Updating all values in a Hash

ruby-talk
1

Does anyone know a good way to update all values in a hash.

E.g. If I have a big hash with each element containing a string as a
value, how can I make each string lower-case?

···

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2

Paul Dugdale wrote:

If I have a big hash with each element containing a string as a
value, how can I make each string lower-case?

hash.each {|k,v| v.downcase!}
Or if you can't use destructive methods:
hash.each {|k,v| hash[k] = v.downcase}

HTH,
Sebastian

···

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3

h = {1 => 'ONE', 2 => 'TWO'}
h.each {|k,v| v.downcase!}

This assumes that all your values really are strings. You'll get some errors if any of the values do not have a downcase! methd.

Blessings,
TwP

···

On May 13, 2008, at 8:20 AM, Paul Dugdale wrote:

Does anyone know a good way to update all values in a hash.

E.g. If I have a big hash with each element containing a string as a
value, how can I make each string lower-case?

4

I found this way

h = Hash.new

def h.(val)
  super.downcase
end

h["ciao"] = "CIOOOO"

puts h["ciao"]
# cioooo

···

On Tue, May 13, 2008 at 2:20 PM, Paul Dugdale <p.dugdale@gmail.com> wrote:

Does anyone know a good way to update all values in a hash.

E.g. If I have a big hash with each element containing a string as a
value, how can I make each string lower-case?
--
Posted via http://www.ruby-forum.com/\.

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5

Hi --

Does anyone know a good way to update all values in a hash.

E.g. If I have a big hash with each element containing a string as a
value, how can I make each string lower-case?
--
Posted via http://www.ruby-forum.com/\.

I found this way

h = Hash.new

def h.(val)
super.downcase
end

h["ciao"] = "CIOOOO"

puts h["ciao"]
# cioooo

That's very resourceful (I'm fully in favor of extending individual
objects), but I'm afraid it doesn't really do what Paul wants. All it
does is downcase what you see when you do h[val]. But the actual value
doesn't change:

h = {}

=> {}

def h.(val); super.downcase; end

=> nil

h["a"] = "ABC"

=> "ABC"

h["a"] # This gives you downcased string.

=> "abc"

h.fetch("a") # But this doesn't.

=> "ABC"

h.values # Neither does this.

=> ["ABC"]

I think also that Paul is dealing with a hash that already has some
values, so the iterative solutions might be better.

David

···

On Tue, 13 May 2008, Sandro Paganotti wrote:

On Tue, May 13, 2008 at 2:20 PM, Paul Dugdale <p.dugdale@gmail.com> wrote:

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6

Sorry! I was trying to keep my post simple, but ended up doing a really
bad job of explaining what I was trying to achieve...

I come from a C programming background, so I'm always thinking in terms
of pass-by-value and pass-by-reference.

What I was trying to find out is - is there any way of passing a
reference to the value to the 'each' block, rather than a copy of the
value.

I.e. some easy way of doing this:
hash.each { |k,v| v = my_value }

Without having to refer to the hash again in the block like this:
hash.each { |k,v| hash[k] = my_value }

···

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

7

Paul Dugdale wrote:

What I was trying to find out is - is there any way of passing a
reference to the value to the 'each' block, rather than a copy of the
value.

You always pass by reference, but you pass a reference to the object (a
pointer if you will) not a reference to the variable. You can't have variable
references in ruby.

I.e. some easy way of doing this:
hash.each { |k,v| v = my_value }

v.replace my_value
If v has a replace method

Without having to refer to the hash again in the block like this:
hash.each { |k,v| hash[k] = my_value }

If your v object is immutable, you'll have to do the above, I'm afraid.

HTH,
Sebastian

···

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8

Sorry! I was trying to keep my post simple, but ended up doing a really bad job of explaining what I was trying to achieve...

I come from a C programming background, so I'm always thinking in terms of pass-by-value and pass-by-reference.

What I was trying to find out is - is there any way of passing a reference to the value to the 'each' block, rather than a copy of the value.

A reference is indeed passed to the block. There is no copying of values going on. You can see it because v.downcase! works; this does change the object referenced by the Hash.

I.e. some easy way of doing this:
hash.each { |k,v| v = my_value }

Without having to refer to the hash again in the block like this:
hash.each { |k,v| hash[k] = my_value }

I'm afraid, you'll have to use that idiom. The only alternative is to change your scenario so that it will modify an object, e.g. create an intermediary:

irb(main):001:0> Holder = Struct.new :val
=> Holder
irb(main):002:0> h={1=>Holder.new(123)}
=> {1=>#<struct Holder val=123>}
irb(main):003:0> h.each {|k,v| v.val += 10}
=> {1=>#<struct Holder val=133>}
irb(main):004:0>

I doubt though that this is more efficient that using any of

h.keys.each {|k| h[k] += 10}
h.each {|k,v| h[k] = v + 10}

What is the greater context of your issue, i.e. what are you trying to achieve?

Kind regards

  robert

···

On 13.05.2008 17:46, Paul Dugdale wrote:

9

Even if it isn't not doing so can be problematic:

a = "Fred"
h = {:flintstone => a}
a # => "Fred"
h # => {:flintstone=>"Fred"}
h.each {|k,v| v.downcase!}
h # => {:flintstone=>"fred"}
# Expected, but...

a # => "fred"
# Expected???!!!!

Understanding how object reference differs from pointer reference and
value semantics is crucial to starting to understand a pure object
language like Ruby.

···

On Tue, May 13, 2008 at 12:04 PM, Sebastian Hungerecker <sepp2k@googlemail.com> wrote:

Paul Dugdale wrote:
> What I was trying to find out is - is there any way of passing a
> reference to the value to the 'each' block, rather than a copy of the
> value.

> Without having to refer to the hash again in the block like this:
> hash.each { |k,v| hash[k] = my_value }

If your v object is immutable, you'll have to do the above, I'm afraid.

--
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

10

Hi --

Paul Dugdale wrote:

What I was trying to find out is - is there any way of passing a
reference to the value to the 'each' block, rather than a copy of the
value.

You always pass by reference, but you pass a reference to the object (a
pointer if you will) not a reference to the variable. You can't have variable
references in ruby.

I'd put it differently, and I know this sounds convoluted but I think
in the end it turns out to be more apt: You're passing by value, but
the value you're passing is a reference to an object.

"Passing by reference" makes it sound (to me) like when you do this:

   s = "hi"
   my_method(s)

s somehow gets wrapped in a reference on its way to my_method, whereas
it's actually really just being handed off. The value assigned to the
corresponding parameter in my_method is the same as the value of s:
namely, a reference to that string.

There's probably some aspect of using literal constructors as method
arguments that needs to be factored in there somewhere....

David

···

On Wed, 14 May 2008, Sebastian Hungerecker wrote:

--
Rails training from David A. Black and Ruby Power and Light:
   INTRO TO RAILS June 9-12 Berlin
   ADVANCING WITH RAILS June 16-19 Berlin
   INTRO TO RAILS June 24-27 London (Skills Matter)
See http://www.rubypal.com for details and updates!