Different function output and print option in r

Question

I have a data frame x and subject function.

DATA

dput(head(x))
structure(list(subjects = c(14L, 14L, 14L, 14L, 14L, 14L), 
visit =  structure(c(1L, 1L, 1L, 1L, 1L, 1L), .Label = c("0", "1", "2"), class = "factor"),
room = structure(c(1L, 1L, 1L, 1L, 1L, 1L), .Label = c("bedroom", 
"den", "dining room", "family  room", "hall", "kitchen", 
"living room", "office", "study room", "tv room"), class = "factor"), 
value = c(6, 6, 2.75, 2.75, 2.75, 2.75), timepoint = 53:58),
row.names = c(NA, 6L), class = c("LongitudinalData", "data.frame"))

Function

subject<-function(x,id) UseMethod("subject")
subject.LongitudinalData<- function(x ,id) { 
    a<- x[x$subjects==id,]
    b<-noquote(paste("Subject ID:",id))
    out<-list(a,b)
    class(out)<-"subject"
    invisible(out)
}

Now i want to get following print resualt (Diserable outpout, not actual):

out<-subject(x,14)
print(out)
Subject ID: 14

And desirable str output would be like below (A data frame not a list)

str(out)
Classes ‘LongitudinalData’ and 'data.frame':    10 obs. of  5 variables:
$ subjects : int  14 14 14 14 14 14 14 14 14 14 ...
$ visit    : Factor w/ 3 levels "0","1","2": 1 1 1 1 1 1 1 1 1 1 ...
$ room     : Factor w/ 10 levels "bedroom","den",..: 1 1 1 1 1 1 1 1 1 1 ...
$ value    : num  6 6 2.75 2.75 2.75 2.75 6 6 2.75 2.75 ...
$ timepoint: int  53 54 55 56 57 58 59 60 61 62 ...

Not working proposed solution and its output

print.subject  <- function(x) {  
x[[2]]
  }
str(out)
List of 2
 $ :Classes ‘LongitudinalData’ and 'data.frame':    6 obs. of  5 variables:
  ..$ subjects : int [1:6] 14 14 14 14 14 14
  ..$ visit    : Factor w/ 3 levels "0","1","2": 1 1 1 1 1 1
  ..$ room     : Factor w/ 10 levels "bedroom","den",..: 1 1 1 1 1 1
  ..$ value    : num [1:6] 6 6 2.75 2.75 2.75 2.75
  ..$ timepoint: int [1:6] 53 54 55 56 57 58
 $ : 'noquote' chr "Subject ID: 14"
 - attr(*, "class")= chr "subject"

I do not want a list of two elements but a data frame

Any idea?

You are getting it as a list, print(out[[2]])# [1] Subject ID: 14 — akrun
– akrun, Commented Dec 12, 2018 at 18:56

Konrad Rudolph · Accepted Answer · 2018-12-12 19:16:27Z

1

Why not the following?

print.subject = function (x) {
    cat(x[[2]], '\n')
    invisible(x)
}

Two things to note:

NextMethod is generally a good idea but the above is simple enough and robust. By contrast, I’m not even exactly sure what your solution does.
You need to use [[, not [ subsetting, to access the correct list element (rather than a list slice).

edited Dec 12, 2018 at 19:16

answered Dec 12, 2018 at 19:10

Konrad Rudolph

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3 Comments

Iman Over a year ago

I am seeking a way to have a function that returns only a data frame, but when I print it, it gives me like "Subject Id : * "

Konrad Rudolph Over a year ago

@imi Yes. That is what my answer does.

Iman Over a year ago

my problem is the str(output) part, I want a data frame, not a list of two items

Iman · Accepted Answer · 2018-12-13 21:59:59Z

No need to save output in a list at first place

subject<-function(x,id) UseMethod("subject")
subject.LongitudinalData<- function(x ,id) { 
   a <- x[x$subjects==id,]
   class(a)<-c("subject","LongitudinalData","data.frame")
   invisible(a)
}


print.subject  <- function(x) {
  if(length(unique(x$subjects)) == 0) {
    noquote(paste("NULL"))
  } else {
      noquote(paste("Subject ID:",unique(x$subjects) ))
      }

}

output

> print(out)
[1] Subject ID: 14
> str(out)
Classes ‘subject’, ‘LongitudinalData’ and 'data.frame': 11945 obs. of  5 variables:
 $ subjects : int  14 14 14 14 14 14 14 14 14 14 ...
 $ visit    : Factor w/ 3 levels "0","1","2": 1 1 1 1 1 1 1 1 1 1 ...
 $ room     : Factor w/ 10 levels "bedroom","den",..: 1 1 1 1 1 1 1 1 1 1 ...
 $ value    : num  6 6 2.75 2.75 2.75 2.75 6 6 2.75 2.75 ...
 $ timepoint: int  53 54 55 56 57 58 59 60 61 62 ...

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