[R] problem in while loop?

[Berend Hasselman]

On 30-08-2013, at 09:44, Jonsson <amen.alyaari at Bordeaux.inra.fr> wrote:

> I have three datasets that I want to compute the errors between them using
> linear regression.for this, I want to iterate to reach certain criteria for
> the calibration. if changes become smaller than eps the iteration is
> successful, hence stop and write parameters into cal:eps=0.00001 if number
> of iterations is > itermax the iteration failed, hence stop and fill cal
> with missing value itermax=400
> 
> So I tried this code:
> 
>                  x= c(5,2,4,2,1)
>                  y= c(5,3,4,6,9)
>                  z= c(5,8,4,7,3)
>              itermax=400
> get initial calibration parameters, here we assume that:x is the reference
> dataset offset x_a=0, slope x_b=1 the other two datasets y, z are
> "calibrated" to x using a simple linear regression
> 
>             res=lm(x~y)
>               y_a=coef(res)[1] ; y_b=coef(res)[2]
>              res1=lm(x~z)
>              z_a=coef(res1)[1] ; z_b=coef(res1)[2]
>              y_t = y/y_b - y_a/y_b  # "calibrate" y
>              z_t = z/z_b - z_a/z_b  #"calibrate" z
>              x_e = sqrt(mean((x-y_t)*(x-z_t)))#calculate error of x
>               iter <- 0
>           while(((x_e-x) > 0.00001)&& (iter < itermax)) { 
>              iter <- 0  ##start iteration
>                   x = x_e 
>                   res=lm(x~y)
>                   y_a=coef(res)[1] ; y_b=coef(res)[2]
>                   res1=lm(x~z)
>                   z_a=coef(res1)[1] ; z_b=coef(res1)[2]
>                   y_t = y/y_b - y_a/y_b  # "calibrate" y
>                   z_t = z/z_b - z_a/z_b  #"calibrate" z
>                   x_e = sqrt(mean((x-y_t)*(x-z_t)))
>                    iter <- iter + 1 # increase iteration counter
>                    } 
> But I got the same result for X_e before and after the loop:
> 
>> x_e
>                         [1] 6.454089
> 
I tried your code and got this error message:

Error in model.frame.default(formula = x ~ y, drop.unused.levels = TRUE) : 
  variable lengths differ (found for 'y')
Calls: lm -> eval -> eval -> <Anonymous> -> model.frame.default

And looking at your code:  x is a vector, x_e is a scalar and in the while loop you are assigning x_e to x so x is then a scalar.

Berend