[R] priority of operators in the FOR ( ) statement

[Patrick Burns]

The command that I think is most useful in this situation
is 'browser()'.

Even a couple decades of programming in the S language
hasn't yet solved the problem of my fingers typing code that
doesn't match what I want to happen.  I quite consistently have
a

browser()

call in functions that I write to make sure that what I am assuming
is the same as what R assumes.


Patrick Burns
patrick at burns-stat.com
+44 (0)20 8525 0696
http://www.burns-stat.com
(home of S Poetry and "A Guide for the Unwilling S User")

Ravi.Vishnu at outokumpu.com wrote:

>Dear All,
>I spent an entire evening in debugging a small, fairly simple program in R 
>- without success. It was my Guru in Bayesian Analysis, Thomas Fridtjof, 
>who was able to diagonose the problem. He said that it took a long time 
>for him also to locate the problem.
>This program illustrates in some ways the shortcomings of the error 
>messages that R responds with. In this case, it was quite misleading and 
>directs attention to a location far removed the actual problem statement.
>Without any more introductory comments, let me directly discuss the 
>essential problem. I am enclosing the entire program after a brief 
>discussion.
>
>The problem arises from the following statement (nr is an integer 
>constant) :
>for ( i in 1:nr-1) {.......}
>The unexpected problem (at least for me) is that R reads the above 
>statement as (i in (1:nr)-1) {.....}. This makes i be initially as zero 
>which leads to an error because the for loop in R starts from 1. The 
>problem is easily fixed by writing the for loop as  ( i in 1:(nr-1)) 
>{.......}. This would be an easy problem to fix if R directly indicates 
>what the problem is. Instead, it gives mystifying error messages which are 
>totally misleading. For example, to the program given below, I got the 
>following error message (these point to commands elsewhere in the program) 
>:
>Error in if ((x >= 0) & (x < s2)) return(x/2) else if ((x >= s2) & (x <  : 
>
>        missing value where TRUE/FALSE needed
>
>I would like clarifications on the following points :
>1. I am just curious to know if the priority of operators in the for 
>statement ( the colon before the minus operator, for example) is a 
>deliberate design decision. I have tested Matlab and found that it 
>interprets my original statement correctly without an extra paranthesis.
>2. Faced with a similiar problem in the future, what is a smart way of 
>debugging in R to locate a problem. With this problem, I checked and 
>double checked every single statement in the program, except the for 
>statement because I just did not expect any problem there. I have seen 
>that there is a debug package but I have not used it. Can such tools be 
>used to locate a problem with greater ease? Can somebody give a concrete 
>example (for the following program, for example) of a debugging routine.
>
>*************************************************************************'
># Bayesian Data Analysis
>## source("M:/programming/Rfolder/Assignments/fortest.txt")
>
># #Remove all objects from the workspace
>rm(list=ls())
># #We will also try to note the time that the program takes
># #We will start the clock at starttime
>starttime <- proc.time()[3];
>
>my.function<-function(x) {
>s2<-sqrt(2);
>if ((x>=0) & (x<s2)) return(x/2)
>else 
>if ((x>=s2) & (x<1+s2)) return(0.2)
>else 
>if ((x>=1+s2) & (x<1.5+s2)) return(0.6)
>else 
>if ((x>1.5+s2) | (x<0)) return(0)
>}
>
>alphayx<-function(y,x) {
>fy<-my.function(y)
>fx<-my.function(x)
>fyx<-fy/fx
># to account for 0/0 division
>if (is.na(fyx)) fyx<-0
>#fyx<-ifelse(is.na(fyx),0,fyx);
>alpha<-min(1,fyx)
>return(alpha)
>}
>
>sigma<-0.5;
>#nr is the number of iterations
>nr<-20
>x<-numeric(nr);
>x[1]<-1;
>t<-1:nr;
>
>for (i in 1:nr-1) {
>xi<-x[i];
>yi<-rnorm(1,mean=xi,sd=sigma);
>ui<-runif(1,0,1);
>ualphai<-alphayx(yi,xi);
>xn<-ifelse(ui<=ualphai,yi,xi);
>x[i+1]<-xn;
>}
>
>plot(t,x,type="p")
>
>endtime<-proc.time()[3];
>elapsedTime<-endtime-starttime;
>cat("Elapsed time is", elapsedTime, "seconds", "\n")
>*****************************************************************************'
>
>
>
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