subtract first value from each subset of dataframe

Question

I want to subtract the smallest value in each subset of a data frame from each value in that subset ie

A <- c(1,3,5,6,4,5,6,7,10)
B <- rep(1:4, length.out=length(A))
df <- data.frame(A, B)
df <- df[order(B),]

Subtracting would give me:

  A B
1 0 1
2 3 1
3 9 1
4 0 2
5 2 2
6 0 3
7 1 3
8 0 4
9 1 4

Answer 1

I think the output you show is not correct. In any case, from what you explain, I think this is what you want. This uses ave base function:

within(df, { A <- ave(A, B, FUN=function(x) x-min(x))})
  A B
1 0 1
5 3 1
9 9 1
2 0 2
6 2 2
3 0 3
7 1 3
4 0 4
8 1 4

Of course there are other alternatives such as plyr and data.table .

Answer 2

Echoing Arun's comment above, I think your expected output might be off. In any event, you should be able to use can use tapply to calculate subsets and then use match to line those subsets up with the original values:

subs <- tapply(df$A, df$B, min)

df$A <- df$A - subs[match(df$B, names(subs))]

df
  A B
1 0 1
5 3 1
9 9 1
2 0 2
6 2 2
3 0 3
7 1 3
4 0 4
8 1 4