Printing 2 dimensionl hash problem


#1

Hi,

busy with my script, I am trying to print my hash, which is built up as
follows:

hash:

key = A, value = [1,2,3]
key = B, value = [4,5,6]

etc.

I try this:

@h.each do |key,value|
print key, " is ", value.each {|x| print x, " – " }, “\n”

The print is strange, becuase it looks like this now:

1 – 2 – 3 – A is 123
4 – 5 – 6 – B is 456

I don’t understand why. If I interpreted this correctly, I’ve got the
following two questions:

  1. How come the code block is executed first
  2. After that the key and value is printed, however, I don’t see the
    print for the values here (except for the print in de the code block,
    which was executed already).

Krekna


#2

From: Krekna M. [mailto:removed_email_address@domain.invalid] :
#----------------------------------------------------------

@h.each do |key,value|

print key, " is ", value.each {|x| print x, " – " }, “\n”

The print is strange, becuase it looks like this now:

1 – 2 – 3 – A is 123

4 – 5 – 6 – B is 456

1. How come the code block is executed first

#----------------------------------------------------------

print expects to print a value of the expression (of course). so before
prints does the writing, it has to get the value first.

#----------------------------------------------------------

2. After that the key and value is printed, however, I don’t see the

print for the values here (except for the print in de the code block,
which was executed already).
#----------------------------------------------------------

"value.each {…} " is an expression that returns a value, in this case
the array “value”. that expression however will perform *first a {|x|
print x, “—”} for every element x in array “value”.

eg,

C:\temp\rubygems-0.9.1>cat test.rb

h = {“A” => [1,2,3], “B” =>[4,5,6]}

puts “sample 1”
h.each do |key,value|
print key, " is "
value.each {|x| print x, " – " }
print “\n”
end
puts
puts “sample 2”
h.each do |key,value|
puts “#{key} is #{value.join(’ – ')}”
end

C:\temp\rubygems-0.9.1>ruby test.rb
sample 1
A is 1 – 2 – 3 –
B is 4 – 5 – 6 –

sample 2
A is 1 – 2 – 3
B is 4 – 5 – 6

hth.
kind regards -botp


#3

2007/1/18, Peña, Botp removed_email_address@domain.invalid:

1. How come the code block is executed first

#----------------------------------------------------------

print expects to print a value of the expression (of course). so before prints does the writing, it has to get the value first.

#----------------------------------------------------------

2. After that the key and value is printed, however, I don’t see the

print for the values here (except for the print in de the code block,
which was executed already).
#----------------------------------------------------------

"value.each {…} " is an expression that returns a value, in this case the array “value”. that expression however will perform *first a {|x| print x, “—”} for every element x in array “value”.

Thank you for your explanation!

I see now, that value.each is an argument to the print, which is not
what I need indeed, because like you said value.each with its code
block is already printing the values, so that’s why they are printed
twice.

So, I’ve put the value.each on a separate line like in your example now.
Thus, the value.each expression is returned first, apparently the
arguments are parse before the (first) print is executed.

Same as this:
irb(main):012:0> print “I”, " am ", "#{print "first “}”
first I am => nil

Krekna


#4

etc.

I don’t understand why. If I interpreted this correctly, I’ve got the
following two questions:

  1. How come the code block is executed first
  2. After that the key and value is printed, however, I don’t see the
    print for the values here (except for the print in de the code block,
    which was executed already).

:slight_smile: I think what you want to actually do is:

@h.each do |key,value|
print key, " is ", value.join(’ – '), “\n”
end

or

@h.each do |key,value|
puts key, " is ", value.join(’ – ')
end

To hopefully answer your questions:

You have asked Ruby to print out the return value of value.each{…},
which is simply value:

[1,2,3] == [1,2,3].each {|n| do_something_with_n_if_you_like(n)}

So that explains the 123 at the end of the line, because [1,2,3].to_s is
“123”

Why the “1 – 2 – 3” before everything else though? Well, before Ruby
can execute that first “print”, it must first evaluate what is being
printed, just as if you had writen “print 1+2+3”. In calculating that,
Ruby calls values.each {…}, and this prints out the “1 – 2 – 3” at
the start of the line.

Hope that helps.


#5

Thank you very much, my answer was posted before I read this.
I think it’s clear to me now.
That’s the best way, learning by doing and talking about it what
actually happens when something went wrong.

Thank you too.

Krekna

2007/1/18, removed_email_address@domain.invalid removed_email_address@domain.invalid: