Debugging Lapply/Sapply Calls

Debugging lapply/sapply calls

Use the standard R debugging techniques to stop exactly when the error occurs:

options(error = browser) 

or

options(error = recover)

When done, revert to standard behaviour:

options(error = NULL)

Unexpected behaviour of function used in conjunction with lapply/sapply?

As we know functions by default return the last line in the function, however, since a value is assigned in this function it doesn't explicitly display the result but if you use print you can see it.

print(mult_six(7))
#[1] 42

Need help converting a for loop to lapply or sapply

You can rbind all elements in a list using do.call, and you can read in all the files into that list using lapply:

mean(
  filter( # here's the filter that will be applied to the rbind-ed data
    do.call("rbind", # call "rbind" on all elements of a list 
            lapply( # create a list by reading in the files from list.files()
              # add any necessary args to read.csv:
              list.files("[::DIR_PATH::]"), function(x) read.csv(file=x, ...)
            ) 
    )
  ), ID %in% id)$pollutant, # make sure id is replaced with what you want
na.rm = TRUE
)

R: sapply / lapply Different Behaviour with Names

Look carefully at this:

lapply(cc, ff)
#> [[1]]
#> [[1]]$myname
#> [1] "1"
#>
#>
#> [[2]]
#> [[2]]$myname
#> [1] "2"

The output of lapply itself doesn't have names. Look:

a <- lapply(cc, ff)
names(a)
#> NULL

The output of the lapply is actually an unnamed list. Each element of a is a named list.

names(a[[1]])
#> [1] "myname"
names(a[[2]])
#> [1] "myname"

So in fact, USE.NAMES will apply, and sapply will assign the contents of cc as names for the output of the lapply for which sapply is a thin wrapper as stated in the documentation. It's quite straightforward to follow the code through:

sapply
#> function (X, FUN, ..., simplify = TRUE, USE.NAMES = TRUE) 
#> {
#>     FUN <- match.fun(FUN)
#>     answer <- lapply(X = X, FUN = FUN, ...)
#>     if (USE.NAMES && is.character(X) && is.null(names(answer))) 
#>         names(answer) <- X
#>     if (!isFALSE(simplify) && length(answer)) 
#>         simplify2array(answer, higher = (simplify == "array"))
#>     else answer
#> }
#> <bytecode: 0x036ae7a8>
#> <environment: namespace:base>

Using lapply to nested list

It seems like a nested list of vectors. We may use recursive apply (rapply)

rapply(qw, f = \(x) norm(matrix(x)))

---

Or use a nested sapply/lapply

as.vector(sapply(qw, \(x) sapply(x, \(y) norm(matrix(y)))))

How to tell lapply to ignore an error and process the next thing in the list?

Use a tryCatch expression around the function that can throw the error message:

testFunction <- function (date_in) {
  return(tryCatch(as.Date(date_in), error=function(e) NULL))
}

The nice thing about the tryCatch function is that you can decide what to do in the case of an error (in this case, return NULL).

> lapply(dates2, testFunction)
[[1]]
[1] "2010-04-06"

[[2]]
NULL

[[3]]
[1] "2010-04-08"

---

Related Topics