Create relative value from the first row of an grouped dataframe-CodePudding

I have

df<-data.frame(year=c(2010, 2010, 2010, 2011, 2011), day=c(1,2,3,1,2), value=c(5,6,7,8,5))

year day value
1 2010   1     5
2 2010   2     6
3 2010   3     7
4 2011   1     8
5 2011   2     5

I would like create a variable with the relative change of value within each year (relative to the first day of the year) and end up with

  year day value rel.value
1 2010   1     5     1.000
2 2010   2     6     1.200
3 2010   3     7     1.400
4 2011   1     8     1.000
5 2011   2     5     0.625

is it possible to do this within the dplyr framework, without looking up to a second table?

I've used slice to obtain a table with the values of day 1 of each year but I don't know how to divide all subsequent values of the same year.

CodePudding user response：

After grouping by 'year', divide the value by the first element of 'value' to create the column

library(dplyr)
df <- df %>% 
  group_by(year) %>%
  mutate(rel.value = value/first(value)) %>% 
  ungroup

-output

df
# A tibble: 5 × 4
   year   day value rel.value
  <dbl> <dbl> <dbl>     <dbl>
1  2010     1     5     1    
2  2010     2     6     1.2  
3  2010     3     7     1.4  
4  2011     1     8     1    
5  2011     2     5     0.625