[R] Need help to locate my mistake

[Louise Hoffman]

>  This is certainly ***NOT*** correct. (If you really got those numbers
>  from Matlab, then Matlab is up to Puttee.)

It was my mistake =) I had calculated the straight line so the edge of
the plot was the y-axis =)

>  Have you plotted your data?
>
>         (1) Fitting a straight line is ridiculous.

Yes, I guess we have to realise that in the project, and explain why
it is not good enough =) Next step is to use exponential smoothing =)

>         (2) If you are so foolish as to fit a straight line, you get
>         theta to have entries -4197.96 (intercept) and 2.16 (slope).
>         The line y = 79.69 + 0.18*x is off the edge of the graph and
>         does not even appear.

It was clearly a bad mistake of mine! =(

>         Yes.  The expression (t(x)%*%x)^(-1) is the matrix of entry
>         by entry reciprocals of the entries of t(x)%*%x.
>
>         You want:
>
>                 theta <- solve(t(x)%*%x))%*%t(x)%*%y

Thanks =)

>         Anyhow, if you're going to use R, why not ***use R***?
>
>         fit <- lm(fpi ~ rtime,data=fuelData)
>         theta <- coef(fit)
>
>         This gives an answer identical to that from the corrected version of
>         your ``from scratch'' expression.  (That expression, while
>  theoretically
>         correct, is numerically ill-advised.  The cognoscenti use either the
>         Choleski or the ``qr'' decomposition of t(x)%*%x to effect the
>  calculations.

This is great stuff!