dplyr: divide all values in group by group's first value

Question

My df looks something like this:

ID    Obs    Value
1     1      26
1     2      13
1     3      52
2     1      1,5
2     2      30

Using dplyr, I to add the additional column Col, which is the result of a division of all values in the column value by the group's first value in that column.

ID    Obs    Value    Col
1     1      26       1
1     2      13       0,5
1     3      52       2
2     1      1,5      1
2     2      30       20

How do I do that?

Answer 1

After grouping by 'ID', use mutate to create a new column by dividing the 'Value' by the first of 'Value'

library(dplyr)
df1 %>%
   group_by(ID) %>%
   mutate(Col = Value/first(Value))

If the first 'Value' is 0 and we don't want to use it, then subset the 'Value' with a logical expression and then take the first of that

df1 %>%
   group_by(ID) %>%
    mutate(Col = Value/first(Value[Value != 0]))

---

Or in base R

df1$Col <- with(df1, Value/ave(Value, ID, FUN = head, 1))

NOTE: The comma in 'Value' suggests it is a character column. In that case, it should be first changed to decimal ( . ) if that is the case, convert to nunmeric and then do the division. It can be done while reading the data

Answer 2

Or, without creating an additional column:

library(tidyverse)

df = data.frame(ID=c(1,1,1,2,2), Obs=c(1,2,3,1,2), Value=c(26, 13, 52, 1.5, 30))

df %>% 
  group_by(ID) %>% 
  mutate_at('Value', ~./first(.))
#> # A tibble: 5 x 3
#> # Groups:   ID [2]
#>      ID   Obs Value
#>   <dbl> <dbl> <dbl>
#> 1     1     1   1  
#> 2     1     2   0.5
#> 3     1     3   2  
#> 4     2     1   1  
#> 5     2     2  20

### OR ###
df %>% 
  group_by(ID) %>% 
  mutate_at('Value', function(x) x/first(x))
#> # A tibble: 5 x 3
#> # Groups:   ID [2]
#>      ID   Obs Value
#>   <dbl> <dbl> <dbl>
#> 1     1     1   1  
#> 2     1     2   0.5
#> 3     1     3   2  
#> 4     2     1   1  
#> 5     2     2  20

^{Created on 2020-01-04 by the reprex package (v0.3.0)}