Method behaviour

Ruby
1

Some methods like

def change(word)
word[0]=‘e’
end
a=“affluent”
puts a
change(a)
puts a

alter the original object outside of the method, where as others don’t.

def change(word)
word=“fish”
end
a=“cow”
puts a
change(a)
puts a

Why the change in behaviour, what determines it? Is the only way I can
write methods that change the original object by using the Ruby methods
that do this?

2

On 06/08/06, Sard A. [email protected] wrote:

alter the original object outside of the method, where as others don’t.
Why the change in behaviour, what determines it? Is the only way I can
write methods that change the original object by using the Ruby methods
that do this?

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

In the second example you are changing the reference to where ‘word’
points. You create a new string (‘fish’) and then set ‘word’ to
reference it. ‘a’ still points to your original word (‘cow’).

3

Hi –

On Mon, 7 Aug 2006, Sard A. wrote:

alter the original object outside of the method, where as others don’t.
Why the change in behaviour, what determines it? Is the only way I can
write methods that change the original object by using the Ruby methods
that do this?

It’s because there’s a distinction between (a) sending a message to an
object, and (b) assigning to a variable.

a = “cow” # assignment
a[0] = ‘w’ # send message “[]” to a
a = “fish” # assignment – reusing the identifier ‘a’

Whenever you assign to a variable, you’re wiping the slate clean: the
variable loses all association with the object it referred to
previously (if any).

The “word[0] = ‘e’” expression looks like assignment, but technically
it’s calling the method “[]=” on the object word, with the single
argument ‘e’. It’s not the same as just plain assignment to an
identifier.

David

4

Your assessment is correct but you got one detail wrong. The message
sent by

  word[0]='e'

is actually []= with arguments 0 and ‘e’. That is, the pseudo-
assignment is really the message

  word.[]=(0, 'e')

As a coating of syntactic sugar, Ruby permits us to write this as
something that looks like an assignment and which, consequently, can
confuse people like the OP.

Regards, Morton

P.S. Don’t think I’m complaining about the syntactic sugar: the real
message underlying the sugar coating is ugly. I don’t want to write
stuff like that.

5

Hi –

On Mon, 7 Aug 2006, Morton G. wrote:

Your assessment is correct but you got one detail wrong. The message sent by

word[0]=‘e’

is actually []= with arguments 0 and ‘e’.

Whoops, right: two arguments.

David