#cran-r

lapply will return a list of the same length as the object (x)

basic argument structure: lapply(x, FUN,..)

wherein:

x= vector (list/atomic) or an expression

FUN= function, for ex: mean

...= optional functions

sapply() more ‘user-friendly’ mode of applying a function to each element of an object--- by default, returns a vector, matrix, or array--that is if simplify=‘array’

basic argument structure: sapply(x, FUN, ..., simplify=T, USE.NAMES=T)

wherein:

x= vector (atom/list)

FUN= function

...= additional functions

simplify= logic/character string 

(can be TRUE/FALSE-- returning a simp. vector/matrix or can be =“array” and return an array)

USE.NAMES= logical, (i.e. T/F) , if =T, and X is a character, will use result as names

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IN ACTION: 

if there is a data frame (af), then: af<-data.frame(x=c(2,4,6,8,NA),y=c(14,23,5,0,1))

lapply() and coercion

(1) run a structure test-- although entered as a data frame, mode= list, but structure should be correct

> str(af) 'data.frame':   5 obs. of  2 variables: $ x: num  2 4 6 8 NA $ y: num  14 23 5 0 1

(2) add new argument lapply, coerce object, and run structural test

> af<-lapply(af,as.integer) > str(af) List of 2 $ x: int [1:5] 2 4 6 8 NA $ y: int [1:5] 14 23 5 0 1

OR > af<-lapply(af,as.character) > str(af) List of 2 $ x: chr [1:5] "2" "4" "6" "8" ... $ y: chr [1:5] "14" "23" "5" "0" ...

(4) reset af , this time use lapply() with [] to preserve original class/object

> af[]<-lapply(af,as.integer) > str(af) 'data.frame':   5 obs. of  2 variables: $ x: int  2 4 6 8 NA $ y: int  14 23 5 0 1

> af[]<-lapply(af,as.character) > str(af) 'data.frame':   5 obs. of  2 variables: $ x: chr  "2" "4" "6" "8" ... $ y: chr  "14" "23" "5" "0" ...

**Note: structural differences between using [] --- it keeps the data frame structure

lapply() using other functions (non-coercive)

(1) start with freshly reset data frame (af) 

(2) add function of choice, print new object (af)

> af<-lapply(af,mean) > af $x [1] NA

$y ---(i.e. 14+23+0+5+1=43/5=8.6) [1] 8.6

note: remember, all operations will default to NA if an element value is NA

(3) reset af again, this time perform expressions with []

sapply()