Perl: Where am I going wrong with postfix/prefix precedence? - perl

Question

The following fragment shows a simple n-perline output procedure.
Two cases are shown, one using prefix ++, the other postfix ++, in a boolean expression.
Since '++' has higher precedence than '==', I expected the results to be the same, but they are not: one does 5 per line, the other 6.
use English;
my #arr = (1,2,3,4,5,6,7,8,9,8,7,6);
my $perline = 5;
my $ndone = 0;
for(#arr) {
print " $ARG";
if(++$ndone == $perline) {
$ndone = 0;
print "\n";
}
}
print "\n---\n";
my $perline = 5;
my $ndone = 0;
for(#arr) {
print " $ARG";
if($ndone++ == $perline) {
$ndone = 0;
print "\n";
}
}
Output:
1 2 3 4 5
6 7 8 9 8
7 6
---
1 2 3 4 5 6
7 8 9 8 7 6

Answer 1

This is not about precedence of operations but about what prefix and postfix ++ return. From perldoc perlop:
"++" and "--" work as in C. That is, if placed before a variable, they
increment or decrement the variable by one before returning the value, and
if placed after, increment or decrement after returning the value.
Essentially you could define these as functions:
sub prefix_plusplus {
$_[0] = $_[0] + 1; # increment value
return $_[0]; # returns value after increment
}
sub postfix_plusplus {
my $before = $_[0];
$_[0] = $_[0] + 1; # increment value
return $before; # returns value before increment
}
my $x = my $y = 5;
printf "%d,%d\n", prefix_plusplus($x), postfix_plusplus($y); # 6,5
printf "%d,%d\n", $x, $y; # 6,6
# and same thing with the ++ operand
$x = $y = 5;
printf "%d,%d\n", ++$x, $y++; # 6,5
printf "%d,%d\n", $x, $y; # 6,6