Newbie has trouble understanding a Why example

Ruby
#1

I’m just learning ruby, so I am working my way through ‘Why’s Poignant
Guide to Ruby’

I’m on the 5th chapter, and I came across this example:

class ArrayMine < Array

Build a string from this array, formatting each entry

then joining them together.

def join( sep = $, format = “%s” )
collect do |item|
sprintf( format, item )
end.join( sep )
end
end

An example of the method in action:

rooms = ArrayMine[3, 4, 6]
print "We have " + rooms.join( ", “, “%d bed” ) + " rooms available.”

Which prints, “We have 3 bed, 4 bed, 6 bed rooms available.”

I’m confused by the line “def join( sep = $, format = “%s” )”. What on
earth is going on with the parameters? What do the sep = $, and format
= “%s” do? If I modify that line to “def join( sep, format)”, the same
thing prints out. So why are $, and %s used? What do they do?

#2

Ca Josephson wrote:

   sprintf( format, item )

I’m confused by the line “def join( sep = $, format = “%s” )”. What on
earth is going on with the parameters? What do the sep = $, and format
= “%s” do? If I modify that line to “def join( sep, format)”, the same
thing prints out. So why are $, and %s used? What do they do?

Its a little confusing. In the method definition, sep= and format=
indicate the default values for those parameters. The default value for
sep is the value of the global variable $, and the the default format
string is “%s”

now $, is the default system output separator and is nil unless
explicitly set

C:\Documents and Settings\Administrator>irb
irb(main):001:0> puts $,
nil
=> nil
irb(main):002:0> %w(a b c d e f g).join
=> “abcdefg”
irb(main):003:0> $,=":"
=> “:”
irb(main):004:0> %w(a b c d e f g).join
=> “a:b:c:d:e:f:g”

does that make more sense?

Steve

#3

On Mon, Aug 10, 2009 at 4:28 PM, Ca Josephson[email protected]
wrote:

  sprintf( format, item )

I’m confused by the line “def join( sep = $, format = “%s” )”. What on
earth is going on with the parameters? What do the sep = $, and format
= “%s” do? If I modify that line to “def join( sep, format)”, the same
thing prints out. So why are $, and %s used? What do they do?

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

This is rather a hint than an answer, if you want a more explicit
answer do not hesitate to ask again:
irb(main):004:0> def a sep=",", format=""
irb(main):005:1> p [ sep, format ]
irb(main):006:1> end
=> nil
irb(main):007:0> a 41, 42
[41, 42]
=> [41, 42]
irb(main):008:0> a 41
[41, ""]
=> [41, “"]
irb(main):009:0> a
[",", "”]
=> [",", “*”]

HTH
Robert

#4

On Mon, Aug 10, 2009 at 10:28 AM, Ca Josephson[email protected]
wrote:

  sprintf( format, item )

I’m confused by the line “def join( sep = $, format = “%s” )”. What on
earth is going on with the parameters? What do the sep = $, and format
= “%s” do? If I modify that line to “def join( sep, format)”, the same
thing prints out. So why are $, and %s used? What do they do?

They provide default values for the parameters.

try:

print "We have " + rooms.join( “, " ) + " rooms available.”
or
print “We have " + rooms.join + " rooms available.”

with and without your modification.

–
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale

#5

On Mon, Aug 10, 2009 at 5:00 PM, steve[email protected] wrote:

def join( sep = $, format = “%s” )

=> “:”
irb(main):004:0> %w(a b c d e f g).join
=> “a:b:c:d:e:f:g”

does that make more sense?

Steve

No idea, but if the implementation of join looks like this, it would

def join by
reduce{ |s,e| “#{s}#{by||$,}#{e}” }
end

HTH
Robert