CODEWITHSUNDEEP
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Reputation: 10133

Subtraction based on two factors

My dataframe looks like so:

group <- c("A", "A", "A", "A", "B", "B", "B", "B", "C", "C", "C", "C", "C", "C")
value <- c(3:6, 1:4, 4:9)
type <- c("d", "d", "e", "e", "g", "g", "e", "e", "d", "d", "e", "e", "f", "f")
df <- cbind.data.frame(group, value, type)

df
   group value type
1      A     3    d
2      A     4    d
3      A     5    e
4      A     6    e
5      B     1    g
6      B     2    g
7      B     3    e
8      B     4    e
9      C     4    d
10     C     5    d
11     C     6    e
12     C     7    e
13     C     8    f
14     C     9    f

Within each level of factor "group" I would like to subtract the values based on "type", such that (for group "A") 3 - 5 (1st value of d - 1st value of e) and 4 - 6 (2nd value of d - 2nd value of d). My outcome should look similarly to this..

A
  group d_e
1     A  -2
2     A  -2

B
  group g_e
1     B  -2
2     B  -2

C
  group d_e d_f e_f
1     C  -2  -4  -2
2     C  -2  -4  -2

So if - as for group C - there are more than 2 types, I would like to calculate the difference between each combination of types.

Reading this post I reckon I could maybe use ddply and transform. However, I am struggling with finding a way to automatically assign the types, given that each group consists of different types and also different numbers of types.

Do you have any suggestions as to how I could manage that?

Upvotes: 2

Views: 1493

Answers (1)

G. Grothendieck
G. Grothendieck

Reputation: 269664

Its not clear why the sample answer in the post has two identical rows in each output group and not just one but at any rate this produces similar output to that shown:

DF <- df[!duplicated(df[-2]), ]
f <- function(x) setNames(
            data.frame(group = x$group[1:2], as.list(- combn(x$value, 2, diff))),
                   c("group", combn(x$type, 2, paste, collapse = "_"))
            )

by(DF, DF$group, f)

giving:

DF$group: A
  group d_e
1     A  -2
2     A  -2
------------------------------------------------------------ 
DF$group: B
  group d_e
1     B  -2
2     B  -2
------------------------------------------------------------ 
DF$group: C
  group d_e d_f e_f
1     C  -2  -4  -2
2     C  -2  -4  -2

REVISED minor improvements.

Upvotes: 4

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