Confusion with Ruby's "case/when" block statement

Ruby uses === operator on the case/when type execution style.Now It
also known that Ruby depending on the type
of the thing present in the when clause, calls the respective .===
method.

Say when statement contains the class names, then the rule is - it will use Module#===, which will return true if the right side is an instance of, or subclass of, the left side. One example with this context is:

Here instance of test occurs

obj = ‘hello’
#=> “hello”
case obj
when String
print ‘It is a string’
when Fixnum
print ‘It is a number’
else
print ‘It is not a string’
end
#It is a string
#=> nil

Here subclass of test occurs

num = 10
#=> 10
case num
when Numeric
puts “Right class”
else
puts “Wrong class”
end
#Right class
#=> nil

Now when contains String literals then String#=== is called, which
in turn checks if left and right handside
literal are same(same chracter in same sequence) or not.

a = “abc”
#=> “abc”
case a
when “def” then p “Hi”
when “abc” then p “found”
else “not found”
end
#“found”
#=> “found”

The all logic is too cool. Now my query is with case/when structure -

How does ruby know if when holding class, or String literals or
anything valid at runtime?

or

What test does it perform before calling the respective .=== operator.

nothing … is just calls ===

in your case:
case num
when Numeric

it calls Numeric === num which returns true for num = 10

this one works too:

case
when false
p “wrong”
when true
p “right”
end
#=> “right”

or you can use other formula, it will break if it find something that is
evaluated into true or when used something in the case statement, wich
is result === case_value

Dude, read a bit. http://phrogz.net/ProgrammingRuby/language.html under
Operator Expressions. And read the source of the method being called.

Duh

Before understanding the Case/when working principal,let me clear the
below which while does when it gets its turn.

String.===("abc") #=> true

Because “abc” is an instance of String class. - Am I right?

Now I tried the below just to check who is whose super class.

10.class #=> Fixnum
Fixnum.superclass #=> Integer
Integer.superclass #=> Numeric
Numeric.superclass #=> Object

Humm. That means the below returns true as Fixnum is also the indirect
subclass of Numeric. - Am I right?

Numeric.===(10) #=> true

But why then the below outputs contradictory to the above?

Numeric.===(Fixnum) #=> false

One more question is -

when we are calling Numeric.===(10) and String.===("abc") . I think
we are sending not "abc" and 10 rather "abc".class and 10.class.

I might be 100% wrong, In that case please correct me.

Thanks

10.===(10) #=> true
Numeric.===(10) #=> true

Now look at the above. Both return true. Does they output true on
the same logic? I think NO. 10.===(10) is just like 10 ==(10)
comparison. But Numeric.===(10) outputs true as class of 10 is the
subclass of Numeric.

“abc”.===(“abc”) #=> true
String.===(“abc”) #=> true

Now look at the above. Both return true. Does they output true on
the same logic? I think NO. "abc".===("abc") is just like simple
string literal comparison "abc" ==("abc") comparison. But
String.===("abc") outputs true as "abc" which is an instance of
String.

Now my question is how ruby detects lefthand side operands types and
apply the proper rule of comparisons ?

On 4/03/2013, at 11:22 AM, “Kumar R.” [email protected] wrote:

Now look at the above. Both return true. Does they output true on
the same logic? I think NO. "abc".===("abc") is just like simple
string literal comparison "abc" ==("abc") comparison. But
String.===("abc") outputs true as "abc" which is an instance of
String.

Now my question is how ruby detects lefthand side operands types and
apply the proper rule of comparisons ?

It doesn’t detect anything.You are calling the === method on String, you
have even written it as such String.===("abc"). Each class is free to
implement it how ever they want.

This is basic OO

Henry

As clearly stated (even within ANY ruby language tutorial explaining
how
ruby works)

“A case expression searches for a match by starting at the first (top
left) comparison, performing comparison === target. When a comparison
returns true, the search stops, and the body associated with the
comparison is executed. case then returns the value of the last
expression
executed. If no comparison matches: if an else clause is present, its
body
will be executed; otherwise, case silently returns nil.”

There’s the ‘test’ it performs, AND what method is run for the test,
AND
what happens afterwards.

READ IT!

On Mon, Mar 4, 2013 at 6:41 PM, Kumar R. [email protected] wrote:

Module#===) operator, which tests if the right-hand side is_a? left-hand
side. The syntax is simple and elegant.

Now, what do you want to know? Or is everything clear now?

Cheers

robert

Actually the below paragraph made me confused: from the link :
http://ruby.about.com/od/control/a/The-Case-Statement.htm

What Type?

A common use for the case statement is to determine the type of the
value and do something different depending on its type. Though this
breaks Ruby’s customary duck typing, it’s sometimes necessary to get
things done. This works by using the Class#=== (technically, the
Module#===) operator, which tests if the right-hand side is_a? left-hand
side. The syntax is simple and elegant.

Robert K. wrote in post #1100204:

On Mon, Mar 4, 2013 at 6:41 PM, Kumar R. [email protected] wrote:

Module#===) operator, which tests if the right-hand side is_a? left-hand
side. The syntax is simple and elegant.

Now, what do you want to know? Or is everything clear now?

Cheers

robert

@Robert - that is_a? test actually made me confused. as when test
for subclass or instance of or any thing valid . But how is_a doing
help to the while clause that is my confusion.

On Tue, Mar 5, 2013 at 7:16 PM, Kumar R. [email protected] wrote:

Robert K. wrote in post #1100204:

On Mon, Mar 4, 2013 at 6:41 PM, Kumar R. [email protected] wrote:

Module#===) operator, which tests if the right-hand side is_a? left-hand
side. The syntax is simple and elegant.

Now, what do you want to know? Or is everything clear now?

@Robert - that is_a? test actually made me confused. as when test
for subclass or instance of or any thing valid . But how is_a doing
help to the while clause that is my confusion.

Yes, you are confused. We were talking about “case … when” and not
“while”.

Module#is_a? and Module#=== implement the same test.

I suggest you get yourself a Ruby tutorial and work through it start
to end - including all the exercises.

Cheers

robert

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