From: Robert Klemme <shortcutter@...>
Date: 2010-07-10T20:55:09+09:00
Subject: Re: if statements and case statements questions

On 10.07.2010 06:30, John Crichton wrote:
> I am fairly new to Ruby and programming and had a couple questions about
> if/else and case statements.  I was wondering if there were benefits to
> using case statements instead of if/elsif/else type statements.  Are
> case statements faster?
>
> If I am reading in a file line by line and doing something similar to
>
> lines = File.open("file.csv")
> FasterCSV.parse(lines) do |row|
>    if ( lines =~ /^blah/)
>
>      puts "blahblahblah"
>      some_method
>    end
>
>    if ( lines =~ /^name/&&  row[1] =="123")
>      puts "ping pong abc"
>      another_method
>    end
>    if ( lines =~ /^address/&&  row[3] =="XYZ")
>      puts "abcdefghijklmnopqrstuvwxyz"
>      method3
>    end
> end
>
> Does writing it with a case statement like so benefit me?
> lines = File.open("file.csv")
> FasterCSV.parse(lines) do |row|
>    case
>    when ( lines =~ /^blah/)
>      puts "blahblahblah"
>      some_method
>
>    when ( lines =~ /^name/&&  row[1] =="123")
>      puts "ping pong abc"
>      another_method
>
>    when ( lines =~ /^address/&&  row[1] =="XYZ")
>      puts "abcdefghijklmnopqrstuvwxyz"
>      method3
>    end
> end

I personally prefer the case because then it is immediately clear that 
there is a set of alternatives that belong together.  The difference to 
if elsif else end isn't too big though.

> While writing this I couldn't figure out how to write my case to be "if
> lines begins with"
>    case "lines =~"
>    when /^blah/
>      puts "blahblahblah"
>      some_method

I think that has been answered already: since you have more conditions 
there is no easy alternative to the case form where the condition is in 
when clause.  If you need only a regexp match as only criterion then you 
can do

case line
when /^blah/
   puts "This is a blah line: #{line}"
when /\.$/
   puts "This line ends with a dot."
end

> Lastly is there a "better" way to write an if statement that is
> conditional on a bunch of things like
>    if row[0] == john&&  row[7] == abc123&&  row[8] != nil&&
> somehashtable.has_key?(row2)

Well, you can encapsulate your condition in something else and use that. 
  Example:

BLAH_LINE = lambda {|line| /^blah/ =~ line && line.length > 87}
class <<BLAH_LINE
   alias :=== :[]
end

case line
when BLAH_LINE
   puts "This is it."
...
end

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/