Split one row after every 3rd column and transport those 3 columns as a new row in r

Question

I have a data frame which is a result of another command. This data frame has only one row with around 40000 entries. My problem is that 3 columns are one connected set of data. Therefore I want to split the row after every third column and transport this as a new row. Example:

Create a test data frame:

df=as.data.frame(matrix(seq(1:12), ncol=12, nrow=1))

Now I have a data frame which looks like this.

V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 V11 V12
1  2  3  4  5  6  7  8  9  10  11  12

But I need it like this:

V1 V2 V3
1  2  3
4  5  6
7  8  9
10 11 12

How can I realise this?

Answer 1

Try

as.data.frame(matrix(unlist(df, use.names=FALSE),ncol=3, byrow=TRUE))
#  V1 V2 V3
#1  1  2  3
#2  4  5  6
#3  7  8  9
#4 10 11 12

Or you could directly use matrix on df

 as.data.frame(matrix(df, ncol=3, byrow=TRUE))

Answer 2

This turned out to be faster than I expected (though still not as fast as the obvious approach that @akrun took), so I'm going to post this (like David) "just for general knowledge". (Plus, "data.table" all the things.) :-)

Create a data.table with three columns:

1. The unlisted values of your single row.
2. A grouping variable to indicate which row the value should be assigned to in the final output.
3. A grouping variable to indicate which column the value should be assigned to in the final output.

Once you have that, you can use dcast.data.table to get the output you mention (plus a bonus column).

For point number 2 above, we can easily define a function like the following to make the process of creating groups easy:

groupMaker <- function(vecLen, perGroup) {
  (0:(vecLen-1) %/% perGroup) + 1
}

Then we can use it as follows:

dcast.data.table(
  data.table(value = unlist(df, use.names = FALSE), 
             row = groupMaker(ncol(df), 3), 
             col = 1:3), 
  row ~ col)
#    row  1  2  3
# 1:   1  1  2  3
# 2:   2  4  5  6
# 3:   3  7  8  9
# 4:   4 10 11 12

---

Now, you mention that you are actually dealing with a single-row ~ 40K column data.frame (I'll assume it to be 39,999 columns since that's nicely divisible by 3 and I don't want to break the other answers).

Keeping that in mind, here are some (useless) benchmarks (useless because we're talking milliseconds here, really).

set.seed(1)
S <- sample(20, 39999, TRUE)
S <- data.frame(t(S))

funAM <- function(indf) {
  dcast.data.table(
    data.table(value = unlist(indf, use.names = FALSE), 
               row = groupMaker(ncol(indf), 3), 
               col = 1:3), 
    row ~ col)
}

funDA <- function(indf) {
  as.data.frame(t(`dim<-`(unlist(indf), c(3, ncol(indf)/3))))
}

funAK <- function(indf) as.data.frame(matrix(indf, ncol=3, byrow=TRUE))

library(microbenchmark)
microbenchmark(funAM(S), funDA(S), funAK(S))
# Unit: milliseconds
#      expr       min        lq      mean    median        uq      max neval
#  funAM(S) 18.487001 18.813297 22.105766 18.999891 19.455812 50.25876   100
#  funDA(S) 37.187177 37.450893 40.393893 37.870683 38.869726 94.20128   100
#  funAK(S)  5.018571  5.149758  5.929944  5.271679  5.536449 26.93281   100

---

Where this might be useful would be in cases where the number of desired columns and your number of input columns are not nicely divisible by each other.

For example, try the following sample data:

set.seed(1)
S2 <- sample(20, 40000, TRUE)
S2 <- data.frame(t(S))

With this sample data:

• funAM would give you a warning but would correctly give you the last two columns of the last row as NA.
• funAK would give you a warning but would (presumably) incorrectly recycle values in the last row.
• funDA would just give you an error.

---

I still think you should just fix the problem at the source though :-)

Answer 3

Could also try using dim<- (just for general knowledge)

as.data.frame(t(`dim<-`(unlist(df), c(3, 4))))
#   V1 V2 V3
# 1  1  2  3
# 2  4  5  6
# 3  7  8  9
# 4 10 11 12