# linear extrapolation to abscissa Classic List Threaded 5 messages Open this post in threaded view
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## linear extrapolation to abscissa Readers, I want to plot a least squares fit linear regression for the following data set: x=0,5,10,15,20; y=16,45,77,101,125 Then I want to perform an extrapolation to obtain a value for x when y=0. How do I perform this please? The documentation is very unfriendly to a statistics and scilab novice! First I tried to read about the interp function but didn't understand the spline references so moved to interpln instead. ;) The help section should have stated that plot2d is prerequisite knowledge. So I read about plot2d. What does the function yi perform? What is the purpose of the apostrophe (') in the example shown in the help? Yours, [hidden email] scilab 411 mandriva 2008
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## Re: linear extrapolation to abscissa 1. get [a, b] in y = ax + b    coefs = regress(x, y); 2. calculate x for y = 0 from    y = ax + b    x = (y-b)/a with y = 0:    x = -b/a that is    x = -coefs(1)/coefs(2) Stefan On 2009-01-14 23:25, e-letter wrote: -------------------- > Readers, > > I want to plot a least squares fit linear regression for the following data set: > > x=0,5,10,15,20; y=16,45,77,101,125 > > Then I want to perform an extrapolation to obtain a value for x when > y=0. How do I perform this please? > > The documentation is very unfriendly to a statistics and scilab > novice! First I tried to read about the interp function but didn't > understand the spline references so moved to interpln instead. ;) > > The help section should have stated that plot2d is prerequisite > knowledge. So I read about plot2d. What does the function yi perform? > What is the purpose of the apostrophe (') in the example shown in the help? > > Yours, > > [hidden email] > > scilab 411 > mandriva 2008 >
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## Re: linear extrapolation to abscissa In reply to this post by e-letter >1. get [a, b] in y = ax + b Entering this into the scilab command resulted in error 276 > >   coefs = regress(x, y); > >2. calculate x for y = 0 from >   y = ax + b >   x = (y-b)/a > >with y = 0: >   x = -b/a >that is >   x = -coefs(1)/coefs(2) Please explain in full detail; I do not understand any of what you wrote! Sorry, These (indented!) are the only commands you should enter (the other lines were meant to be explanations): 1. The first command gives a vector coefs, which contains [a, b] a = coefs(1) b = coefs(2)    coefs = regress(x, y); 2. The second command uses these: coefs(1) (which is a) and coefs(2) (which is b) to compute x when y = 0 by rearranging the first equation (y = ax + b):    x = -coefs(1)/coefs(2) I hope that you should be able to understand (or at least use) what I wrote (trying to explain) earlier with this addition. Stefan PS  You have to know some basic mathematics to use Scilab... ;-) On 2009-01-15 00:15, e-letter wrote: -------------------- >> 1. get [a, b] in y = ax + b > Entering this into the scilab command resulted in error 276 >>   coefs = regress(x, y); >> >> 2. calculate x for y = 0 from >>   y = ax + b >>   x = (y-b)/a >> >> with y = 0: >>   x = -b/a >> that is >>   x = -coefs(1)/coefs(2) > > Please explain in full detail; I do not understand any of what you wrote! > In reply to this post by e-letter For the benefit of other novices, below are my commands in full: x=[0 5 10 15 20]; y=[[16 45 77 101 125] x=-coefs(1)/coefs(2) scilab reports results: x=-3.28... How do I copy text in the scilab window into other programs?