<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=Windows-1252">
<meta content="text/html; charset=Windows-1252">
</head>
<body bgcolor="#FFFFFF">
<div>One thing to watch out for in cases like this is the old extended precision 80-bit floating point format. Scilab 5 doesn't use it, but the Fortan program might do I guess. Scilab 4 used it too, and when I switched from version 4 to 5 some numerical results
related to resonant filters changed in my setup.</div>
<div><br>
</div>
<div>Cheers,</div>
<div>Arvid<br>
<br>
<div class="acompli_signature">Get <a href="https://aka.ms/o0ukef">Outlook for iOS</a></div>
<br>
</div>
<hr tabindex="-1" style="display:inline-block; width:98%">
<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> users <users-bounces@lists.scilab.org> on behalf of Paul Bignier <paul.bignier@scilab-enterprises.com><br>
<b>Sent:</b> Monday, August 22, 2016 9:06:50 AM<br>
<b>To:</b> Users mailing list for Scilab<br>
<b>Subject:</b> Re: [Scilab-users] Numerical precision</font>
<div> </div>
</div>
<div>
<p><br>
</p>
<p>Hello Claus,</p>
<p>If I get this right, after finding discrepancies between Scilab & Fortran, you identified the origin of the problem: the input was different.</p>
<p>So you fed Scilab the same input you had in Fortran and got equal results, so no problem so far.</p>
<p>Have you tried feeding Fortran the Scilab input? See if it gives the same results as Scilab.</p>
<p>As for the input/output variation, it's not that surprising to me if your system is poorly conditioned, especially at some points of interest (near the resonance frequency).</p>
<p>Best regards,</p>
<p>Paul<br>
</p>
<br>
<div class="moz-cite-prefix">On 08/21/2016 06:51 PM, Claus Futtrup wrote:<br>
</div>
<blockquote type="cite">
<div class="moz-cite-prefix">Hi there<br>
<br>
OK, so I went ahead and took some Fortran input ... yes, I do get the same output as Fortran, so it seems there's no calculation error per se in Scilab. It's just tiny differences in the input which generate large differences in output. It's scary.<br>
<br>
If you compare the input (dz1 in scilab vs. dz1 in fortran, you get differences of around :<br>
<br>
1->dz1s./dz1f<br>
ans =<br>
<br>
0.9998331 + 0.0000339i <br>
0.9998132 + 0.0000391i <br>
0.9998197 + 0.0000391i <br>
<br>
<br>
And if you compare input for dz2, you get similar tiny differences :<br>
<br>
-1->dz2s./dz2f<br>
ans =<br>
<br>
0.9999629 + 0.0000126i <br>
0.9999600 + 0.0000145i <br>
0.9999635 + 0.0000145i <br>
<br>
The difference in magnitude is on the scale of less than 0.02%.<br>
<br>
Here's a complete script, including comments which shows the results and explains. I only include the three datapoints around fs, which are of interest:<br>
<br>
<pre style="font-family:Monospaced; font-style:normal; font-size:12.0"><span style="color:rgb(100,174,100); font-style:italic">// dz_test.sce</span>
<span style="color:rgb(100,174,100); font-style:italic">// Scilab input/output for zm_star:</span>
<span style="color:rgb(0,0,0)">dz1s</span> <span style="color:rgb(92,92,92)">=</span> <span style="color:rgb(74,85,219)">[</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.503483777422459</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.5757953650438843</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span>
<span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.5114164682962974</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.5883274915921053</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span>
<span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.5189524114871444</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.6013228498050527</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span><span style="color:rgb(74,85,219)">]</span>
<span style="color:rgb(0,0,0)">dz2s</span> <span style="color:rgb(92,92,92)">=</span> <span style="color:rgb(74,85,219)">[</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.9956247120533783</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">1.1585555511721832</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span>
<span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">1.019729681961281</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">1.1864195137402458</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span>
<span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">1.0436439978573127</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">1.2149095747738672</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span><span style="color:rgb(74,85,219)">]</span>
