I hadn't realized this, but unary minus has an interesting behaviour in some cases.

What do you think this code prints if you type in -1 and hit enter? 

perl -ne'chomp; $x=$_; print "x=$x, -x=",-$x,"\n";'

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I was expecting

x=-1, -x=1

which seemed like a reasonable guess.

What I got was

x=-1, -x=+1

The unexpected "+" sign goes away if $x isn't a string, or if you force a numeric context by printing "0-$x" rather than just -$x.

The behaviour is documented in perlop, where it says:

Unary "-" performs arithmetic negation if the operand is numeric. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned. One effect of these rules is that "-bareword" is equivalent to "-bareword".

I guess my question is "But why???". This doesn't seem to serve any useful purpose, does it? For instance, if you change the script to read 

perl -ne'chomp; $x=0+$_; print "x=$x, -x=",-$x,"\n";'

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then the oddity disappears, since $x is no longer a string. That doesn't seem very DWIM-ish to me...

Edit: Added bold to highlight the part of the perlop passage I'm questioning...


Mike

In reply to Interesting unary - oddity by RMGir

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