looping in perl - perl

Question

I am writing this loop where in the initializing i am intializing using a variable and not a absolute value.Why doesn't the value change? Or is it not allowed in the for loop?
enter code here
for($i = $one, $i > $top_level,$i--)
{
print $i,"\n";
print "One is:",$one,"\n";
}
Why can't i assign $i= $one.
The variable $one keeps changing so thats the reason why i declared $i to $one.When i print the individual values of the variables the values are correct, its just that in the for loop the value is not being assigned.

Answer 1

Use semicolons, not commas.
for($i = $one; $i > $top_level; $i--)
{
print $i,"\n";
print "One is:",$one,"\n";
}
Avoid C-Style loops in perl, if possible.
for my $i (reverse $top_level..$one) {
print $i,"\n";
print "One is:",$one,"\n";
}

Answer 2

The separator inside the for() statement is ";", not ",".

Answer 3

You've accidentally stumbled on using for with a list, by using commas instead of semi-colons. Watch what happens when you add a third print statement:
print "\$_=[$_]\n";
with $one as 1, I get:
$_=[0]
...
$_=[]
...
$_=[1]
And that is because there are three items in the list. First of all what you wanted to do, would have looked like this:
for my $i ( $one..( $top_level - 1 )) {
In this loop, $i is set to each member of the range and then the loop is executed. If we don't suppy the variable, perl assigns it to $_.
for ( $one..( $top_level - 1 )) {
And perl does not wait to compile the list, so before it ever execute the first time it goes through all the terms.
$i = $one;
# push actual $i returned by expression $i = $one
push #a, $i;
# result = [ $i=1 ]
# push boolean false => ''
push #a, ( $i > $top_level );
# result = [ $i=1, '' ]
# push the return of $i--, NOT $i
push #a, ( $i );
# result = [ $i=1, '', 1 ]
# decrement $i
$i--;
# result = [ $i=0, '', 1 ]
You can find this out using a TIE-ed scalar:
package Monitored;
sub TIESCALAR {
my ( $class, $name, $value ) = #_;
return bless { name => $name, value => $value }, $class;
}
sub FETCH {
my $self = shift;
Carp::carp "Reading \$$self->{name}...";
return $self->{value};
}
sub STORE {
my $self = shift;
my $value = shift;
Carp::carp "Storing \$$self->{name}=${\(defined( $value ) ? $value : 'undef')} ";
$self->{value} = $value;
}
And this initialization in the mainline:
my $one = 1;
tie my $i, 'Monitored', 'i';
tie my $top_level, 'Monitored', 'top_level', 5;
for($i = $one, $i > $top_level,$i--) # line 30
{
print "*LOOP*\n";
#print "\$i=$i\n"; <-- commented out to reduce noise
#print "\$one=$one\n";
print "\$_=[$_]\n"; # line 35
}
Then in running the loop the output is:
Storing $i=1 at - line 30
Reading $top_level... at - line 30
Reading $i... at - line 30
Reading $i... at - line 30
Reading $i... at - line 30
Storing $i=0 at - line 30
*LOOP*
Reading $i... at - line 35
$_=[0]
*LOOP*
$_=[]
*LOOP*
$_=[1]
Note that only one time, at line 35, do we access $i after the looping starts.

Answer 4

What on earth are you trying to do? Is this a normal 'for' loop or are you trying to do something exotic. Note use of semicolon, NOT comma. Using a comma in any loop does something completely different: runs each bit of code each time.
Normally it would be:
$one = 1;
for($i = $one; $i > $top_level;$i--)
{
print $i,"\n";
print "One is:",$one,"\n";
}
It is Perl right?