Aha...interesting. Thanks!
On Aug 10, 2011, at 11:44 AM, Robert Young wrote:
> Hi Ralph,
>
> If you run your program with "use strict;", it'll catch the problem --
> when you assign to $xdata[$i], that's the $i-th element of the Perl
> array @xdata, not the pdl value $xdata.
>
> To access an individual value, you need to use at, set, or the slicing
> functions in PDL::NiceSlice. So you can replace your loop with
> something like:
>
> for $i (0..15) {
> $xdata->set($i,$i);
> $ydata->set($i,$i * $i);
> print "i ", $i, " x ", $xdata->at($i), " y ", $ydata->at($i), "\n";
> }
>
> or
>
> use PDL::NiceSlice;
> for $i (0..15) {
> $xdata($i)=$i;
> $ydata($i)=$i * $i;
> }
> print "x: $xdata\ny: $ydata\n";
>
> or you can do it with no loop at all:
>
> $xdata=sequence float, 16;
> $ydata=$xdata*$xdata;
>
> Best,
> Robert
>
> On Wed, Aug 10, 2011 at 12:30 PM, Ralph Castain <[email protected]> wrote:
>> Hi folks
>>
>> Afraid I have to ask an embarrassingly noobie question. I'm trying to get a
>> fitting program to work, and thought I would use the fitpoly1d function to
>> do it. However, I'm having trouble getting a very basic example to work,
>> probably because I'm not understanding the PDL data system.
>>
>> I thought I would just try a simple quadratic fit for the example. The usual
>> perl methods for generating the data arrays don't seem to work, but having
>> perused the PDL site plus various examples I found on the web, I managed to
>> get something to at least run (see below).
>>
>> The problem is that the fit comes out as linear, and I can't seem to access
>> the output values. I'm sure I'm just being stupid, but can someone point out
>> the obvious mistake?
>>
>> Thanks
>> Ralph
>>
>> #!/opt/local/bin/perl
>> #
>>
>> use PDL;
>> use PDL::MATH;
>> use PDL::Fit::Polynomial;
>>
>> my $xdata = sequence float, 16;
>> my $ydata = sequence float, 16;
>>
>> my $i = 0;
>> while ($i < 16) {
>> $xdata[$i] = $i;
>> $ydata[$i] = $i * $i;
>> print "i ", $i, " x ", $xdata[$i], " y ", $ydata[$i], "\n";
>> $i++;
>> }
>>
>> ($yfit, $coeffs) = fitpoly1d $xdata, $ydata, 3;
>>
>> print "Fit complete with coeffs ", $coeffs, "\n";
>>
>> print "Fitted values ", $yfit, "\n";
>>
>> $i = 0;
>> while ($i < 16) {
>> print "i ", $i, " yfit ", $yfit[$i], "\n";
>> $i++;
>> }
>>
>>
>> Output:
>>
>> $ ./pdl.pl
>> i 0 x 0 y 0
>> i 1 x 1 y 1
>> i 2 x 2 y 4
>> i 3 x 3 y 9
>> i 4 x 4 y 16
>> i 5 x 5 y 25
>> i 6 x 6 y 36
>> i 7 x 7 y 49
>> i 8 x 8 y 64
>> i 9 x 9 y 81
>> i 10 x 10 y 100
>> i 11 x 11 y 121
>> i 12 x 12 y 144
>> i 13 x 13 y 169
>> i 14 x 14 y 196
>> i 15 x 15 y 225
>> Fit complete with coeffs [-2.6645353e-14 1 -1.4210855e-15]
>> Fitted values [-2.6645353e-14 1.0000001 2.0000001 3 4.0000002
>> 5.0000001 6.0000001 7 8.0000004 9.0000004 10 11
>> 12 13 14 15]
>> i 0 yfit
>> i 1 yfit
>> i 2 yfit
>> i 3 yfit
>> i 4 yfit
>> i 5 yfit
>> i 6 yfit
>> i 7 yfit
>> i 8 yfit
>> i 9 yfit
>> i 10 yfit
>> i 11 yfit
>> i 12 yfit
>> i 13 yfit
>> i 14 yfit
>> i 15 yfit
>>
>>
>>
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>>
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