[R] if ... problem with compound instructions

[Duncan Murdoch]

On 12/31/2006 9:35 PM, Richard Rowe wrote:
> I am having problems with the 'if' syntax.
> 
> I have an n x 4 matrix, X say.  The first two columns hold x, y values and 
> I am attempting to fill the second two columns with the quadrant in which 
> the datapoint (x, y) is and with the heading angle.
> 
> So I have two problems
> 1) how to do this elegantly (at which I've failed as I can't seem to 
> vectorize the problem) and

You would use the ifelse() function for a vectorized solution.

> 2) how to accomplish the task in a for loop ...
> 
> for (i in 1: length(X[,1])) (
>   if ((X[i,1] ==0) & (X[i,2] ==0)) (X[i,3] <- NA; X[i,4] <-NA) else (
> removing the pathological case ... then a series of nested if statements 
> assigning quadrant and calculating heading
> 
> e.g.
> if ((X[i,1] < 0) & (X[i,2] >= 0)) (X[i,3] <- 4; X[i,4] <- 
> atan(X[i,1]/X[i,2]) + 2*pi) else (

Use {} rather than () around blocks of statements, and &&
rather than & for most logical constructions, e.g.

if ((X[i,1] < 0) && (X[i,2] >= 0)) {
   X[i,3] <- 4; X[i,4] <- atan(X[i,1]/X[i,2]) + 2*pi
} else {

...

Duncan Murdoch

> 
> 
> In the first instance the ';'  seems to the source of a syntax 
> error.  Removing the second elements of the compound statement solves the 
> syntax problem and the code runs.
> 
> As the R syntax is supposed to be 'Algol-like' I had thought
> 
> if <A> then <B> else <C>
> should work for compound <B> ... ie that the bracket (X[i,3] <- NA; X[i,4] 
> <-NA) should be actioned
> 
> 1) any elegant solutions to what must be a common task?
> 
> 2) any explanations for the ';' effect ?
> 
> thanks
> 
> Richard Rowe
> 
> Dr Richard Rowe
> Zoology & Tropical Ecology
> School of Tropical Biology
> James Cook University
> Townsville 4811
> AUSTRALIA
> 
> ph +61 7 47 81 4851
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> 
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