How to select only first non NA value of each group in R?

December 1, 2021

I have a data frame like

mydata <- data.frame(Id=c(01,01,01,01,01,01,02,02,02,02),
                     VISIT=c("Screeing","Baseline","Baseline","Baseline","Week 9","Week 9","Baseline","Week 2",
                             "Week 2","Week 2"),
                    Score=c(1,2,4,5,78,9,5,NA,3,4))

> mydata
   Id    VISIT Score
1   1 Screeing     1
2   1 Baseline     2
3   1 Baseline     4
4   1 Baseline     5
5   1   Week 9    78
6   1   Week 9     9
7   2 Baseline     5
8   2   Week 2     NA
9   2   Week 2     3
10  2   Week 2     4

What I am trying to do is to group by Id and VISIT and choose the first non NA value of each group as

> mydata

      Id VISIT    Score 
   <dbl> <fct>    <dbl> 
 1     1 Screeing     1     
 2     1 Baseline     2     
 5     1 Week 9      78    
 7     2 Baseline     5     
 9     2 Week 2       3

This came to my mind

mydata<-mydata %>%
 group_by(Id,VISIT) %>% 
 mutate(first = dplyr::first(na.omit(Score)))

But it does not remove other rows, and it just create a new column with repeated values of first non NA of each group.

>Solution :

A dplyr alternative. Assuming that by "first" you simply mean the first row, in the order given, by group.

Note that (Id, VISIT) in your example data gives 2 groups for Baseline.

library(dplyr)

mydata %>% 
  group_by(Id, VISIT) %>% 
  filter(!is.na(Score)) %>% 
  slice(1) %>% 
  ungroup()

Result:

# A tibble: 5 x 3
     Id VISIT    Score
  <dbl> <chr>    <dbl>
1     1 Baseline     2
2     1 Screeing     1
3     1 Week 9      78
4     2 Baseline     5
5     2 Week 2       3