p.constrain {simecol} | R Documentation |
Transform Data Between Unconstrained and Box-constrained Scale
Description
These functions can be used to transform a vector of data or parameters between unconstrained [-Inf, Inf] and box-constrained representation (interval [lower, upper]).
Usage
p.constrain(p, lower = -Inf, upper = Inf, f = 1)
p.unconstrain(p, lower = -Inf, upper = Inf, f = 1)
Arguments
p |
vector of data (e.g. model parameters), |
lower , upper |
vectors with lower resp. upper bounds used for transformation, |
f |
optional scaling factor. |
Details
These functions are employed by fitOdeModel
ssqOdeModel
in order to be able to use the unconstrained
optimizers of optim
for constrained optimization.
The transformation functions are
p' = \tan(\pi/2 \cdot (2 p - upper - lower) / (upper - lower)) \cdot 1/f
and its inverse
p = (upper + lower)/2 + (upper - lower) \cdot \arctan(f \cdot p')/\pi
.
Value
vector with transformed (resp. back-transformed) values.
References
This trick seems to be quite common, but in most cases it is preferred to apply optimizers that can handle constraints internally.
Reichert, T. (1998) AQUASIM 2.0 User Manual. Computer Program for the Identification and Simulation of Aquatic Systems. Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH - 8600 Duebendorf Switzerland, https://www.eawag.ch/de/abteilung/siam/software/.
See Also
Examples
xx <- seq(-100, 100, 2)
plot(xx, yy<-p.constrain(xx, -20, 45), type="l")
points(p.unconstrain(yy, -20, 45), yy, col="red")