# Why is "nil" being included in an array?

**URL:** <https://www.ruby-forum.com/t/why-is-nil-being-included-in-an-array/64174>\
**Category:** Ruby\
**Created:** [June 22, 2006, 8:22pm UTC](https://www.ruby-forum.com/t/why-is-nil-being-included-in-an-array/64174 "2006-06-22T20:22:44Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\
**Post date:** [June 22, 2006, 8:22pm UTC](https://www.ruby-forum.com/t/why-is-nil-being-included-in-an-array/64174/1 "2006-06-22T20:22:44Z")

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I’m trying to simply lower-case all of the files in a directory. It  
doesn’t work because RUBY complains about not being able to convert  
“nil” to a string. Why in the world is it even including “nil” in my  
array?

files = Dir.glob(’\*.pdf’)  
files.each do |file|  
File.rename(file, file.downcase!)  
end

It says:  
“…can’t convert nil into String (Type error)…”

Thanks,  
Peter

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<div class="post-metadata">

**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\
**Post date:** [June 22, 2006, 8:40pm UTC](https://www.ruby-forum.com/t/why-is-nil-being-included-in-an-array/64174/2 "2006-06-22T20:40:05Z")

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On 6/22/06, Peter B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> It says:  
> “…can’t convert nil into String (Type error)…”

I think you’re mistaking the source of your error here. Check this out  
(ri String#downcase!):

```
Downcases the contents of _str_, returning +nil+ if no changes were
made.

```

So you’re call to +downcase!+ is returning nil in the cases where  
+file+ was already downcased. When +File.rename+ sees that nil and  
tries to convert it to a String, it raises the error you see.

Try using +String#downcase+ instead (note the lack of the exclamation  
point).

Jacob F.

---

<div class="post-metadata">

**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\
**Post date:** [June 22, 2006, 8:40pm UTC](https://www.ruby-forum.com/t/why-is-nil-being-included-in-an-array/64174/3 "2006-06-22T20:40:14Z")

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On Jun 22, 2006, at 19:22, Peter B. wrote:

> I’m trying to simply lower-case all of the files in a directory. It  
> doesn’t work because RUBY complains about not being able to convert  
> “nil” to a string. Why in the world is it even including “nil” in my  
> array?
> 
> files = Dir.glob(’\*.pdf’)  
> files.each do |file|  
> File.rename(file, file.downcase!)  
> end

An excellent illustration of why side-effects can be harmful.

file.downcase! has to be evaluated before File.rename can be  
evaluated. #downcase! returns nil if no changes are made, so for any  
file that’s already in lower case, you’re making this call:

File.rename(file, nil)

You can fix this if you change the ! method to the normal #downcase.  
This will also help you avoid a more subtle bug:

files = Dir.glob(‘Test’)  
=\> [“Test”]  
files.each do |file|  
File.rename(file, file.downcase!)  
end  
files = Dir.glob(‘Test’)  
=\> [“Test”] # What the hell? shouldn’t that be gone?

It’s the same bug biting you in a different way: this time, because  
#downcase! is evaluated before the call to #rename, it’s modifying  
the file variable in-place, so #rename is effectively being called  
like this:

File.rename(‘test’, ‘test’)

Where ‘test’ obviously doesn’t exist in the first place (or even if  
it did, it’s still not what you wanted).

matthew smillie.

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<div class="post-metadata">

**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\
**Post date:** [June 22, 2006, 11:31pm UTC](https://www.ruby-forum.com/t/why-is-nil-being-included-in-an-array/64174/4 "2006-06-22T23:31:04Z")

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On 6/22/06, Matthew S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> It’s the same bug biting you in a different way: this time, because  
> #downcase! is evaluated before the call to #rename, it’s modifying  
> the file variable in-place, so #rename is effectively being called  
> like this:
> 
> File.rename(‘test’, ‘test’)

I had no idea that Ruby evaluated it that way. Thank you for this  
detailed explanation.

Pat

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<div class="post-metadata">

**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\
**Post date:** [June 23, 2006, 2:50pm UTC](https://www.ruby-forum.com/t/why-is-nil-being-included-in-an-array/64174/5 "2006-06-23T14:50:35Z")

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Jacob F. wrote:

> On 6/22/06, Peter B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:
> 
> > It says:  
> > “…can’t convert nil into String (Type error)…”
> 
> I think you’re mistaking the source of your error here. Check this out  
> (ri String#downcase!):
> 
> ```
> Downcases the contents of _str_, returning +nil+ if no changes were
> made.
> 
> ```
> 
> So you’re call to +downcase!+ is returning nil in the cases where  
> +file+ was already downcased. When +File.rename+ sees that nil and  
> tries to convert it to a String, it raises the error you see.
> 
> Try using +String#downcase+ instead (note the lack of the exclamation  
> point).
> 
> Jacob F.

I see. Thanks a lot, Jacob. There’s danger in my scripting there. I’ll  
try it without the exclamation.

-Peter