How to replace multiple values at once

Question

I would like to replace different values in a vector with specific other values at once.

In the problem I'm working through:

• 1 should be replaced with 2,
• 2 with 4,
• 3 with 6,
• 4 with 8,
• 5 with 1,
• 6 with 3,
• 7 with 5 and
• 8 with 7.

So that:

x <- c(4, 2, 0, 7, 5, 7, 8, 9)
x
[1] 4 2 0 7 5 7 8 9

would be converted to:

[1] 8 4 0 5 1 5 7 9

after the replacements.

I have tried using:

x[x == 1] <- 2
x[x == 2] <- 4

and so on, but that results in 1 getting replaced with 7.

What is the simplest solution to this problem without using any packages?

Answer 1

A possible solution using match:

old <- 1:8
new <- c(2,4,6,8,1,3,5,7)

x[x %in% old] <- new[match(x, old, nomatch = 0)]

which gives:

> x
[1] 8 4 0 5 1 5 7 9

What this does:

• Create two vectors: old with the values that need to be replaced and new with the corresponding replacements.
• Use match to see where values from x occur in old. Use nomatch = 0 to remove the NA's. This results in an indexvector of the position in old for the x values
• This index vector can then be used to index new.
• Only assign the values from new to the positions of x that are present in old: x[x %in% old]

Answer 2

If one can define conversion pair for all values then converting to factor and then back to integer can be an option.

old <- 0:9
new <- c(0,2,4,6,8,1,3,5,7,9)

as.integer(as.character(factor(x, old, new)))
# [1] 8 4 0 5 1 5 7 9