For example, I have a data frame:
df <- data.frame(grp = c(1,1,1,1,1,2,2,2,2,2),
idx = c(1,2,3,4,5,1,2,3,4,5),
val = c(4,6,1,7,2,8,5,3,9,1))
I want to divide the val of each row by the val of the first row in each group. The only way I found is to introduce a new column:
df %>% group_by(grp) %>%
arrange(idx) %>%
mutate(t = ifelse(row_number(idx) == 1, val, 0)) %>%
mutate(val = val / sum(t))
Is there any simple way to do this?
We can do
df %>%
group_by(grp) %>%
arrange(idx) %>%
mutate(val = val/sum((row_number() == 1)*val))
# A tibble: 10 x 3
# Groups: grp [2]
# grp idx val
# <dbl> <dbl> <dbl>
# 1 1 1 1.000
# 2 2 1 1.000
# 3 1 2 1.500
# 4 2 2 0.625
# 5 1 3 0.250
# 6 2 3 0.375
# 7 1 4 1.750
# 8 2 4 1.125
# 9 1 5 0.500
#10 2 5 0.125
If we need to divide by the first 'val' observation, just do val[1L]
df %>%
group_by(grp) %>%
arrange(idx) %>%
mtuate(val = val/val[1L])
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