<span style="color:rgb(0,0,0)">mu</span> <span style="color:rgb(92,92,92)">=</span> <span style="color:rgb(188,143,143)">1.997015667744342</span><span style="color:rgb(0,0,0)">;</span>
<span style="color:rgb(0,0,0)">zm_star</span> <span style="color:rgb(92,92,92)">=</span> <span style="color:rgb(74,85,219)">(</span><span style="color:rgb(188,143,143)">1</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(0,0,0)">mu</span><span style="color:rgb(74,85,219)">)</span><span style="color:rgb(92,92,92)">*</span> <span style="color:rgb(0,0,0)">dz1s</span> <span style="color:rgb(92,92,92)">.*</span> <span style="color:rgb(0,0,0)">dz2s</span> <span style="color:rgb(92,92,92)">./</span> <span style="color:rgb(74,85,219)">(</span><span style="color:rgb(0,0,0)">dz2s</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(0,0,0)">mu</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(0,0,0)">dz1s</span><span style="color:rgb(74,85,219)">)</span><span style="color:rgb(0,0,0)">;</span>
<span style="color:rgb(100,174,100); font-style:italic">// =</span>
<span style="color:rgb(100,174,100); font-style:italic">// 67.582546 - 57.549843i </span>
<span style="color:rgb(100,174,100); font-style:italic">// 104.56363 + 0.9610187i </span>
<span style="color:rgb(100,174,100); font-style:italic">// 64.846465 + 47.016802i</span>
<span style="color:rgb(100,174,100); font-style:italic">// Fortran input:</span>
<span style="color:rgb(0,0,0)">dz1f</span> <span style="color:rgb(92,92,92)">=</span> <span style="color:rgb(74,85,219)">[</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.50358736782056468</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.57587442109900699</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span>
<span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.51153502148422891</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.58841744553307240</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span>
<span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.51906952093260994</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.60141095056474114</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span><span style="color:rgb(74,85,219)">]</span>
<span style="color:rgb(0,0,0)">dz2f</span> <span style="color:rgb(92,92,92)">=</span> <span style="color:rgb(74,85,219)">[</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">0.99567617243878548</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">1.1585859673221321</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span>
<span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">1.0197876382178386</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">1.1864521484345998</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span>
<span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">1.0436997434456954</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(188,143,143)">1.2149387979780446</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(218,112,214)">%i</span><span style="color:rgb(74,85,219)">]</span>
<span style="color:rgb(0,0,0)">zm_star</span> <span style="color:rgb(92,92,92)">=</span> <span style="color:rgb(74,85,219)">(</span><span style="color:rgb(188,143,143)">1</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(0,0,0)">mu</span><span style="color:rgb(74,85,219)">)</span><span style="color:rgb(92,92,92)">*</span> <span style="color:rgb(0,0,0)">dz1f</span> <span style="color:rgb(92,92,92)">.*</span> <span style="color:rgb(0,0,0)">dz2f</span> <span style="color:rgb(92,92,92)">./</span> <span style="color:rgb(74,85,219)">(</span><span style="color:rgb(0,0,0)">dz2f</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(0,0,0)">mu</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(0,0,0)">dz1f</span><span style="color:rgb(74,85,219)">)</span><span style="color:rgb(0,0,0)">;</span>
<span style="color:rgb(100,174,100); font-style:italic">// Scilab output:</span>
<span style="color:rgb(100,174,100); font-style:italic">// =</span>
<span style="color:rgb(100,174,100); font-style:italic">// 66.547108 - 58.427427i </span>
<span style="color:rgb(100,174,100); font-style:italic">// 105.67638 - 0.8443203i </span>
<span style="color:rgb(100,174,100); font-style:italic">// 66.018415 + 47.279403i </span>
<span style="color:rgb(100,174,100); font-style:italic">// Fortran output:</span>
