using paste with a list

Question

I'm trying to understand the behavior of strsplit and paste, which are inverse functions. However, when I strsplit a vector, a list is returned, like so:

> strsplit(c("on,e","tw,o","thre,e","fou,r"),",")
[[1]]
[1] "on" "e" 

[[2]]
[1] "tw" "o" 

[[3]]
[1] "thre" "e"   

[[4]]
[1] "fou" "r"  

I tried using lapply to cat the elements of the list back together, but it doesn't work:

> lapply(strsplit(c("on,e","tw,o","thre,e","fou,r"),","),cat)
on etw othre efou r[[1]]
NULL

[[2]]
NULL

[[3]]
NULL

[[4]]
NULL

The same formula with paste instead of cat actually does nothing at all! Why am I getting these results? and how can I get the result I want, which is the original vector back again?

(Obviously, in my actual code I'm trying to do more with the strsplit and cat than just return the original vector, but I think a solution to this problem will work for mine. Thanks!)

Answer 1

While yes, cat will concatenate and print to the console, it does not actually function in the same way paste does. It's result best explained in help("cat")

The collapse argument in paste is effectively the opposite of the split argument in strsplit. And you can use sapply to return the simplified pasted vector.

x <- c("on,e","tw,o","thre,e","fou,r")

( y <- sapply(strsplit(x, ","), paste, collapse = ",") )
# [1] "on,e"   "tw,o"   "thre,e" "fou,r" 
( z <- vapply(strsplit(x, ","), paste, character(1L), collapse = ",") )
# [1] "on,e"   "tw,o"   "thre,e" "fou,r"

identical(x, y)
# [1] TRUE
identical(x, z)
# [1] TRUE

Note that for cases like this, vapply will be more efficient than sapply. And adding fixed = TRUE in strsplit should increase efficiency as well.