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Substrings in Ruby

The other day I was wondering why some really simple Ruby code of mine wasn’t working. The code under test looked something like this:

    def find_directories
        directories = []
        entries = `ls -l`.split/\n/  # get long-format directory listing
        entries.shift   # toss away the totals line
        entries.each do |entry|
            is_directory = (entry[0] == 'd')  # is it a directory?
            directories << dir if is_directory

I figured that my ls -l.split/\n/ trickery wasn’t working. So I fired up irb and tried it manually. It worked fine.
Then I sprinkled a few puts statements throughout. entries was fine, and each entry was perfect, looking something like this:
drwx------ 24 jcohen jcohen 816 Aug 19 11:04 Documents

I couldn’t figure out what was wrong. All I had to do was look at the first character of entry; if it was a ‘d’, then I knew I had a directory. But for some reason, is_directory was always false. I googled, checked the RDocs for the String class, thumbed through the PickAxe, and something I read triggered one of those ah-ha moments.
Rats: My C# brain is still alive and kicking.

To confirm my fears, I fired up irb again:

irb> s = "Jeff" 
=> "Jeff" 
irb> s[0]
=> 74

That’s right, folks: on a string, the bracket syntax – when given only one parameter – will return the ASCII value of the character inside. Not the actual character, as it will in C#.
To correctly grab a one-character substring from a string, you have to supply two parameters:

irb> s[0,1]
=> "J" 
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