Calculate groupwise ratio of consecutive values in R

r

Solution

Try dplyr:

install.packages(dplyr)
require(dplyr)
mdata <- data.frame(group = c("A","A","A","B","B","C","C"), x = c(2,3,5,6,3,7,6))   
mdata <- group_by(mdata, group)
mutate(mdata, ratio = x / lag(x))

# Source: local data frame [7 x 3]
# Groups: group

#   group x     ratio
# 1     A 2        NA
# 2     A 3 1.5000000
# 3     A 5 1.6666667
# 4     B 6        NA
# 5     B 3 0.5000000
# 6     C 7        NA
# 7     C 6 0.8571429

Your diff would simplify to:

mutate(mdata, diff = x - lag(x))

# Source: local data frame [7 x 3]
# Groups: group

#   group x diff
# 1     A 2   NA
# 2     A 3    1
# 3     A 5    2
# 4     B 6   NA
# 5     B 3   -3
# 6     C 7   NA
# 7     C 6   -1

Problem

I want to calculate the ratio between consecutive values within groups. It is easy for differences using `diff`: ``` mdata <- data.frame(group = c("A","A","A","B","B","C","C"), x = c(2,3,5,6,3,7,6)) mdata$diff <- unlist(by(mdata$x, mdata$group, function(x){c(NA, diff(x))})) mdata group x diff 1 A 2 NA 2 A 3 1 3 A 5 2 4 B 6 NA 5 B 3 -3 6 C 7 NA 7 C 6 -1 ``` Is there an equivalent function to calculate ratios? Desired output would be: ``` group x ratio 1 A 2 NA 2 A 3 1.5000000 3 A 5 1.6666667 4 B 6 NA 5 B 3 0.5000000 6 C 7 NA 7 C 6 0.8571429 ```

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