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
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
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)
Thanks
Case statements are not faster. Both Case and if statements are
constant
time operations so there is no speed benefit of one over the other. It
really just comes down to your preference in any given situation.
Depending
on the code one or the other can lead to cleaner and easier to
understand
code.
Oh and cool name btw 
Ruby switches expand to the === operator, for example:
case string
when “” then puts “Empty”
when /^a/ then puts “Starts with a”
end
is equivalent to:
if string === “”
puts “Empty”
elsif string === /^a/
puts “Contains a”
end
Your first way of doing things, with the three if statements, you should
change the last two to elsifs.
Also since you are not just comparing lines but also row numbers, you
can’t use
case lines
when …
when …
end
I guess you should stick with the if elsif … end
On 10.07.2010 06:30, John Crichton wrote:
method3
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
Thank you all for your replies/answers. I appreciate the help and
explanations.
Kenneth    wrote:
Ruby switches expand to the === operator, for example:
case string
when “” then puts “Empty”
when /^a/ then puts “Starts with a”
end
is equivalent to:
if string === “”
puts “Empty”
elsif string === /^a/
puts “Contains a”
end
Actually the arguments are the other way round:
if “” === string
puts “Empty”
elsif /^a/ === string
puts “Starts with a”
end
Another example:
case arg
when String; puts “it’s a string”
when Numeric; puts “it’s a number”
end
The first expands to String === arg, which is functionally equivalent to
arg.is_a?(String)