Ask a Jedi: "Object of type class.lang.Boolean cannot be used as an array"

This post is more than 2 years old.

Darren asks:

is there anything wrong with this function because when i invoke it i get a weird error i don't understand which is "Object of type class.lang.Boolean cannot be used as an array"

all i'm doing is query the db and compiling it into a 1 dimensional array. heres the function

(some stuff deleted)
<cfquery name="getExtraImages" datasource="#request.dsn#"> SELECT * FROM extraimages </cfquery>

<cfset imageArray = arrayNew(1)>

<cfloop query="getExtraImages" startrow="1" endrow="#getExtraImages.recordcount#">
<cfset imageArray = arrayAppend(imageArray, "#extraImagePath#")>
</cfloop>

This is a simple problem, and one I see often. I'm pretty sure I answered this question before on the blog before, but as I said - it shows up often. The problem is the arrayAppend function. You would think that it takes an array, a value to add to it, and returns the new (larger) array. Instead, the function changes the array you pass to it directly and returns true or false. Now - I've never seen arrayAppend return false before, so it isn't something you have to worry about checking, but you do need to ensure you use it the right way. I'd change his line above to:

<cfset arrayAppend(imageArray, extraImagePath)>

Note that I removed the unnecessary pound signs as well. If that line confuses you, you can think of it like running a function and assigning to nothing. Imagine this:

<cfset temp = arrayAppend(imageArray, extraImagePath)>

Now imagine you don't need temp. That's where the shorter syntax is useful.

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Archived Comments

Comment 1 by Nathan Mische posted on 1/2/2008 at 6:57 PM

I think people tend to get confused due to the inconsistency between arrayAppend() and listAppend(). Unlike arrayAppend() you do need to assign the result of listAppend() to a new variable as it returns a new list and leaves the original list unchanged.

Comment 2 by Shane Zehnder posted on 1/2/2008 at 8:51 PM

<cfdump> is your friend. Or CFQuickDocs ;)

I have had that problem before. It's not always easy to remember which array functions return a boolean or a reference to the array.

Comment 3 by James White posted on 5/21/2009 at 12:03 AM

Glad i found this post. I know you cover this last year, but it certainly helped me this year :-).

Thanks,

JW

Comment 4 by Gary Stanton posted on 12/18/2013 at 9:57 PM

Wish that ListAppend worked this way... makes more sense to me.