Strings in Ruby - Accessing Strings
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You can extract and manipulate segments of a string using the String method[]. It’s an alias of theslicemethod: any place you use[], you can useslice, with the same arguments.slice!performs in-place changes and is a counterpart to[]=.
We’ll access several strings in the examples that follow:
line = "A horse! a horse! my kingdom for a horse!"
cite = "Act V, Scene IV"
speaker = "King Richard III"
If you enter a string as the argument to[], it will return that string, if found:
speaker['King'] # => "King"
Otherwise, it will returnnil—in other words, it’s trying to break the news to you: “I didn’t find the string you were looking for.” If you specify aFixnum(integer) as an index, it returns the decimal character code for the character found at the index location:
line[7] # => 33
At the location7,[]found the character33(!). If you add thechrmethod (from theIntegerclass), you’ll get the actual character:
line[7].chr # => "!"
You can use an offset and length (twoFixnums) to tell[]the index location where you want to start, and then how many characters you want to retrieve:
line[18, 23] # => "my kingdom for a horse!"
You started at index location 18, and then scooped up 23 characters from there, inclusive. You can capitalize the result with thecapitalizemethod, if you want:
line[18, 23].capitalize # => "My kingdom for a horse!"
(More oncapitalizeand other similar methods later in the chapter.)
Enter a range to grab a range of characters. Two dots (..) means include the last character:
cite[0..4] # => "Act V"
Three dots (...) means exclude the last value:
cite[0...4] # => "Act "
You can also use regular expressions (see the end of the chapter), as shown here:
line[/horse!$/] # => "horse!"
The regular expression/horse!$/asks, “Does the wordhorse, followed by!come at the end of the line ($)?” If this is true, this call returnshorse!;nilif not. Adding another argument, aFixnum, returns that portion of the matched data, starting at0in this instance:
line[/^A horse/, 0] # => "A horse"
Theindexmethod returns the index location of a matching substring. So if you useindexlike this:
line.index("k") # => 21
21 refers to the index location where the letterkoccurs inline.
See if you get what is going on in the following examples:
line[line.index("k")] # => 107
line[line.index("k")].chr # => "k"
If you figured out these statements, you are starting to catch on! It doesn’t take long, does it? If you didn’t understand what happened, here it is: when line.index("k") was called, it returned the value21, which was fed as a numeric argument to[]; this, in effect, calledline[21].
Please check back next week for the continuation of this article.
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 This article is excerpted from chapter four of Learning Ruby, written by Michael Fitzgerald (O'Reilly, 2007; ISBN: 0596529864). Check it out today at your favorite bookstore. Buy this book now.
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