Scope and Variable Resolution in Ruby Blocks

Question

There is something that I am missing in the following code.

hostnames =  []
ip_addrs  =  []
hw_addrs  =  []

File.open("/etc/dhcp/dhcpd.conf", "r").each_line do |line|

  unless line.match('#')                        # Make sure the line is not commented
    if line.match("host-name")
        hostname = line.scan(/"([^"]*)"/)       # extract the Hostname
    elsif line.match("fixed-address")
        ip_addr = line.scan(/(\S*);/)           # Extract IP addr
    elsif line.match("ethernet")
      hw_addr = line.scan(/(\S*);/)             # Extract the HW address
    end
  end

  hostnames + hostname.to_a if hostname         # Protect against `nil' values
  ip_addrs  + ip_addr.to_a  if ip_addr          #  Same
  hw_addrs  + hw_addr.to_a  if hw_addr          #  Same

end

puts hostnames.inspect                          # Should be a list of hostnames...

This should populate the arrays with the values found in the dhcpd.conf file. If I print the values inside the File.open.each_line block then I get the complete list to STDOUT. When I try to get the values outside of the block I get empty arrays.

I think that the block generates a copy of my variables and works on those, but they don't get passed back out of the block. I'm not sure how the internals work, just a guess.

You didn't add it to the hostnames array. Try hostnames << hostname.to_a — Shobhit
– Shobhit, Commented Aug 7, 2013 at 19:28
Is this an exact copy of your code? The three statements before the last end don't make sense. Were those supposed to be assignments? — Peter Alfvin
– Peter Alfvin, Commented Aug 7, 2013 at 19:29

tadman · Accepted Answer · 2013-08-07 19:29:13Z

1

If you want to add something to an array you need to use the proper operator:

hostnames += hostname.to_a if hostname

What you're doing is creating a temporary result and discarding it: a + b is not a permanent modification, but a = a + b is, where the short-hand is a += b.

Generally you use << to add single elements to an array, it avoids creating a new array and just appends to the existing one. For example:

hostnames << hostname if hostname

That's generally sufficient. Casting everything to arrays just to concatenate them is wasteful unless you are actually appending several values at once.

answered Aug 7, 2013 at 19:29

tadman

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1 Comment

Devon Finninger Over a year ago

Right, as soon as I read this I remembered. Thanks. I knew I was missing something simple.

Shobhit · Accepted Answer · 2013-08-07 19:31:49Z

1

Change hostnames + hostname.to_a to hostnames << hostname.to_a

In the current version of your code, you don't actually add the hostname to the hostnames array. << pushes the hostname element to hostnames

answered Aug 7, 2013 at 19:31

Shobhit

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