From: Matthew Smillie <M.B.Smillie@...>
Date: 2006-06-23T03:39:36+09:00
Subject: Re: Why is "nil" being included in an array?

On Jun 22, 2006, at 19:22, Peter Bailey 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.