do-while loop in R

问题:

I was wondering about how to write do-while-style loop?

I found this post:

you can use repeat{} and check conditions whereever using if() and exit the loop with the "break" control word.

I am not sure what it exactly means. Can someone please elaborate if you understand it and/or if you have a different solution?

回答1:

Pretty self explanatory.

repeat{
  statements...
  if(condition){
    break
  }
}

Or something like that I would think. To get the effect of the do while loop, simply check for your condition at the end of the group of statements.

回答2:

See ?Control or the R Language Definition:

> y=0
> while(y <5){ print( y<-y+1) }
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5

So do_while does not exist as a separate construct in R, but you can fake it with:

repeat( { expressions}; if (! end_cond_expr ) {break} )

If you want to see the help page you cannot type ?while or ?repeat at the console but rather need to use ?'repeat' or ?'while'. All the "control-constructs" including if are on the same page and all need character quoting after the "?" so the interpreter doesn't see them as incomplete code and give you a continuation "+".

回答3:

Building on the other answers, I wanted to share an example of using the while loop construct to achieve a do-while behaviour. By using a simple boolean variable in the while condition (initialized to TRUE), and then checking our actual condition later in the if statement. One could also use a break keyword instead of the continue <- FALSE inside the if statement (probably more efficient).

  df <- data.frame(X=c(), R=c())  
  x <- x0
  continue <- TRUE

  while(continue)
  {
    xi <- (11 * x) %% 16
    df <- rbind(df, data.frame(X=x, R=xi))
    x <- xi

    if(xi == x0)
    {
      continue <- FALSE
    }
  }

回答4:

Noticing that user 42-'s perfect approach {
* "do while" = "repeat until not"
* The code equivalence:

do while (condition) # in other language
..statements..
endo

repeat{ # in R
  ..statements..
  if(! condition){ break } # Negation is crucial here!
}

} did not receive enough attention from the others, I'll emphasize and bring forward his approach via a concrete example. If one does not negate the condition in do-while (via ! or by taking negation), then distorted situations (1. value persistence 2. infinite loop) exist depending on the course of the code.

In Gauss:

proc(0)=printvalues(y);
DO WHILE y < 5;    
y+1;
 y=y+1;
ENDO;
ENDP;
printvalues(0); @ run selected code via F4 to get the following @
       1.0000000 
       2.0000000 
       3.0000000 
       4.0000000 
       5.0000000 

In R:

printvalues <- function(y) {
repeat {
 y=y+1;
print(y)
if (! (y < 5) ) {break}   # Negation is crucial here!
}
}
printvalues(0)
# [1] 1
# [1] 2
# [1] 3
# [1] 4
# [1] 5

I still insist that without the negation of the condition in do-while, Salcedo's answer is wrong. One can check this via removing negation symbol in the above code.