Ruby case statement

[tobyclemson]

Hi,

I've read through some of the other case statement posts but can't
find any that answer me this question.

I know that the case doesn't allow the follow through aspect of case
statements from other languages but is the following valid:

case type
when 1
# do stuff for type 1
when 1,2
# do stuff for both type 1 and 2
end

If it isn't valid, what is the easiest way to obtain this
functionality

Thanks,
Toby

 

[Robert Klemme]

Hi,

I've read through some of the other case statement posts but can't
find any that answer me this question.

I know that the case doesn't allow the follow through aspect of case
statements from other languages but is the following valid:

case type
when 1
# do stuff for type 1
when 1,2
# do stuff for both type 1 and 2
end

If it isn't valid, what is the easiest way to obtain this
functionality

Why don't you just try it out?

Btw, what is the "type" that you are checking? I ask because if you
want to do something based on the class of an instance there are
different means.

Kind regards

robert

 

[Sharon Phillips]

case type

when 1
# do stuff for type 1
when 1,2
# do stuff for both type 1 and 2
end

If it isn't valid, what is the easiest way to obtain this
functionality

i=1
case i
when 1: puts "1"
when 1,2: puts "1 or 2"
when 3: puts "3"
else puts "other"
end

1

so it looks like the second one gets skipped.

The best i can come up with (but I'm pretty sure someone else will
come up with a much better alternative) is

i=1
case i
when 1,2
puts "1 or 2"
puts "1" if i==1
when 3
puts "3"
else
puts "other"
end

1 or 2
1

Cheers,
Dave

 

[tobyclemson]

Sorry type was misleading - the variable could be anything, I'm not
doing anything to do with the class of an instance. I had tried it out
but I'm quite new to ruby so I doubted myself and thought I'd ask
those in the know!

 

[tobyclemson]

Thanks, that's neater than the ways of doing it that I have found
since first posting.

 

[Robert Klemme]

Sorry type was misleading - the variable could be anything, I'm not
doing anything to do with the class of an instance. I had tried it out
but I'm quite new to ruby so I doubted myself and thought I'd ask
those in the know!

I probably was too quick on the trigger, I am sorry. I completely
overlooked your "fall through" issue. Personally I have used that in
other languages but have yet to miss it in Ruby.

It's sometimes difficult to just translate an idiom from another
programming language. And more often than not the resulting code is
either ugly, inefficient or could be done more elegantly. Of course
there are solutions using "case" and other Ruby constructs that would
achieve what you want but it's probably more helpful for you if you can
present a real problem that you are trying to solve and we can come up
with a solution.

This is a simple solution, i.e. separate the task into several steps and
do control flow for each step:

type = 1
puts 1 if type == 1
case type
when 1,2: puts "1 or 2"
when 3: puts 3
else puts "other"
end

Kind regards

robert

 

[tobyclemson]

Basically I had a situation where dependent on the formatting of a
block of text, different actions had to be carried out but some
actions were common to each of the formattings whereas others weren't
so I really wanted to be able to apply the bits that were specific to
say the type 1 format first and then follow down into the actions
common to both type 1 and type 2 such as is the case in other
languages when you don't 'break' out of each 'case' clause. I
understand now that all 'case' clauses in ruby automatically 'break'
when they finish but I was wondering whether there was a way around
that such as in my code example. It appears though that in my code
block, the first 'when' clause fulfilling the condition is executed
and then the 'case' is exited. I wanted to know whether there was a
way to stop the 'case' exiting so that it continues and carries out
the actions in the next 'when' clause fulfilling the condition. And if
there isn't a way to do this, what is the best way to achieve the
functionality I am trying to obtain.

Thanks
Toby

I agree. --Not so sure a case statement makes sense here. Tell us what you
are trying to accomplish. Is it to get clarity on ruby's case statements? or
to really follow the logic you've detailed?

puts i if [1,3].include? i
puts "1 or 2" if [1,2].include? i

when 1: puts "1"
when 1,2: puts "1 or 2"

Logically does'nt make much sense.
Here "when 1" is redundant, your puts "1 or 2" will never be printed even
when value is either 1 or 2 as you want.

