why cannot i put conditions in when clause

[Junkone]

irb(main):003:0> case a
irb(main):004:1> when 0
irb(main):005:1> puts "0"
irb(main):006:1> when >0
irb(main):007:1> puts "greater"
irb(main):008:1> end
SyntaxError: compile error
(irb):6: syntax error, unexpected '>'
when >0
^
from (irb):8

 

[Phillip Gawlowski]

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Junkone wrote:
| irb(main):003:0> case a
| irb(main):004:1> when 0
| irb(main):005:1> puts "0"
| irb(main):006:1> when >0
| irb(main):007:1> puts "greater"
| irb(main):008:1> end
| SyntaxError: compile error
| (irb):6: syntax error, unexpected '>'
| when >0
| ^
| from (irb):8
|

That's what if..else..end is there for.

In a case statement you usually check against 'known' and well-defined
states. Also, once a condition is met in a case statement, no other
branches are evaluated. So, if you had 'when < 1' that wouldn't be
evaluated, even id a = 2.

- --
Phillip Gawlowski
Twitter: twitter.com/cynicalryan
Blog: http://justarubyist.blogspot.com

10 years old is a good age to get stuck at.
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[Joel VanderWerf]

irb(main):003:0> case a
irb(main):004:1> when 0
irb(main):005:1> puts "0"
irb(main):006:1> when >0
irb(main):007:1> puts "greater"
irb(main):008:1> end
SyntaxError: compile error
(irb):6: syntax error, unexpected '>'
when >0
^
from (irb):8

It's not an expression.

You can do this:

case
when a==0
when a>0
end

or

case a
when 0
when 0..MAX_EXPECTED_VALUE_FOR_A
end

 

[James Gray]

irb(main):006:1> when >0

Idea #1:

when 0..(1.0/0.0)

Idea #2:

case # note that I have removed the a
# ...
when a > 0

Hope that helps.

James Edward Gray II

 

[Une Bévue]

Idea #1:

when 0..(1.0/0.0)

;-)

 

[Robert Klemme]

irb(main):003:0> case a
irb(main):004:1> when 0
irb(main):005:1> puts "0"
irb(main):006:1> when >0
irb(main):007:1> puts "greater"
irb(main):008:1> end
SyntaxError: compile error
(irb):6: syntax error, unexpected '>'
when >0
^
from (irb):8

You can:

$ ruby -ce 'case;when a == 0;puts "0";when a > 0;puts "greater";end'
Syntax OK


This is the second form of "case".

Btw, there is also another way: define a criteria that implements === as

0:

irb(main):001:0> POS = Object.new
=> #<Object:0x7ff9e244>
irb(main):002:0> def POS.===(x) x > 0 end
=> nil
irb(main):003:0> a=10
=> 10
irb(main):004:0> case a
irb(main):005:1> when 0
irb(main):006:1> puts "0"
irb(main):007:1> when POS
irb(main):008:1> puts "positive"
irb(main):009:1> end
positive
=> nil
irb(main):010:0>

Kind regards

robert

 

[Avdi Grimm]

You can do this:

case
when a==0
when a>0
end

First of all: huh. Learn something new about Ruby (almost) every day.

That said - why would you do this rather than an if/elsif? I'm
genuinely curious. E.g.

if a == 0 then ...
elsif a>0 then ...
end

--
Avdi

Home: http://avdi.org
Developer Blog: http://avdi.org/devblog/
Twitter: http://twitter.com/avdi
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