CODEWITHSUNDEEP
rgroup-bytapplysplit-apply-combine
Taylor
Taylor

Reputation: 2077

improvement on tapply (shifting groups of vectors)

The order of the return object from tapply() is ambiguous, so I've started to worry about this bit of code:

#d <- data.frame(value = c(1,2,3,5), 
#               source = c("a","a","b","b"))
d$value <- unlist(tapply(d$value, d$source, function(v) v-v[1]))

The idea is to split value into groups, then and then shift each group of its element so that the group starts off at 0.

What's a better way to do this? I have several ideas, but I can't think of anything that

Upvotes: 2

Views: 75

Answers (2)

ThomasIsCoding
ThomasIsCoding

Reputation: 101054

Hope ave makes sense

> transform(d,value = ave(value,source, FUN = \(x) x-x[1]))
  value source
1     0      a
2     1      a
3     0      b
4     2      b

Upvotes: 4

Allan Cameron
Allan Cameron

Reputation: 173793

A little-known trick in base R is the split(x, y) <- lapply(split(x, y), f) paradigm, so the following one-liner meets all your requirements:

d <- data.frame(value = c(1,2,3,5), 
                source = c("a","a","b","b"))

split(d$value, d$source) <- lapply(split(d$value, d$source), \(x) x - x[1])

Resulting in:

d
#>   value source
#> 1     0      a
#> 2     1      a
#> 3     0      b
#> 4     2      b

Whatever the ordering of source

Created on 2023-06-06 with reprex v2.0.2

Upvotes: 4

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