By using commas in your original code, that is the type of loop you created. Your first attempt also demonstrates how this "for (list...)" structure evaluates (creates) the entire list before doing the first iteration. In effect, the OP code is sort of equivalent to:

$i = 500;
$i = 900;
$i += 100;

for ( $i, $i, $i ) {
    ....
}
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    my $theta = $T * (( 1013 / $_ ) ** ( 287/1004 ));
    push ( @press, $_ );
    push ( @thetas, $theta );
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I was taught that for structure was like this: (starting value, ending value, increment). But I infer from your response that it must be: (starting value, continue while this is true, increment)?

It's for (EXPR1; EXPR2; EXPR3) {...}. Note that there are semi-colons between the expressions, not commas.

The first expression is run before entering the loop. Before each iteration the second expression is run; if false, the loop terminates. After each iteration, the third expression is run. For further details, see man perlsyn.

Perls three-argument for loop is exactly like in C: initializer, test, and counting. See also http://en.wikipedia.org/wiki/For_loop#Three-expression_for_loops. Other languages may implement for loops differently, notably BASIC and its derivates often have constructs like for i=1 to 100 step 3 (counting up from 1 with increments to 3 until 100 is reached or exceeded) or even for i=100 downto 1 step 3 (counting down in steps of 3, starting at 100, stopping when 1 is reached or exceeded). Other languages have even stranger for loops, see the wikipedia article.

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

I was taught that for structure was like this: (starting value, ending value, increment). But I infer from your response that it must be: (starting value, continue while this is true, increment)?

Won't hurt you to read the docs...

What's wrong:

• The first expression has nothing to do with a "starting value". (Starting value of what??? It's not like C-style loops have counters.)

• The last expression has nothing to do with an "increment". (Incrementing what??? It's not like C-style loops have counters.)

What it really means

• The first expression (if specified) is evaluated unconditionally before the loop is started.

• The second expression is evaluated at the start of every loop pass (incl the first). If an expression is specified and it evaluates to something false, the look exits.

• The third expression (if specified) is evaluated at the end of every loop pass (even if next is called).

A common usage:

for (my $i = 0; $i < $n; ++$i) {
   ...
}
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The above is equivalent to the following, except $i is scoped to the loop.

my $i = 0;
while ($i < $n) {
   ...
} continue {
   ++$i
}
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use strict;
use warnings;

# no need for your comment! it is clear!!
my $outfilename = "pot_temp.txt";  # define an output filename

# always check the return values of file operations !!!
# there a bunch of syntax'es for this, the main thing is to do it!

#open(TFILE,">$outfilename") || die ("unable to open $outfilename $!")
+;
# below I don't use TFILE, this is just a test to stdout

#there are shorter ways to do this, but this is staightforward. 
#Maybe these are pressure increments? And I have wrong name?
#whatever, this is one way to do it...

my @temp_increments;
for (my $i=500; $i<=900 ; $i += 100)
{
   push @temp_increments, $i;
}

#my @temp_increments = qw( 500, 600, 700, 800, 900);
#would be equivalent for example

print "What is the temperature (K)?:";
my $T = <STDIN>;
chomp ($T);

print "T results are:\n";
foreach my $temp (@temp_increments)
{
  my $theta = $T*((1013/$temp)**(287/1004));
  print "temp = $temp  theta=$theta \n";
}

#Example output......
#C:\TEMP>perl temp.pl
#What is the temperature (K)?:26
#T results are:
#temp = 500  theta=31.8147304436198
#temp = 600  theta=30.1990838700515
#temp = 700  theta=28.8972548674746
#temp = 800  theta=27.8150092740013
#temp = 900  theta=26.8940943585568
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The C style for loop is rare in Perl. Strive to use this in as limited of scope and context as possible.