# What does this do?

**URL:** <https://www.ruby-forum.com/t/what-does-this-do/180213>\
**Category:** Ruby\
**Created:** [December 14, 2009, 11:23pm UTC](https://www.ruby-forum.com/t/what-does-this-do/180213 "2009-12-14T23:23:02Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![rogerdpack](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/45deac/32.png) [@rogerdpack](https://www.ruby-forum.com/u/rogerdpack)\
**Post date:** [December 14, 2009, 11:23pm UTC](https://www.ruby-forum.com/t/what-does-this-do/180213/1 "2009-12-14T23:23:02Z")

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> > “abcd”[0…2] = ‘ggg’  
> > =\> “ggg”

What is going on here, anybody have any ideas?  
-r

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**Author:** ![rogerdpack](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/45deac/32.png) [@rogerdpack](https://www.ruby-forum.com/u/rogerdpack)\
**Post date:** [December 14, 2009, 11:31pm UTC](https://www.ruby-forum.com/t/what-does-this-do/180213/2 "2009-12-14T23:31:15Z")

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On Mon, Dec 14, 2009 at 3:23 PM, Roger P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> > > “abcd”[0…2] = ‘ggg’  
> > > =\> “ggg”
> 
> What is going on here, anybody have any ideas?

Ruby lets you invoke #[]= on literals. They then go immediately out of  
scope and will be garbage collected, I assume. #[]= will always return  
the  
value on the RHS.

Perhaps this clears things up:

=\> “abcd”

> > x[0…2] = ‘ggg’  
> > =\> “ggg”  
> > x  
> > =\> “gggd”

So that’s what would happen if you used a variable instead of a literal.  
What you’re doing isn’t much different from:

{:foo =\> 42}[:foo] = ‘ggg’

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**Author:** ![rogerdpack](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/45deac/32.png) [@rogerdpack](https://www.ruby-forum.com/u/rogerdpack)\
**Post date:** [December 14, 2009, 11:33pm UTC](https://www.ruby-forum.com/t/what-does-this-do/180213/3 "2009-12-14T23:33:00Z")

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On Mon, Dec 14, 2009 at 4:23 PM, Roger P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> > > “abcd”[0…2] = ‘ggg’  
> > > =\> “ggg”
> 
> ## What is going on here, anybody have any ideas? -r
> 
> Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).

var = “abcd” # =\> “abcd”  
var[0…2] = ‘ggg’ # =\> “ggg”  
var # =\> “gggd”

It just replaces indexes 0 through 2 with the string “ggg” When you  
assign  
it to a variable, it is easier to see.

It returned “ggg” in irb (as seen on the second line) because it was  
assigned through the equal sign, and assignments through equal signs  
always  
return whatever the input was.

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**Author:** ![rogerdpack](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/45deac/32.png) [@rogerdpack](https://www.ruby-forum.com/u/rogerdpack)\
**Post date:** [December 15, 2009, 4:31pm UTC](https://www.ruby-forum.com/t/what-does-this-do/180213/4 "2009-12-15T16:31:23Z")

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Tony A. wrote:

> On Mon, Dec 14, 2009 at 3:23 PM, Roger P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
> wrote:
> 
> > > > “abcd”[0…2] = ‘ggg’  
> > > > =\> “ggg”
> > 
> > What is going on here, anybody have any ideas?
> 
> Ruby lets you invoke #[]= on literals.

Thanks for your and Josh’s replies. It makes sense now.  
“abcd”[0…2] = ‘ggg’

is replacing abc with ggg – it’s just that the result isn’t stored  
anywhere 🙂  
-r

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<div class="post-metadata">

**Author:** ![rogerdpack](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/45deac/32.png) [@rogerdpack](https://www.ruby-forum.com/u/rogerdpack)\
**Post date:** [December 16, 2009, 6:57am UTC](https://www.ruby-forum.com/t/what-does-this-do/180213/5 "2009-12-16T06:57:32Z")

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On Tue, Dec 15, 2009 at 3:31 PM, Roger P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> On Mon, Dec 14, 2009 at 3:23 PM, Roger P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
> wrote:
> 
> > > “abcd”[0…2] = ‘ggg’  
> > > =\> “ggg”  
> > > What is going on here, anybody have any ideas?

I hesitate to add to the above replies, but I might learn something, so:  
IRB  
class Q  
def z=( v ) ; @y = 42 ; end  
def y() ; @y ; end  
end  
#=\> nil  
q = Q.new #=\> #\<Q:0x4484dd4\>  
q.y #=\> nil  
q.z = 113 #=\> 113  
q.y = #=\> 42  
q #=\> #\<Q:0x4524d84 @y=42\>

From Programming Ruby, 1st Edition, 2000, page 76:  
[Programming Ruby: The Pragmatic Programmer's Guide](http://ruby-doc.org/docs/ProgrammingRuby/html/tut_expressions.html)  
An assignment statement sets the variable or attribute on its left  
side (the lvalue)  
to refer to the value on the right (the rvalue).  
It then returns that value as the result of the assignment expression.

“abcd”[0…2] = ‘ggg’  
is an assignment expression, and the rvalue is “ggg”, so that’s what’s  
returned.

Whilst usually (but not always, as in the Q example above)  
the assignment “sets” the lvalue target to the rvalue,  
that’s (sort of) “irrelevant” to what the assignment expression returns?