Question

This code produces an error:

Error in while (diff < limit) { : missing value where TRUE/FALSE
needed

i <- 1
j <- 1
limit <- 0.05
diff <- 0
ppm <- rep(NA,20)
while(i[1]<=nrow(list)) 
{
  while(diff < limit) ##This is where the error is occurring
  {
    if(j==0)
       ppm <- rep(NA,length(ppm))
    ppm[j] <- list$ppm[i]      
    j <- j+1
    if(j > 1)
       diff <- ppm[j]-ppm[j-1]
  }
  print(ppm)
}

Now i have found that this is usually because the condition in while is evaluating to NA and hence while cannot check for TRUE/FALSE. But in this case i really don’t see what the error is.

Answer 1

On your first time through the while loop:

• j is 1, ppm[1] gets set to the ith (first) element of list$ppm
• j gets incremented to 2
• j is now > 1, so you take the difference between ppm[2] which is still NA, since ppm was initialized to NA and ppm[2] hasn’t been touched yet and ppm[1]; the result is NA

Now go back to the top of the while loop and compare diff (now NA) with limit (fixed at 0.05) … boom.

i <- 1
j <- 1
limit <- 0.05
diff <- 0     ## diff() is a built-in, probably avoid ...
ppm <- rep(NA,20)
list <- data.frame(ppm=1:10)  ## list is a built-in, probably avoid ...
while(i[1]<=nrow(list))   ## why i[1] ??? why not just i ???
{
  while(diff < limit) ##This is where the error is occurring
  {
    if(j==0)  ## as DWin says: never true, j is initialized to 1
       ppm <- rep(NA,length(ppm))
    ppm[j] <- list$ppm[i]      
    j <- j+1
    if(j > 1) {  ## always true if j is initialized to 1 and incremented the first time
      diff <- ppm[j]-ppm[j-1]  ## will be NA if ppm[j-1] is NA
      cat(j,ppm[j-1],ppm[j],diff,"\n")  ## debugging statement!
     }
  }
  print(ppm)
}