How do I make a loop in R that uses a different variable each time it loops?

Question

I want to make a loop that uses a different variable each time the loop repeats. Let's say I have 3 different variables: x, y, and z:

x <- 1
y <- 2
z <- 3

I want to perform the same calculation on x, y, and z. I want to multiply by 2:

B <- A*2

Before each loop, I want A to be set equal to the new variable. The long way to do this would be to write it all out manually:

A <- x
B <- A*2
A <- y
B <- A*2
A <- z
B <- A*2

I want to the result to be a vector of this form.

C <- c(2,4,6)

My calculation and variables are much longer than the ones I've shown here, so writing it out manually would make my code very messy. I would prefer to use a loop so I only have to write the calculation once.

Answer 1

Often you can avoid loops by storing things in a list and calling a function from the apply family:

mylist <- list(x, y, z)
sapply(mylist, function(x) x*2)

Notice that this is simple and useful if your variables are in a dataframe:

df <- data.frame(x, y, z)
sapply(df, function(x) x*2)

Answer 2

This is basic R programming, and can be solved with a form of *apply .

Start by putting your variables in a list .

x <- 1
y <- 2
z <- 3

my_list <- list(x, y, z)

First, the mapply in my comment. It's not strictly necessary to assign a new function, this is just to use a function, f , that looks like a function. The function '*' does not.

f <- `*`    # assign the function '*' to function f

mapply(f, my_list, list(2))
#[1] 2 4 6

Another way would be to use lapply or sapply . Read the help page (it's the same for both these forms of *apply ).

g <- function(v){
    2*v
}

sapply(my_list, g)
#[1] 2 4 6

lapply(my_list, g)