R: fill in cells with values from different rows - r

Question

I’m trying to fill NAs in a row with values from a different row. These rows are “linked” by a case number. I want to write an if loop that goes through the entire data frame and does this. But I think I don’t grasp the R language well enough. Can anybody help me?
The data frame:
CASE <- c(1, 2, 3, 4, 5, 6)
SERIAL <-c("AB",NA, NA, "CD", NA, NA)
REF <- c(NA, 1, 1, NA, 4, 4)
PA <- c(4, NA, NA, 2, NA, NA)
PE <- c(NA, 2, NA, NA, 1, NA)
PE2 <- c(NA, NA, 3, NA, NA, 3)
df <- data.frame (CASE, SERIAL, REF, PA, PE, PE2)
CASE SERIAL REF PA PE PE2
1 AB NA 4 NA NA
2 <NA> 1 NA 2 NA
3 <NA> 1 NA NA 3
4 CD NA 2 NA NA
5 <NA> 4 NA 1 NA
6 <NA> 4 NA NA 3
In the row CASE = 1, I want to fill in the empty PE and PE2 with the values from the rows below, which reference the line (by REF = 1). In the line CASE = 4, I want to fill in the empty PE and PE2 with the values from the rows below, which reference the line (by REF = 4). The lines with no serial number only serve to fill the lines 1 and 4, so to speak. There is no way to collect the data directly into the corresponding lines. I tried this for loop, but don't know how to refrence the values correctly?
for (i in 1:dim(df)[1]{
if (data$SERIAL[i]==NA){
[data$CASE[data$REF[i]],PE] <- data$PE[i]
[data$CASE[data$REF[i]],PE2] <- data$PE2[i]}
}
)
Expected output:
CASE SERIAL REF PA PE PE2
1 1 AB NA 4 2 3
2 2 <NA> 1 NA 2 NA
3 3 <NA> 1 NA NA 3
4 4 CD NA 2 1 3
5 5 <NA> 4 NA 1 NA
6 6 <NA> 4 NA NA 3

Answer 1

This is a dplyr solution, but perhaps it would work:
df %>%
mutate(REF = ifelse(is.na(REF), CASE, REF)) %>%
group_by(REF) %>%
summarise(SERIAL = first(SERIAL),
across(c(PA, PE, PE2), ~sum(.x, na.rm=TRUE))) %>%
rename("CASE" = "REF")
# # A tibble: 2 x 5
# CASE SERIAL PA PE PE2
# <dbl> <chr> <dbl> <dbl> <dbl>
# 1 1 AB 4 2 3
# 2 4 CD 2 1 3

Answer 2

withSerial = subset(df, !is.na(SERIAL))
withSerial
# CASE SERIAL REF PA PE PE2
#1 1 AB NA 4 NA NA
#4 4 CD NA 2 NA NA
noSerialwithRef = subset(df, is.na(SERIAL) & !is.na(REF))
noSerialwithRef
# CASE SERIAL REF PA PE PE2
#2 2 <NA> 1 NA 2 NA
#3 3 <NA> 1 NA NA 3
#5 5 <NA> 4 NA 1 NA
#6 6 <NA> 4 NA NA 3
withSerial$PE = subset(noSerialwithRef, !is.na(PE))$PE
withSerial$PE2 = subset(noSerialwithRef, !is.na(PE2))$PE2
withSerial
# CASE SERIAL REF PA PE PE2
#1 1 AB NA 4 2 3
#4 4 CD NA 2 1 3

Answer 3

Update: Added library(tidyr) thanks to Martin Gal and added alternative code suggested by Martin Gal:
Here is another dplyr way:
fill SERIAL
use lead in the grouped_columns
keep only first rows of gorups with slice(1)
library(dplyr)
library(tidyr)
df %>%
fill(SERIAL, .direction = "down") %>%
group_by(SERIAL) %>%
mutate(PE = lead(PE),
PE2 = lead(PE2,2)) %>%
slice(1)
# Alternative and better (suggested by Martin Gal):
df %>% fill(-c(CASE, SERIAL), .direction = "up") %>% drop_na()
CASE SERIAL REF PA PE PE2
<dbl> <chr> <dbl> <dbl> <dbl> <dbl>
1 1 AB NA 4 2 3
2 4 CD NA 2 1 3