How to use loop to create multiple columns

Question

I am struggling with creating multiple columns in a DRY way.
I have searched google and stack exchange and I am still struggling with the below.

df <- data.frame(red = 1:10, blue=seq(1,30,3))

myfunction <- function(x){
  log(x) + 10
}

df$green <- myfunction(df$red)
df$yellow <- myfunction(df$blue)

My questions are:
how can I create the green and yellow columns using a for loop?
how can I create the green and yellow using an apply function?

`lapply` would be standard practice for that – talat Jul 06 '17 at 14:21 — talat, Jul 06 '17 at 14:21

score 1 · Answer 1 · answered Jul 06 '17 at 14:25

1

I've spent a bit of time working on these kinds of things. Most of the time you're going to want to either know all the names of the new variables or have them work in an orderly pattern so you can just paste together the names with an indexing variable.

df <- data.frame(red = 1:10, blue=seq(1,30,3))

myfunction <- function(x){
  log(x) + 10
}

newcols = apply(X = df, MARGIN = 2, FUN = myfunction)
colnames(newcols) = c("greeen","yellow")
df = cbind(df,newcols)

# Alternative
df <- data.frame(red = 1:10, blue=seq(1,30,3))
colors = c("green", "yellow")
for(i in 1:length(colors)){
  df[,colors[i]] = myfunction(df[,i])
}

answered Jul 06 '17 at 14:25

svenhalvorson

1,090
8
21

It's better not to use `apply` on data.frames. The usual operations are `sapply` or `lapply` – Sotos Jul 06 '17 at 14:31
I use all of those frequently. Why do you mention this @Sotos – svenhalvorson Jul 06 '17 at 14:32
Because `apply` will coerce to matrix – Sotos Jul 06 '17 at 14:33
Okay... at the end of the script `df` is a data frame as intended! – svenhalvorson Jul 06 '17 at 14:34
Yes, I know. Have a [look at this](https://stackoverflow.com/a/7141669/5635580) It will clarify it – Sotos Jul 06 '17 at 14:35
<3 that post and have used it many times. I still don't get it but have fun! – svenhalvorson Jul 06 '17 at 14:36
It is not efficient to be coersing to matrix before doing operations. That is why `apply` is not advised for data frames. – Sotos Jul 06 '17 at 14:37
My life is inefficient – svenhalvorson Jul 06 '17 at 14:46
I have no say in that. I m talking about R – Sotos Jul 06 '17 at 14:47

score 1 · Accepted Answer · answered Jul 06 '17 at 16:11

1

As pointed out by Sotos, apply is slower than lapply. So I believe the optimal solution is:

df[,c("green","yellow")] <- lapply(df, myfunction)

answered Jul 06 '17 at 16:11

Max888

3,089
24
55

How to use loop to create multiple columns

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