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Dear Community:
I am a very new user of MathCad, and I need help understanding the syntax of the linfit function. (At least I think it's a syntax issue).
I built a small worksheet to look at various curve fitting functions (it grew out of a need from a work related worksheet; I have hand entered some of the data and changed to units to make the example sheet self-contained). I successfully parroted the linfit example for a quadratic fit (a*x^2+b*x+c), but I could not extend it successfully for a cubic fit (a*x^3+b*x^2+cx+d). I could successfully build a cubic fit using polyc.
I enclose the worksheet:
Page 1: Data
Page 2: Cubic Spline
Page 3: Straight Line
Page 4: Quadratic linfit (and polyc)
Page 6: Cubic linfit (and polyc) <---- This is where it goes wrong
Any insights would be appreciated.
Regards,
Bob
Solved! Go to Solution.
Thank you for your answers. However, I am still perplexed. I have enclosed a PDF of my worksheet for those with Prime 3.
Page 6, from the PDF is repeated here:
(ps, qres) are x,y values. I print out the coefficients from both linfit and polyc (they come out in reverse order, but you can look at the units to compare). The coefficients for x^3 and x^2 are arguably zero. But the linear and constant terms are very, very different. This difference is plotted on page 7 (reproduced here):
The results from polyc seem reasonable, those from linfit do not.
I think that I'm using the correct syntax - but clearly that isn't so!
Regards,
Bob
I can't open your file, so I can't see what you did. But this is linfit in Prime 3.0:
If we knew that the linear term wasn't there, we could modify the function:
This document is for Mathcad Prime 4.0 version.
Thank you for your answers. However, I am still perplexed. I have enclosed a PDF of my worksheet for those with Prime 3.
Page 6, from the PDF is repeated here:
(ps, qres) are x,y values. I print out the coefficients from both linfit and polyc (they come out in reverse order, but you can look at the units to compare). The coefficients for x^3 and x^2 are arguably zero. But the linear and constant terms are very, very different. This difference is plotted on page 7 (reproduced here):
The results from polyc seem reasonable, those from linfit do not.
I think that I'm using the correct syntax - but clearly that isn't so!
Regards,
Bob
Hi Robert,
Are you still having the issue or has it been solved?
I see you have marked your last post as correct, but within, your text hints that you are still having the issue.
Thank you for clarifying.
Best,
Toby
I can duplicate this exact problem in Prime 3.0, Linfit gives an incorrect response.
I think you found a bug!
Note that "polyfit" (without the "c") will create a function: Y := poltfit (ps,qres,3) then Y(pres)= will give the function evaluated at "pres". But you probably already knew that! 😉