The question is - What are you actually trying to do?

 

[Robert Klemme]

Basically I had a situation where dependent on the formatting of a
block of text, different actions had to be carried out but some
actions were common to each of the formattings whereas others weren't
so I really wanted to be able to apply the bits that were specific to
say the type 1 format first and then follow down into the actions
common to both type 1 and type 2 such as is the case in other
languages when you don't 'break' out of each 'case' clause. I
understand now that all 'case' clauses in ruby automatically 'break'
when they finish but I was wondering whether there was a way around
that such as in my code example. It appears though that in my code
block, the first 'when' clause fulfilling the condition is executed
and then the 'case' is exited. I wanted to know whether there was a
way to stop the 'case' exiting so that it continues and carries out
the actions in the next 'when' clause fulfilling the condition. And if
there isn't a way to do this, what is the best way to achieve the
functionality I am trying to obtain.

I do not know how complex your operations are. From what you write I
guess there could be multiple actions and they could be complex. Also,
there might be numerous formats - if not today then maybe in a later
stage. Going from there the solution I'd probably favor is a class
hierarchy of formatters. That way you can define some basic
formatting functionality in methods in the base class and use those
methods from all sub class methods.

You then only need to map your format code in some way to the name of
a specific formatter class and send the request off to that to get
your specific formatting. For the mapping you can use various
mechanisms, the easiest probably being the class name of the
formatter. But you can as well use some specific mapping using a
Hash. There are other methods as well.

Kind regards

robert

 

[John Joyce]

I do not know how complex your operations are. From what you write I
guess there could be multiple actions and they could be complex. Also,
there might be numerous formats - if not today then maybe in a later
stage. Going from there the solution I'd probably favor is a class
hierarchy of formatters. That way you can define some basic
formatting functionality in methods in the base class and use those
methods from all sub class methods.

You then only need to map your format code in some way to the name of
a specific formatter class and send the request off to that to get
your specific formatting. For the mapping you can use various
mechanisms, the easiest probably being the class name of the
formatter. But you can as well use some specific mapping using a
Hash. There are other methods as well.

Kind regards

robert

you mean you want it to fall through like in C?

 

[Robert Klemme]

you mean you want it to fall through like in C?

No. This is a completely different approach which has nothing to do
with "case".

robert

 

[Robert Klemme]

Apparently my newsgroup reply has not made it to the mailing list
(yet). Apologies if you receive two answers.

you mean you want it to fall through like in C?

No. My suggestion has nothing to do with "case" statements. It's an
OO way of doing this.

Kind regards

robert

 

[tobyclemson]

Well it was actually quite a small one off script but it led me to
wondering... Yes John, I want it to fall through like C - is that
possible? Thanks Robert for the useful alternative method. I'm pretty
new to Ruby and haven't actually got so far as to thinking of
everything in terms of classes so this was just a small script written
procedurally rather than in a class based way.

 

[John Joyce]

Well it was actually quite a small one off script but it led me to
wondering... Yes John, I want it to fall through like C - is that
possible? Thanks Robert for the useful alternative method. I'm pretty
new to Ruby and haven't actually got so far as to thinking of
everything in terms of classes so this was just a small script written
procedurally rather than in a class based way.

Nope. That's why I asked.
The fall through in C's switch-case block is exactly something that
doesn't happen in Ruby.
The Ruby switch-case is of course different, perhaps that's why Matz
changed the naming of it to case-when.
It has no fall through.
case-when(s)-else-end

The else provides a default if no matching 'when'
It's not quite the same as a fall through.

It's different from C, but this control mechanism is usually
different in different languages, it seems.

C's switch-case block is really nothing more than a series of if-else
statements.
C's else is sometimes more like 'also' (as far as I remember, it's
been a while)
Ruby's case-when is not the same.