r - Debugging lapply/sapply calls

Question

Code written using lapply and friends is usually easier on the eyes and more Rish than loops. I love lapply just as much as the next guy, but how do I debug it when things go wrong? For example:

> ## a list composed of numeric elements 
> x <- as.list(-2:2)
> ## turn one of the elements into characters
> x[[2]] <- "what?!?"
> 
> ## using sapply
> sapply(x, function(x) 1/x)
Error in 1/x : non-numeric argument to binary operator

Had I used a for loop:

> y <- rep(NA, length(x))
> for (i in 1:length(x)) {
+     y[i] <-  1/x[[i]]
+ }
Error in 1/x[[i]] : non-numeric argument to binary operator

But I would know where the error happened:

> i
[1] 2

What should I do when using lapply/sapply?

Answer 1

If you wrap your inner function with a try() statement, you get more information:

> sapply(x, function(x) try(1/x))
Error in 1/x : non-numeric argument to binary operator
[1] "-0.5"                                                    
[2] "Error in 1/x : non-numeric argument to binary operator
"
[3] "Inf"                                                     
[4] "1"                                                       
[5] "0.5"

In this case, you can see which index fails.