R repeat function until condition met

Question

I am trying to generate a random sample that excludes certain "bad data." I do not know whether the data is "bad" until after I sample it. Thus, I need to make a random draw from the population and then test it. If the data is "good" then keep it. If the data is "bad" then randomly draw another and test it. I would like to do this until my sample size reaches 25. Below is a simplified example of my attempt to write a function that does this. Can anyone please tell me what I am missing?

df <- data.frame(NAME=c(rep('Frank',10),rep('Mary',10)), SCORE=rnorm(20))
df

random.sample <- function(x) {
  x <- df[sample(nrow(df), 1), ]
  if (x$SCORE > 0) return(x)
 #if (x$SCORE <= 0) run the function again
}

random.sample(df)

Answer 1

Here is a general use of a while loop:

random.sample <- function(x) {
  success <- FALSE
  while (!success) {
    # do something
    i <- sample(nrow(df), 1)
    x <- df[sample(nrow(df), 1), ]
    # check for success
    success <- x$SCORE > 0
  }
  return(x)
}

An alternative is to use repeat (syntactic sugar for while(TRUE)) and break:

random.sample <- function(x) {
  repeat {
    # do something
    i <- sample(nrow(df), 1)
    x <- df[sample(nrow(df), 1), ]
    # exit if the condition is met
    if (x$SCORE > 0) break
  }
  return(x)
}

where break makes you exit the repeat block. Alternatively, you could have if (x$SCORE > 0) return(x) to exit the function directly.

Answer 2

use this after your first sample

while (any(bad <- (x$SCORE <= 0)))
   x[bad, ] <- df[sample(nrow(df), sum(bad)), ]

Answer 3

You can just select the rows to sample directly like so (just 5):

> df <- data.frame(NAME=c(rep('Frank',10),rep('Mary',10)), SCORE=rnorm(20))
> df[sample(which(df$SCORE>0), 5),]


 NAME     SCORE
14  Mary 1.0858854
10 Frank 0.7037989
16  Mary 0.7688913
5  Frank 0.2067499
17  Mary 0.4391216

this is without replacement, for bootstrap put in replace=T.

Answer 4

 random.sample <- function(x) {
   x <- df[sample(nrow(df), 1), ]
   if (x$SCORE > 0) return(x)
   Recall(x)# run the function again
 }

 random.sample(df)
#   NAME    SCORE
#14 Mary 1.252566

It seems to me that this should work as well:

 df$SCORE[ df$SCORE > 0 ][ sample(1:sum(df$SCORE > 0), 1) ]
#[1] 0.6579631