Create columns in R within a for loop

Question

I have a data frame in R and I would like to create new columns within a for loop. I have tried many things for the last 2 days but without success. At the end, I did find a solution that seems to work, but it doesn't seem very straight forward. I was wondering if anybody has a more elegant way to do this.
Sorry if this has already been addressed but I couldn't find similar question on SO
Here is my example.

x <- runif(20)
a <- as.data.frame(x)
for (i in 1:100){
  d <- x + i
  a <- cbind(a, d)
}

c <- 1
for (i in 1:100){
  c[i] <- paste0("colum", i)
    }
colnames(a) <- c("x", c)

Thanks in advance for any good suggestions to make this process all within one loop instead of 2.

For one thing, you can remove the second for loop completely. paste0("colum", 1:100) gives the same result. — data paRty
– data paRty, Commented Apr 1, 2015 at 20:58
Oh yes. thanks for that. I'm slightly editing the questions then. — Franky
– Franky, Commented Apr 1, 2015 at 21:00

IRTFM · Accepted Answer · 2015-04-01 23:03:38Z

2

Why not:

a[2:11] <- sapply(1:10, "+", a[[1]])

The names would be:

names(a) <- c("x", paste0("column", 1:10))

answered Apr 1, 2015 at 23:03

IRTFM

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Comments

konvas · Accepted Answer · 2015-04-01 21:04:25Z

1

In this case you can avoid both loops. In R you should always try to use vectorised statements when possible. In this case you can do this using addition fairly easily, one way is

result <- matrix(x + rep(0:100, length(x)), ncol = 101, byrow = TRUE)
result <- as.data.frame(result)
names(result) <- c("x", paste0("column", 1:100))

You can also keep the matrix structure if you only want to store numbers. Also, you should avoid using cbind, rbind and other functions that increase the size of an object with each iteration in loops. The R inferno is a good book to read.

answered Apr 1, 2015 at 21:04

konvas

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Comments

data paRty · Accepted Answer · 2015-04-01 21:05:03Z

1

Perhaps what you are looking for is eval and parse, as such:

for (i in 1:100) {
    eval(parse(text = paste0('a$colum', i, ' <- whatever_you_want_your_column_to_contain'))
}

This will create named columns and their contents in a single statement.

answered Apr 1, 2015 at 21:05

data paRty

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2 Comments

Ben Over a year ago

how can I take care of assigning only single rows in that context? For instance, when I create the different columns and their names in each loop I would like to assign different numbers to the certain rows of each column.

Roland Over a year ago

You are using eval(parse()) as a work around for the non-standard evaluation of $. That is just bad advice. Instead you should use [[. I suggest reading help("$") which mentions this as the recommended approach for passing element names programmatically.

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