Base R - match

I’ve sort of rediscovered base R’s match() function recently, so I figured I’d add a page here about it.

Per it’s documentation, match “returns a vector of the positions of (first) matches of its first argument in its second.”

So let’s illustrate this with the following data:

x <- 1
y <- c(2, 3, 4, 1, 5)

#we expect this to return 4 -- the index of the value 1 in y
match(x, y)

[1] 4

And if x contains multiple elements, it does the same thing for each element:

x <- 1:2

match(x, y) 

[1] 4 1

And if y contains repeats of the same element, we still just get the first match:

y <- c(y, 1) 

match(x, y)

[1] 4 1

The cool thing for me here is what we can use this for replacements.

The following code:

1. draws x, 100 samples (with replacement) from a uniform distribution 1:10;
2. determines k, the number of unique values in x;
3. draws y, which is k samples from LETTERS (a vector containing all capital letters);
4. creates a new vector, x_replace that replaces each unique value in x with a corresponding value of y. If we truly wanted to replace x, we would just assign this last result to back to x

set.seed(0408)
n <- 100
x <- sample.int(10, size = n, replace = TRUE) 
k <- length(unique(x))
y <- sample(LETTERS, k, replace = FALSE)

x_replace <- y[match(x, unique(x))]

And to show that this does what we think, let’s look at the first 10 values of x:

x[1:10]

 [1] 10  9  9  8  7  1  1  5  5  3

as well as the first 10 values of x_replace:

x_replace[1:10] 

 [1] "S" "W" "W" "L" "K" "U" "U" "E" "E" "C"

and we can see that “S” corresponds to 10, “W” corresponds to 9, “U” corresponds to 1, etc.