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[Acceleo] How to test for empty/null reference? [message #639585]

Wed, 17 November 2010 03:08

Glenview Jeff

Messages: 79
Registered: September 2010

Member

This should be a really easy question to answer, but I can't figure it out from the Acceleo manual.

I can't figure out how to test for a null reference so I can skip it. I have some code that iterates through a structure. The top level type is called an Activity and it contains a reference 0..1 of menu. Not all activities have menus in the model and thus I get the error I pasted below the code.

Thanks for your help!

line 8: [for (anActivity : Activity | myActivities.activities)]
line 9: 	[for (aMenu : Menu | anActivity.menu)]

Invalid loop iteration at line 9 in Module Strings for block for (anActivity.menu.items). Last recorded value of self was flashcards.activities.impl.AnyMemoImpl@14c816b (name: AnyMemo).

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Re: [Acceleo] How to test for empty/null reference? [message #639588 is a reply to message #639585]

Wed, 17 November 2010 04:08

Glenview Jeff

Messages: 79
Registered: September 2010

Member

Update: I noticed that if I change the Activity to contain 0..* menus then the for loops work fine. But I want to limit there to be only 0 or 1 menus. Is this an Acceleo bug or am I misunderstanding something?

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Re: [Acceleo] How to test for empty/null reference? [message #639611 is a reply to message #639588]

Wed, 17 November 2010 07:59

Laurent Goubet

Messages: 1902
Registered: July 2009

Senior Member

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Hi Jeff,

This will work well if you set it to be 0..* because EMF will never
return "null" in such cases : it is a collection, whether it is empty or
not. In the case of "0..1" multiplicity, you have either "null" or an
object.

The problem you have is that you tried to do a "for" loop on such a 0..1
reference. Acceleo accepts "for" on an object which is not a Collection
(it will simply iterate once), but it will refuse iterating on an Object
which is either "null" or "invalid"... which is what you get, and will
not be changed.

I don't understand why you are using a "for" loop at this point. If the
reference is 0..1, you either have something or not. Why not use an "if"?

[for (anActivity : Activity | myActivities.activities)]
[if (not anActivity.oclIsUndefined())]

Laurent Goubet
Obeo

On 17/11/2010 05:08, glenviewjeff wrote:
> Update: I noticed that if I change the Activity to contain 0..* menus
> then the for loops work fine. But I want to limit there to be only 0 or
> 1 menus. Is this an Acceleo bug or am I misunderstanding something?

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Re: [Acceleo] How to test for empty/null reference? [message #639751 is a reply to message #639611]

Wed, 17 November 2010 16:39

Glenview Jeff

Messages: 79
Registered: September 2010

Member

Thanks so much for the prompt help Laurent. I had only used the for loop since I couldn't figure out the syntax to determine if a reference was null.

For future readers, the correct code for my example is:

	[if (not anActivity.menu.oclIsUndefined())]

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Re: [Acceleo] How to test for empty/null reference? [message #639778 is a reply to message #639751]

Wed, 17 November 2010 18:08

Ed Willink

Messages: 7655
Registered: July 2009

Senior Member

Hi

The not very obvious OCL idiom is

aVariable->notEmpty()

if aVariable is invalid result is invalid
else if aVariable is null result is false
else result is true
-- a real object

Regards

Ed Willink

On 17/11/2010 16:39, glenviewjeff wrote:
> Thanks so much for the prompt help Laurent. I had only used the for
> loop since I couldn't figure out the syntax to determine if a
> reference was null.
>
> For future readers, the correct code for my example is:
>
> [if (not anActivity.menu.oclIsUndefined())]
>

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Re: [Acceleo] How to test for empty/null reference? [message #643185 is a reply to message #639778]

Sun, 05 December 2010 20:02

Glenview Jeff

Messages: 79
Registered: September 2010

Member

Thanks very much Ed. Your method is far more readable. Javadoc or the equivalent for the methods auto-completion on variables would be really helpful.

Jeff

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