubrique.children.empty?  
– this one may be faster sometimes (if size computation is slow)  
and it more explicitly shows what are you asking

> > ```
> > 0.step(rubrique.level) { result += '&nbsp;'}
> > 
> > ```

(rubrique.level + 1).times { result \<\< ’&nbsp;‘}  
or  
result \<\< (’&nbsp;’ \* (rubrique.level +1))  
– the first one is more descriptive, IMO

Jano

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<div class="post-metadata">

**Author:** ![Zouplaz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/a4c791/32.png) [@Zouplaz](https://www.ruby-forum.com/u/Zouplaz)\
**Post date:** [April 18, 2007, 10:26pm UTC](https://www.ruby-forum.com/t/recursive-method-not-working/92362/4 "2007-04-18T22:26:01Z")

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le 18/04/2007 21:18, Brian C. nous a dit:

> Well, the only recursive call I can see is line 10. You append the return  
> values from these calls onto ‘result’. But then in line 12 you go back  
> around the loop, and in line 3 you reinitialise result, thereby throwing  
> away everything you’ve calculated so far.

You got it ! Thank you !

---

<div class="post-metadata">

**Author:** ![Zouplaz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/a4c791/32.png) [@Zouplaz](https://www.ruby-forum.com/u/Zouplaz)\
**Post date:** [April 18, 2007, 9:19pm UTC](https://www.ruby-forum.com/t/recursive-method-not-working/92362/5 "2007-04-18T21:19:53Z")

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On Thu, Apr 19, 2007 at 01:05:05AM +0900, Zouplaz wrote:

> Hi, I’m trying to display a hierarchical tree but there’s something  
> wrong with the method below.  
> The result var is ‘lost’ between the calls - I mean, each “#{result}”  
> contains the right value at the end of the method (I checked that), but  
> that value is lost when returning to the upper level.
> 
> I don’t see why…
> 
> Thanks for your help !

1\> def affiche\_arbre(rubriques)  
2\> for rubrique in rubriques  
3\> result = ‘  
’  
4\> 0.step(rubrique.level) { result += ’&nbsp;‘}  
5\> result += rubrique.libelle + ’ ’ + link\_to(’+’, :action =\>  
‘new’, :parent\_id =\> rubrique) + ’ ’  
6\> unless rubrique.parent\_id == nil  
7\> result += link\_to(‘E’, :action =\> ‘edit’, :id =\> rubrique) +  
’ ’ + link\_to(‘D’, :action =\> ‘delete’, :id =\> rubrique )  
8\> end  
9\> if rubrique.children.size \> 0  
10\> result += affiche\_arbre(rubrique.children)  
11\> end  
12\> end  
13\> “#{result}”  
14\> end

Well, the only recursive call I can see is line 10. You append the  
return  
values from these calls onto ‘result’. But then in line 12 you go back  
around the loop, and in line 3 you reinitialise result, thereby throwing  
away everything you’ve calculated so far.

---

<div class="post-metadata">

**Author:** ![Zouplaz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/a4c791/32.png) [@Zouplaz](https://www.ruby-forum.com/u/Zouplaz)\
**Post date:** [April 19, 2007, 9:15am UTC](https://www.ruby-forum.com/t/recursive-method-not-working/92362/6 "2007-04-19T09:15:55Z")

</div>

On Apr 18, 9:01 am, Zouplaz [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> ```
> if rubrique.children.size > 0
> result += affiche_arbre(rubrique.children)
> end
> end
> "#{result}"
> 
> ```
> 
> end

This code could also be:  
============ code ============  
links(rubrique)  
[link\_to(‘+’, :action =\> ‘new’, :parent\_id =\> rubrique)] +  
( rubrique.parent\_id ?  
[ link\_to(‘E’, :action =\> ‘edit’, :id =\> rubrique),  
link\_to(‘D’, :action =\> ‘delete’, :id =\> rubrique) ] : [] )  
end

def affiche\_arbre(rubriques)  
rubriques.collect do |rubrique|  
‘  
’ + (’&nbsp;’ \* rubrique.level) +  
[rubrique.libelle].concat(links(rubrique)).join(’ ') +  
( rubrique.children.size.zero? ? ‘’ :  
affiche\_arbre(rubrique.children) )  
end.join()  
end  
=========== /code ===============  
Please excuse the Anglicized new function – I know no French. Notice  
that this code allows you to test correctness of link generation  
separately from correctness of the generated HTML. By restricting  
myself to expressions – there are no variable assignments in this  
code, let alone mutations – I protect myself from the kind of bug  
that marred the original.