<span style="color:rgb(100,174,100); font-style:italic">// ( 66.547108467527437 , -58.427426595183157 )</span>
<span style="color:rgb(100,174,100); font-style:italic">// ( 105.67638116311093 ,-0.84432029182632362 )</span>
<span style="color:rgb(100,174,100); font-style:italic">// ( 66.018415098214774 , 47.279402998470466 )</span></pre>
<br>
As can be seen. Scilab gives the same output as Fortran, when the input is the same.<br>
<br>
I'm just totally surprised how such small differences in input can generate such large differences in output.<br>
<br>
Best regards,<br>
Claus<br>
<br>
On 21-08-2016 16:11, Claus Futtrup wrote:<br>
</div>
<blockquote type="cite">
<p>Dear Scilab Users</p>
<p>I have a script, which imitates a Fortran script. I can see there's a difference in calculation of about 1%, which is very strange to me. Input parameters (dz1 and dz2 vectors, length 1200) to the equation seem to agree within 0,1% ... so right now my theory
is that the precision slips away when I do the following calculation:</p>
<p>Scilab: <span style="color:rgb(0,0,0)">zm_star</span> <span style="color:rgb(92,92,92)">
=</span> <span style="color:rgb(74,85,219)">(</span><span style="color:rgb(188,143,143)">1</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(0,0,0)">mu</span><span style="color:rgb(74,85,219)">)</span><span style="color:rgb(92,92,92)">*</span>
<span style="color:rgb(0,0,0)">dz1</span> <span style="color:rgb(92,92,92)">.*</span>
<span style="color:rgb(0,0,0)">dz2</span> <span style="color:rgb(92,92,92)">./</span>
<span style="color:rgb(74,85,219)">(</span><span style="color:rgb(0,0,0)">dz2</span><span style="color:rgb(92,92,92)">-</span><span style="color:rgb(0,0,0)">mu</span><span style="color:rgb(92,92,92)">*</span><span style="color:rgb(0,0,0)">dz1</span><span style="color:rgb(74,85,219)">)</span><span style="color:rgb(0,0,0)">;</span>
<span style="color:rgb(100,174,100); font-style:italic">// model free mech. impedance</span></p>
<p>Fortran: zm_star = (1-mu)*dz1*dz2/(dz2-mu*dz1)<br>
</p>
<p>Do you see anything in the Scilab formulation, which should worry me / which would give me such a high error?</p>
<p>P.S. mu is a mass-ratio = 1.9970156677443420... and it's exactly the same value in both Scilab and Fortran.<br>
</p>
<p>Most of the calculations in the output vector are OK, but in particular around the resonance frequency, I can list the following three datapoints (zm_star(47:49)), to show what is worrying me:</p>
<p>In Fortran, data no. 47-48-49 (near fs):<br>
<br>
( 66.547108467527437 , -58.427426595183157 )<br>
( 105.67638116311093 ,-0.84432029182632362 )<br>
( 66.018415098214774 , 47.279402998470466 )<br>
<br>
Whereas in Scilab I get:<br>
<br>
67.58254632254881 - 57.549843258298814*%i<br>
104.56362768103634 + 0.9610187273575014*%i<br>
64.84646498264745 + 47.01680213507681*%i</p>
<p>In particular the imaginary part is different at the middle data point (near fs) where from Fortran the value is negative, whereas in Scilab the value is positive. It seems that the calculation of zm_star involves some math operations that could be critical
to the precision.</p>
<p>If you study the impedance magnitude (pythagoras...), the results from Fortran are about 1% higher in value - IMHO that's non-negligible.</p>
<p>Studying the Nyquist circle plot of the data, it seems to me that in general the Fortran calculation is more correct.</p>
<p>What is going wrong with the Scilab equation? Please let me know if you have any ideas how to increase the precision of the calculation. Thanks.</p>
<p>P.S. I'm using Scilab 5.5.0 (64 bit, Windows 10). Could it be the Intel Math Kernel that's doing this wrong?<br>
</p>
<p>/Claus<br>
</p>
</blockquote>
<p><br>
</p>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset> <br>
<pre>_______________________________________________
users mailing list
<a class="moz-txt-link-abbreviated" href="mailto:users@lists.scilab.org">users@lists.scilab.org</a>
<a class="moz-txt-link-freetext" href="http://lists.scilab.org/mailman/listinfo/users">http://lists.scilab.org/mailman/listinfo/users</a>
</pre>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Paul BIGNIER
Development engineer
-----------------------------------------------------------
Scilab Enterprises
143bis rue Yves Le Coz - 78000 Versailles, France
Phone: +33.1.80.77.04.68
<a class="moz-txt-link-freetext" href="http://www.scilab-enterprises.com">http://www.scilab-enterprises.com</a></pre>
</div>
</body>
</html>