[SOLVED] Ruby: $_ not getting changed as expected

grail · 10-02-2011, 05:30 AM

So it is probably simple and a question of scoping but it is bugging me.

Here is a simple example of my question:

Code:

$ echo "hello world" | ruby -pe '$_.split[1] = "help"'
hello world

Now to me the expected output should have been:

Code:

hello help

My thinking is that $_ is defined as follows:

Quote:

$_ The last input line of string by gets or readline.

So in our example this would mean it equals "hello world".

Now the split will use the default separator ($;) which is whitespace as this
value is set to nil. Therefore I would expect the second element of the array
created by split to be "world" which bears true and can be tested with:

Code:

$ echo "hello world" | ruby -pe 'puts $_.split[1]'
world
hello world

So my question is, if I am setting the second part of the array to "help"
why does this not show in the results?

crts · 10-02-2011, 12:50 PM

Hi grail,

I found some documentation here:
http://www.troubleshooters.com/codec...l.htm#_Strings

If you scroll down a bit you will find this for split:

Code:

mystring.split(pattern, limit)

Returns a new array with parts of the string split wherever pattern was encountered as a substring.
If limit is given, returns at most that many elements in the array.

So $_.split does not manipulate "$_" itself, i.e. it does not turn $_ into an array. Instead it returns a new array with the contents of $_ as its elements.
When you do

Code:

ruby -pe '$_.split[1] = "help"'

you get as a first step the array

Code:

[ "hello", "world" ]

in return. In a second step the element at position [1] gets the value "help" assigned. However, since there is no variable to store the returned array those changes are lost.
Consider the following:

Code:

$ echo "hello world"|ruby -ne '$F = $_.split;$F[1] = "help";puts "field [0]: "+$F[0];puts "field [1]: "+$F[1];puts $_'
field [0]: hello
field [1]: help
hello world

As you can see the array does get changed but $_ is never touched.

grail · 10-02-2011, 07:32 PM

Thanks crts ... it was one of those things I looked at for ages and it was making my head hurt so I thought I would ask a question as no amount of searching or altering of the code was helping

I am still learning Ruby but pretty impressed so far

Thanks again.