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.

Check out help("mapply") as in mapply('*', list(x, y, z), list(2)). — Rui Barradas
– Rui Barradas, Commented Aug 23, 2018 at 18:48

DanY · Accepted Answer · 2018-08-23 19:08:52Z

2

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)

edited Aug 23, 2018 at 19:08

answered Aug 23, 2018 at 18:56

DanY

6,1331 gold badge17 silver badges36 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Rui Barradas · Accepted Answer · 2018-08-23 20:24:07Z

0

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)

edited Aug 23, 2018 at 20:24

answered Aug 23, 2018 at 18:59

Rui Barradas

78k8 gold badges41 silver badges75 bronze badges

Collectives™ on Stack Overflow

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

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related