simple question about r language in naming - r

Question

I am a developing beginner in r. I have a simple question about r language.
Thanks to many experts in this site, I am improving a lot.
I am always grateful for that, and anyone who's giving hand with this question, thank you in advance.
This is the code.
Data=sample(1:5,size=25,replace=T)
names(Data)=c("a","b","c","d","e")
I want to name each of 1,2,3,4,5 to a,b,c,d,e.
so I thought I could accomplish this by using the upper code.
I know that the right code is
Data=c("a","b","c","d","e")[Data]
But I can't understand why this is the right code and why I need the last [Data].
Any help would be really appreciated!! Thank you so much in advance!!:)

Answer 1

The last Data provides an index to subset values from c("a","b","c","d","e").
Let's take a simple example :
Consider,
a <- 1:10
Now to get the first value in a you can do
a[1]
#[1] 1
To get 3rd value in a you can do
a[3]
#[1] 3
To get 6th and 8th value in a you can do
a[c(6, 8)]
#[1] 6 8
What will happen if you repeat a certain index? Say you select 1 twice and 3 once.
a[c(1, 1, 3)]
#[1] 1 1 3
As you can see the first value is selected two times and third one time.
Now ,Data that you have serves as that index to subset whereas a becomes c("a","b","c","d","e")
a <- c("a","b","c","d","e")
set.seed(123)
Data=sample(1:5,size=25,replace=T)
Data
#[1] 3 3 2 2 3 5 4 1 2 3 5 3 3 1 4 1 1 5 3 2 2 1 3 4 1
Now you use this Data values to subset from a giving
a[Data]
#[1] "c" "c" "b" "b" "c" "e" "d" "a" "b" "c" "e" "c" "c" "a" "d" "a" "a" "e" "c" "b" "b" "a" "c" "d" "a"
A side note, there is an inbuilt constant letters and LETTERS which gives 26 lower and upper case alphabets.
letters
#[1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z"
LETTERS
# [1] "A" "B" "C" "D" "E" "F" "G" "H" "I" "J" "K" "L" "M" "N" "O" "P" "Q" "R" "S" "T" "U" "V" "W" "X" "Y" "Z"

Answer 2

Here is a way that takes advantage of how objects of class "factor" are coded internally in R. In R, factors are coded as consecutive integers starting at 1, and what the user sees is their labels and levels, not the integer values. But the integer values do not go away, they are still there.
First, create a vector of integers like in the question but setting the RNG seed in order to make the results reproducible. This vector is saved for later.
set.seed(123)
Data <- sample(1:5, size = 25, replace = TRUE)
Saved <- Data
Now create the factor. Note the labels atribute is set to the letters "a" to"e".
Data <- factor(Data, labels = c("a","b","c","d","e"))
Data
# [1] c c b b c e d a b c e c c a d a a e c b b a c d a
#Levels: a b c d e
See the internal representation.
as.integer(Data)
# [1] 3 3 2 2 3 5 4 1 2 3 5 3 3 1 4 1 1 5 3 2 2 1 3 4 1
And compare with the initial values.
identical(Saved, as.integer(Data))
# [1] TRUE

Answer 3

This is because Data contains the numbers you want to name in the order you want to name them. By adding [Data] to the end you are selecting the letters in the order of Data. To understand this, try what c("a","b","c","d","e")[c(1, 2)] does; it selects just the two first letters. If you instead type c("a","b","c","d","e")[c(5, 4)] it will select the two last letters, but in reverse order. Then if you print just Data, you'll see that it contains the numbers from 1 to 5, which is the amount of unique letters. So it will select the letters according to that order. You can see that all the numbers correspond to the letters in order by printing the correctly named Data.
Using names(Data)=c("a","b","c","d","e") does not work correctly since you aren't naming all 25 of the numbers, but rather just the first five of them.