CODEWITHSUNDEEP
rdataframe
mon
mon

Reputation: 22254

R - Data Frame is a list of columns?

Question

  1. Is a data frame in R is a list (list is, in my understanding, a sequence of objects) of columns?
  2. What is the design decision in R to have made a data frame a column-oriented (not row-oriented) structure?
  3. Any reference to related design document or article of data structure design would be appreciated.

I am just used to row-as-a-unit/record and would like to know why it is column oriented. Or if I misunderstood something, kindly suggest.

Background

I had thought a dataframe was a sequence of row, such as (Ozone, Solar.R, Wind, Temp, Month, Day).

> c ## data frame created from read.csv()
  Ozone Solar.R Wind Temp Month Day
1    41     190  7.4   67     5   1
2    36     118  8.0   72     5   2
3    12     149 12.6   74     5   3
4    18     313 11.5   62     5   4
7    23     299  8.6   65     5   7
8    19      99 13.8   59     5   8

> typeof(c)
[1] "list"

However when lapply() is applied against c to show each list element, it was a column.

> lapply(c, function(arg){ return(arg) })
$Ozone
[1] 41 36 12 18 23 19
$Solar.R
[1] 190 118 149 313 299  99
$Wind
[1]  7.4  8.0 12.6 11.5  8.6 13.8
$Temp
[1] 67 72 74 62 65 59
$Month
[1] 5 5 5 5 5 5
$Day
[1] 1 2 3 4 7 8

Whereas I had expected was

[1]    41     190  7.4   67     5   1
[1]    36     118  8.0   72     5   2
…

Upvotes: 2

Views: 2990

Answers (1)

kliron
kliron

Reputation: 4663

1) Is a data frame in R a list of columns?

Yes. 

df <- data.frame(a=c("the", "quick"), b=c("brown", "fox"), c=1:2)
is.list(df)        # -> TRUE
attr(df, "name")   # -> [1] "a" "b" "c"
df[[1]][2]         # -> "quick"

2) What is the design decision in R to have made a data frame a column-oriented (not row-oriented) structure?

A data.frame is a list of column vectors. 

is.atomic(df[[1]])     # -> TRUE
mode(df[[1]])          # -> [1] "character"
mode(df[[3]])          # -> [1] "numeric"

Vectors can only store one kind of object. A "row-oriented" data.frame would demand data frames be composed of lists instead. Now imagine what the performance of an operation like 

df[[1]][20000]

would be in a list-based data frame keeping in mind that random access is O(1) for vectors and O(n) for lists.

3) Any reference to related design document or article of data structure design would be appreciated.

http://adv-r.had.co.nz/Data-structures.html#data-frames

Upvotes: 6

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