Why does !1 give me nothing in Perl?

Question

This is strange. The following:

$sum = !0;
print $sum;

prints out 1 as you would expect. But this

$sum = !1;
print $sum;
         


        

Answer 1

See perldoc perlsyn:

Truth and Falsehood

The number 0, the strings '0' and '' , the empty list () , and undef are all false in a boolean context. All other values are true. Negation of a true value by ! or not returns a special false value. When evaluated as a string it is treated as '' , but as a number, it is treated as 0.

There, if you print the value as a number, you will get 0 rather than the empty string:

printf "%d\n", $_ for map { !$_ } (1, 0);

or

print 0 + $_, "\n" for map { !$_ } (1, 0);

Compare those to

printf "%s\n", $_ for map { !$_ } (1, 0);

and

print $_, "\n" for map { !$_ } (1, 0);