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Fitting 2 equations to different ranges of x

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T O P I C R E V I E W
CMcRitchie	Posted - 07/28/2016 : 08:37:47 AM Hi, I wondered if it was possible to use the Fitting Function Organiser in the following way: Using x,y data where 0<x<2000 Create a function that applies two or more equations to the experimental data over different ranges of x e.g. Equation 1: y = a + (bx) where 0<x<40 Equation 2: y = a + (bx) where 40<x<2000 I know I can manually select different parts of the experimental data and apply each function in turn, however it would be really beneficial for me if this could be done using a user created function. Any help would be much appreciated, Thanks
5 L A T E S T R E P L I E S (Newest First)
Hideo Fujii	Posted - 07/28/2016 : 11:15:45 AM Hi CMcRitchie, > if (x<x0) y=function1(x); > if (x0<=x<=xa) y=function2(x); > else y=function3(x); > what is the correct command for the 2nd line? > I assume repeating "if" is not correct. The syntax should be like (assuming function type is Origin C): if(x<x0) y=function1(x); //x<x0 else if(x<xa) y=function2(x); //x0<=x<xa else y=function3(x); //xa<=x Hope this helps. --Hideo Fujii OriginLab
CMcRitchie	Posted - 07/28/2016 : 10:59:02 AM quote: Originally posted by Hideo Fujii Hi CMcRitchie, > Do you know if there is a similar alternative that would accommodate non-linear functions? Currently, there is no built-in piecewise non-linear functions. So, you need to make a user-defined function generally in the following way: y0=function1(x0); if( x<x0 ) y=function1(x); else y=function2(x); //function2 is a curve which passes (x0,y0) to connect to function1 The hard (if not impossible) part may be to find the form of function2 by solving y0=function2(x0) analytically. Good luck. --Hideo Fujii Again thanks for your help. I have started to play around with using 2 equations, if I were to introduce a third equation, what do I write as the code to introduce an additional condition on the range of x? i.e. go from PWL2 to PWL3 e.g. if (x<x0) y=function1(x); if (x0<=x<=xa) y=function2(x); else y=function3(x); what is the correct command for the 2nd line? I assume repeating "if" is not correct. Many thanks, CMcRitchie
Hideo Fujii	Posted - 07/28/2016 : 10:48:58 AM Hi CMcRitchie, > Do you know if there is a similar alternative that would accommodate non-linear functions? Currently, there is no built-in piecewise non-linear functions. So, you need to make a user-defined function generally in the following way: y0=function1(x0); if( x<x0 ) y=function1(x); else y=function2(x); //function2 is a curve which passes (x0,y0) to connect to function1 The hard (if not impossible) part may be to find the form of function2 by solving y0=function2(x0) analytically. Good luck. --Hideo Fujii
CMcRitchie	Posted - 07/28/2016 : 10:06:52 AM quote: Originally posted by Hideo Fujii Hi CMcRitchie, I think you can consider the built-in functions in the Piecewise category such as PWL2 for two segments (or PWL3 for three segments): http://www.originlab.com/doc/Origin-Help/PWL2 Please take a look. --Hideo Fujii OriginLab Hi Hideo Fujii, Thanks for your suggestion. Do you know if there is a similar alternative that would accommodate non-linear functions? Thanks, CMcRitchie
Hideo Fujii	Posted - 07/28/2016 : 09:50:20 AM Hi CMcRitchie, I think you can consider the built-in functions in the Piecewise category such as PWL2 for two segments (or PWL3 for three segments): http://www.originlab.com/doc/Origin-Help/PWL2 Please take a look. --Hideo Fujii OriginLab

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