Detecting default used in method calls

Ruby
1

I would like to know if a function has been called with or without an
optional argument.

If I have -

def the_Function (a, b = ‘’)
end

and I invoke it like this -

the_Function(‘a’)
the_Function(‘a’, ‘’)

is there any way for me to tell within the_Function which form of
invocation was used?

2

On Jan 2, 2007, at 10:31 AM, Peter L. wrote:

the_Function(‘a’)
the_Function(‘a’, ‘’)

is there any way for me to tell within the_Function which form of
invocation was used?

Not sure if there’s something more graceful, but my first instinct
would be to define it like

def the_function(*args)
…
end

Then the function could contain the defaulting logic in the event of
only one element in the args array.
-Mat

3

Peter L. wrote:

I would like to know if a function has been called with or without an
optional argument.

Not very pretty:

def fun(a, b = (no_b = true; 5))
if no_b then
“Fun(%p): %p” % [a, b]
else
“Fun(%p, %p)” % [a, b]
end
end

fun(1) # => “Fun(1): 5”
fun(1, 2) # => “Fun(1, 2)”

4

Peter L. wrote:

I would like to know if a function has been called with or without an
optional argument.

def foo( a, b=nil )
if b.nil?
# whoa
b=’’
end

…

end

5

def myfunction(a, b = nil)
if (b):
puts “There’s a B!”
else
puts “Business as usual…”
end
end

If you have more than one argument, then do something like this…

def myfunction(args={})

a = args[:a] || ‘my default A’

b = args[:b] || ‘my default B’

and so on…

end

myfunction(:a => ‘b’, :b => ‘c’)

You can do this with merge too (i.e., build a hash with the keys A, B,
and so on, and then merge it with the argument).

I think there’s a more graceful way to do this, but I’m too tired to
find it! :frowning:

–Jeremy

On 1/2/07, Peter L. [email protected] wrote:

the_Function(‘a’)
the_Function(‘a’, ‘’)

is there any way for me to tell within the_Function which form of
invocation was used?

–
Posted via http://www.ruby-forum.com/.

–
My free Ruby e-book:
http://www.humblelittlerubybook.com/book/

My blogs:

http://www.rubyinpractice.com/

6

On 02.01.2007 17:20, Florian G. wrote:

Peter L. wrote:

I would like to know if a function has been called with or without an
optional argument.

Not very pretty:

But clever!

def fun(a, b = (no_b = true; 5))
if no_b then
“Fun(%p): %p” % [a, b]
else
“Fun(%p, %p)” % [a, b]
end
end

fun(1) # => “Fun(1): 5”
fun(1, 2) # => “Fun(1, 2)”

The only other reasonable alternative I can see is this:

def fun(a,*bb)
if bb.empty?
puts “no b”
else
puts “b=#{b}”
end
end

Note that the other approach that has been mentioned cannot reliably
detect whether the parameter was set or not:

def fun(a,b=nil)
if b.nil?
puts “no b”
else
puts “b=#{b}”
end
end

irb(main):013:0> fun 1
no b
=> nil
irb(main):014:0> fun 1,2
b=2
=> nil
irb(main):015:0> fun 1,nil
no b
=> nil

(The last one should have printed “b=”.)

You get more options if you want to use named parameters (i.e. a Hash):

def fun(args={})
a = args[:a]
b = args[:b]

if args.has_key? :b
puts “b=#{b}”
else
puts “no b”
end
end

irb(main):053:0> fun(:a=>1)
no b
=> nil
irb(main):054:0> fun(:a=>1, :b=>2)
b=2
=> nil
irb(main):055:0> fun(:b=>2)
b=2
=> nil

Kind regards

robert
7

On Jan 2, 2007, at 07:31, Peter L. wrote:

the_Function(‘a’)
the_Function(‘a’, ‘’)

is there any way for me to tell within the_Function which form of
invocation was used?

No need to do those complicated things that everybody else is
trying. Instead use an object that nobody else will pass you as a
sentinel:

class X

SENTINEL = Object.new

def the_function(a, b = SENTINEL)
if b == SENTINEL then
…
end
end

end

–
Eric H. - [email protected] - http://blog.segment7.net

I LIT YOUR GEM ON FIRE!

–
Eric H. - [email protected] - http://blog.segment7.net

I LIT YOUR GEM ON FIRE!

–
Eric H. - [email protected] - http://blog.segment7.net

I LIT YOUR GEM ON FIRE!

8

Eric H. wrote:

   ...
 end

end

end

You don’t need to create your own, Exception works quite well:

class X

def the_function(a, b = Exception)
  if b == Exception then
    ...
  end
end

end

T.

9

Florian G. wrote:

fun(1) # => “Fun(1): 5”
fun(1, 2) # => “Fun(1, 2)”

Huh… That’s pretty cool. Maybe prettier is we reduce it even more?

def fun(a, b = (_b = 5))
if _b then
“Fun(%p): %p” % [a, b]
else
“Fun(%p, %p)” % [a, b]
end
end

fun(1) # => “Fun(1): 5”
fun(1, 2) # => “Fun(1, 2)”

Think that works as long as b doesn’t need to default to nil or false.

Oh… and I’m sitting here pronouncing _b as “un-b”.

T.

10

On Wed, 3 Jan 2007, Peter L. wrote:

the_Function(‘a’)
the_Function(‘a’, ‘’)

is there any way for me to tell within the_Function which form of
invocation was used?

def the_function *a, &b
case a.size
when 0
p ‘zero args’
when 1
p ‘one arg’
when 2
p ‘two args’
else
raise ArgumentError
end

 # ...

end

but methods like this are often confusing to read

the_function ‘arg’

the_function ‘arg’, ‘wtf does this do?’

it’s often better and more rubyish to use options

def the_function arg, opts = {}, &block
b = opts[‘b’] || opts[:b]
# …
end

regards.

-a