Perl to count current value based on next value - perl

Question

Currently I'm learning Perl and gnuplot. I would like to know how to count certain value based on the next value. For example I have a text file consist of:
#ID(X) Y
1 1
3 9
5 11
The output should show the value of the unknown ID as well. So, the output should show:
#ID(X) Y
1 1
2 5
3 9
4 10
5 11
The Y of ID#2 is based on the following:
((2-3)/(1-3))*1 + ((2-1)/(3-1))*9 which is linear algebra
Y2=((X2-X3)/(X1-X3))*Y1 + ((X2-X1)/(X3-X1)) * Y3
Same goes to ID#5
Currently I have this code,
#! /usr/bin/perl -w
use strict;
my $prev_id = 0;
my $prev_val = 0;
my $next_id;
my $next_val;
while (<>)
{
my ($id, $val) = split;
for (my $i = $prev_id + 1; $i < $next_id; $i++)
{
$val = (($id - $next_id) / ($prev_id - $next_id)) * $prev_val + (($id - $prev_id) / ($next_id - $prev_id)) * $next_val;
printf ("%d %s\n", $i, $val);
}
printf ("%d %s\n", $id, $val);
($prev_val, $prev_id) = ($val, $id);
($next_val, $next_id) = ($prev_val, $prev_id);
}

Answer 1

Your formula seems more complicated than I would expect, given that you are always dealing with integer spacings of 1.
You did not say whether you want to fill gaps for multiple consecutive missing values, but let's assume you want to.
What you do is read in the first line, and say that's the current one and you output it. Now you read the next line, and if its ID is not the expected one, you fill the gaps with simple linear interpolation...
Pseudocode
(currID, currY) = readline()
outputvals( currID, currY )
while lines remain do
(nextID, nextY) = readline()
gap = nextID - currID
for i = 1 to gap
id = currID + i
y = currY + (nextY - currY) * i / gap
outputvals( id, y )
end
(currID, currY) = (nextID, nextY)
end
Sorry for the non-Perl code. It's just that I haven't been using Perl for ages, and can't remember half of the syntax. =) The concepts here are pretty easy to translate into code though.

Answer 2

Using an array may be the way to go. This will also make your data available for further manipulation.
** Caveat: will not work for multiple consecutive missing values of y; see #paddy's answer.
#!/usr/bin/perl
use strict;
use warnings;
my #coordinates;
while (<DATA>) {
my ($x, $y) = split;
$coordinates[$x] = $y;
}
# note that the for loop starts on index 1 here ...
for my $x (1 .. $#coordinates) {
if (! $coordinates[$x]) {
$coordinates[$x] = (($x - ($x + 1)) / (($x - 1) - ($x + 1)))
* $coordinates[$x - 1]
+ (($x - ($x - 1)) / (($x + 1) - ($x - 1)))
* $coordinates[$x + 1];
}
print "$x - $coordinates[$x]\n";
}
__DATA__
1 1
3 9
5 11

Answer 3

You indicated your problem is getting the next value. The key isn't to look ahead, it's to look behind.
my $prev = get first value;
my ($prev_a, $prev_b) = parse($prev);
my $this = get second value;
my ($this_a, $this_b) = parse($this);
while ($next = get next value) {
my ($next_a, $next_b) = parse($next);
...
$prev = $this; $prev_a = $this_a; $prev_b = $this_b;
$this = $next; $this_a = $next_a; $this_b = $next_b;
}

Answer 4

#! /usr/bin/perl -w
use strict;
my #in = (1,9,11);
my #out;
for (my $i = 0; $i<$#in; $i++) {
my $j = $i*2;
my $X1 = $i;
my $X2 = $i+1;
my $X3 = $i+2;
my $Y1 = $in[$i];
my $Y3 = $in[$i+1];
my $Y2 = $Y1*(($X2-$X3)/($X1-$X3))
+ $Y3*(($X2-$X1)/($X3-$X1));
$out[$j] = $in[$i];
$out[$j+1] = $Y2;
}
$out[$#in*2] = $in[$#in];
print (join " ",#out);