Question

I need to split/divide up a continuous variable into 3 equal sized groups.

Example data frame:

das <- data.frame(anim = 1:15,
                  wt = c(181,179,180.5,201,201.5,245,246.4,
                         189.3,301,354,369,205,199,394,231.3))

After being cut up (according to the value of wt), I would need to have the 3 classes under the new variable wt2 like this:

> das 
   anim    wt wt2
1     1 181.0   1
2     2 179.0   1
3     3 180.5   1
4     4 201.0   2
5     5 201.5   2
6     6 245.0   2
7     7 246.4   3
8     8 189.3   1
9     9 301.0   3
10   10 354.0   3
11   11 369.0   3
12   12 205.0   2
13   13 199.0   1
14   14 394.0   3
15   15 231.3   2

This would be applied to a large data set.

Answer 1

try this:

split(das, cut(das$anim, 3))

if you want to split based on the value of wt, then

library(Hmisc) # cut2
split(das, cut2(das$wt, g=3))

anyway, you can do that by combining cut, cut2 and split.

UPDATED

if you want a group index as an additional column, then

das$group <- cut(das$anim, 3)

if the column should be index like 1, 2, …, then

das$group <- as.numeric(cut(das$anim, 3))

UPDATED AGAIN

try this:

> das$wt2 <- as.numeric(cut2(das$wt, g=3))
> das
   anim    wt wt2
1     1 181.0   1
2     2 179.0   1
3     3 180.5   1
4     4 201.0   2
5     5 201.5   2
6     6 245.0   2
7     7 246.4   3
8     8 189.3   1
9     9 301.0   3
10   10 354.0   3
11   11 369.0   3
12   12 205.0   2
13   13 199.0   1
14   14 394.0   3
15   15 231.3   2