dynamically expand a data frame columns using cbind

Question

I would like to build a data frame in a loop adding a new column each time using cbind. I try the following:

test <- NULL
df <- data.frame(x=c(1,2,3,4))
test <- cbind(test, df)

This generates an error:

Error in data.frame(..., check.names = FALSE) : arguments imply differing number of rows: 0, 4

What is the correct way to instantiate a blank data frame in R and then bind to it in a loop?

Thanks

Answer 1

You need to create test as a structure that has the same number of rows so that cbind.data.frame will not throw an error:

 test <-data.frame(row.names=1:4)
 df <- data.frame(x=c(1,2,3,4))
 test <- cbind(test, df)

 test
  x
1 1
2 2
3 3
4 4

Two other methods:

> test <-data.frame(row.names=1:4)
>      test[['x']] <-c(1,2,3,4)
> test
  x
1 1
2 2
3 3
4 4


> test <-data.frame(row.names=1:4)
>      test[1] <-list(x=c(1,2,3,4))
> test
  x
1 1
2 2
3 3
4 4

Answer 2

As Roman Luštrik pointed out it may be inefficient to use cbind. You can start with empty list and after loop convert it to data.frame.

test <- list()
# inner loop assigment
test <- c(test,list(c(1:4)))
# after loop
test <- as.data.frame(test)