[R] Difficulties with R.oo (static fields, etc.)

[Henrik Bengtsson]

This is not really an rhelp question. I'll get back to you later today
with answers related to R.oo.

Henrik Bengtsson
The R.oo author

> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of 
> Gabriel Baud-Bovy
> Sent: den 27 oktober 2003 01:56
> To: r-help at stat.math.ethz.ch
> Subject: [R] Difficulties with R.oo (static fields, etc.)
> 
> 
> I would like to use R.oo and tcltk to implement a Turtle 
> World. I have 
> encountered
> many problems because:
> 
> 1) I am not sure how to implement static fields with R.oo
> 2) I am not sure how to implement a constructor that would
>      call a function only for the first instance of a class (i.e., to 
> initialize
>      value of static fields only once)
> 3) I am not sure how to remove/delete cleanly existing instances. In 
> particular,
>      I don't know how to reset tcktk when the last instance 
> is deleted.
> 
> Here is the sort of behavior I would like to get:
> 
> t1<-TurtleBasic()   # a window (the Turtle world) with a 
> turtle appears
> forward(t1,50)       #  the turtle moves (turtle's state is 
> also updated)
> turn(t1,pi/2)
> forward(t1,50)
> t2<-TurtleBasic()  # a second turtle in the Turtle world appears
> turn(t2,pi/2)  # second turtle moves
> forward(t1,50)
> delete(t1) # first turtle disappears
> delete(t2) # second turtle and the world disappears
> 
> If possible, I would like to keep this syntax. In fact, I am 
> using R.oo to create mutable objects so as to avoid syntax 
> like  tu<-forward(tu,10) which would be more typical of a 
> functional language like R. From the doc/help 
> files of R.oo,
> I believe that one could do it but I don't find many examples 
> to guide me.
> 
> I pasted my current code with comments below. Thank you for your help.
> 
> Gabriel Baud-Bovy
> 
> #-------------------------------------------------------------
> --------------------------------------------------------
> 
> Description of Turtle Basic Class:
> 
> private fields:
>    .x, .y, .a (turtle position and heading)
>    .turtle: ID of canvas item representing turtle
> static fields:
>    .canvas: tcltk canvas widget
>    .top: tcktk toplevel widget
> methods:
>     TurtleBasic: constructor
>     plot: display turtle in Turtle World
>     forward, turn: move turtle
>     delete: delete turtle (not implemented)
> 
> Note: My current code is very buggy:
>   - Can't see turtle movements
>   - Does not define method to delete turtles
>   - I get an tcltk error when I try to recreate a turtle 
> world after having 
> tried to destroy it
>     (for the moment, I need to detach tcltk to reset it)
> 
> #-------------------------------------------------------------
> --------------------------------------------------------
> 
> library("R.oo")
> 
> setConstructorS3("TurtleBasic",function() {
>    new<-extend(Object(),"TurtleBasic",.x=100,.y=100,.a=0,.turtle=NA)
>    ## Create a new Turtle World if necessary
>    require(tcltk) || stop("tcl/ library not available")
>    if(!is.tkwin(new$.canvas)) { # check to see if it is first 
> instance or not
>      cat("Create Turtle World\n")
>      top <- tktoplevel()
>      tktitle(top) <- "Turtle World"
>      canvas <- tkcanvas(top, relief="raised", width=200, height=200)
>      tkpack(canvas, side="top", fill="both",expand="1")
>      new$.canvas<-canvas 	 			# store 
> canvas widget in static field
>      new$.top<-top
>    }
>    plot.TurtleBasic(new)					
> # plot Turtle
>    new
> })
> 
> setMethodS3("plot","TurtleBasic",function(turtle) {
>      ## delete old item (if it exists)
>      if(is.tkwin(turtle$.canvas) && is.tclObj(turtle$.turtle)) 
> tkdelete(turtle$.canvas,turtle$.turtle)
>      ## a new canvas item representing the turtle is created
>      ## (note: it's necessary to make new item because 
> turtle's heading can 
> change)
>      x <- c(-10,10)
>      y <- c(-10,10)
>      aux<-   cos(turtle$.a)*x+sin(turtle$.a)*y + turtle$.x
>      y  <-  -sin(turtle$.a)*x+cos(turtle$.a)*y + turtle$.y
>      x  <- aux
>      turtle$.turtle <- tkcreate(turtle$.canvas, "line", 
> x[1],-y[1],x[2],-y[2],width=1)
> })
> 
> setMethodS3("forward","TurtleBasic",function(turtle,length){
>    aux       <- turtle$.x + length*cos(turtle$.a)
>    turtle$.y <- turtle$.y + length*sin(turtle$.a)
>    turtle$.x <- aux
>    plot(turtle)
> })
> 
> setMethodS3("turn","TurtleBasic",function(turtle,angle){
>     turtle$.a<-turtle$.a+angle
>     plot(turtle)
> })
> 
> 
> if(0) {
> 
> t1<-TurtleBasic()  # a window (the Turtle world) with a turtle appears
> forward(t1,50)
> turn(t1,pi/2)
> forward(t1,50)
> t2<-TurtleBasic()  # a second turtle in the Turtle world
> turn(t2,pi/2)
> 
> # manually destroy first turtle (would be great to define 
> some sort of 
> method for it)
> tkdelete(t1$.canvas,t1$.turtle)
> rm(t1)
> # destroy second turtle and world
> tkdelete(t2$.canvas,t2$.turtle)
> tkdestroy(t2$.top)
> rm(t2)
> 
> }
> --------------------------------------------------------------------
> Gabriel Baud-Bovy
> Assistant Professor
> UHSR University
> via Olgettina, 58	tel:  (+39) 02 2643 4839
> 20132 Milan, Italy	fax: (+39) 02 2643 4892
> 
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