print list names when iterating lapply

Question

I have a time series (x,y,z and a) in a list name called dat.list. I would like to apply a function to this list using lapply. Is there a way that I can print the element names i.e., x,y,z and a after each iteration is completed in lapply. Below is the reproducible example.

## Create Dummy Data

x <- ts(rnorm(40,5), start = c(1961, 1), frequency = 12)
y <- ts(rnorm(50,20), start = c(1971, 1), frequency = 12)
z <- ts(rnorm(50,39), start = c(1981, 1), frequency = 12)
a <- ts(rnorm(50,59), start = c(1991, 1), frequency = 12)

dat.list <- list(x=x,y=y,z=z,a=a)

## forecast using lapply

abc <- function(x) {
  r <- mean(x)
  print(names(x))
  return(r)
}

forl <- lapply(dat.list,abc)

Basically, I would like to print the element names x,y,z and a every time the function is executed on these elements. when I run the above code, I get null values printed.

Solution 1

You can use some deep digging (which I got from another answer on SO--I'll try to find the link) and do something like this:

abc <- function(x) {
  r <- mean(x)
  print(eval.parent(quote(names(X)))[substitute(x)[[3]]])
  return(r)
}

forl <- lapply(dat.list, abc)
# [1] "x"
# [1] "y"
# [1] "z"
# [1] "a"
forl
# $x
# [1] 5.035647
# 
# $y
# [1] 19.78315
# 
# $z
# [1] 39.18325
# 
# $a
# [1] 58.83891

---

Our you can just lapply across the names of the list (similar to what @BondedDust did), like this (but you lose the list names in the output):

abc <- function(x, y) {
  r <- mean(y[[x]])
  print(x)
  return(r)
}

lapply(names(dat.list), abc, y = dat.list)

Solution 2

The item names do not get passed to the second argument from lapply, only the values do. So if you wanted to see the names then the calling strategy would need to be different:

> abc <- function(nm, x) {
+   r <- mean(x)
+   print(nm)
+   return(r)
+ }
> 
> forl <- mapply(abc, names(dat.list), dat.list)
[1] "x"
[1] "y"
[1] "z"
[1] "a"