CODEWITHSUNDEEP
r
Bratt Swan
Bratt Swan

Reputation: 1146

How to understand a specific function in R

Here I have a simple function in R below it is:

no.dimnames <- function(a) {
## Remove all dimension names from an array for compact printing.
d <- list()
l <- 0
for(i in dim(a)) {
d[[l <- l + 1]] <- rep("", i)
}
dimnames(a) <- d
a
}

The goal for this function is to drop all array names. However, I dont know what does the following indexing do.

d[[l <- l + 1]]

In this case, d is a null list initially, and l<- 0 so then d[[0<- 1]] implies what?

> x <- matrix(sample(1:5,20,replace=TRUE),nrow = 5)
> x
     [,1] [,2] [,3] [,4]
[1,]    5    4    5    3
[2,]    2    1    5    1
[3,]    1    3    4    4
[4,]    3    1    4    3
[5,]    5    3    5    5
> no.dimnames(x)

 5 4 5 3
 2 1 5 1
 1 3 4 4
 3 1 4 3
 5 3 5 5

Upvotes: 1

Views: 74

Answers (2)

Pierre L
Pierre L

Reputation: 28441

It looks like you understand the increment code d[[l <- l + 1]] but are still asking about the empty spaces rep("", i). They are replacing the dimension names with blanks. The i is used to indicate the amount of spaces that are needed.

If we had a 4x5 matrix. We would have four row names and five column names. To make them all blank, we would need four spaces in rows rep("", 4) and five in columns rep("", 5). The code aims to accomplish that:

mat <- matrix(1:20, 4,5)
rownames(mat) <- month.abb[1:4]
colnames(mat) <- letters[1:5]

mat
#     a b  c  d  e
# Jan 1 5  9 13 17
# Feb 2 6 10 14 18
# Mar 3 7 11 15 19
# Apr 4 8 12 16 20


dimnames(mat)
# [[1]]
# [1] "Jan" "Feb" "Mar" "Apr"
# 
# [[2]]
# [1] "a" "b" "c" "d" "e"

#What we need
list(rep("", 4), rep("", 5))
# [[1]]
# [1] "" "" "" ""
# 
# [[2]]
# [1] "" "" "" "" ""

dimnames(mat) <- list(rep("", 4), rep("", 5))

mat
# 
# 1 5  9 13 17
# 2 6 10 14 18
# 3 7 11 15 19
# 4 8 12 16 20

Upvotes: 1

Cayce K
Cayce K

Reputation: 2338

d[[0<- 1]] isn't valid... you are saying set 0 to 1 which can't be done. In this case l is being set to l + 1 where it is initially 0 so it is l <- 0 + 1

Forget about what a is or what it could be.

Just type this into Rstudio or w/e you are using you will see what happens if you check each variable.

> d <- list()
> l <- 0
> d[[l <- l + 1]] <- rep("", 1)
> d
> l

The one part I should explain is in this case when you type the d[[l <- l + 1]] it is assigning l + 1 to l and then using l as the parameter for the [[]].

So the d[[l <- l + 1]] breaks down to this...

l <- l + 1
l <- 0 + 1
l <- 1
[l]
[[l]]
d[[l]]
d[[1]]

Upvotes: 0

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