$a <=> $b

truthseeker66 has asked for the wisdom of the Perl Monks concerning the following question:

I see a line, "@sorted = sort {$a <=> $b} @array;"

The text book I read says, "When you need to sort data in
numerical order, you have to add an argument to the sort
statement. What confuses me is '$a <=> $b'.
Could you explain what this line do? Can I use '$c <=> $d'
instead?

Thank you

Comment on $a <=> $b

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Re: $a <=> $b by roboticus (Chancellor) on Oct 09, 2012 at 18:18 UTC
truthseeker66: Inside the body of the sort block, $a and $b refer to the items being compared. If you use $c and $d, sort isn't going to do what you want it to do. ...roboticus When your only tool is a hammer, all problems look like your thumb.	[reply]
Re: $a <=> $b by tobyink (Canon) on Oct 09, 2012 at 18:57 UTC
roboticus' answer is correct. The documentation for the special variables `$a` and `$b` is in perlvar. `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l] [select]
Re: $a <=> $b by GrandFather (Saint) on Oct 10, 2012 at 01:31 UTC
roboticus answered the immediate question, but note that this is why you should avoid using $a and $b a general purpose variables: they are magical and should really only be used inside sort. Note too that using sort without a {...} bit is like `sort {$a cmp $b}`. That is, sort's default is to do a string sort. True laziness is hard work	[reply] [d/l]