Conditional Column Formatting - r

Question

I have a data frame that looks like this:
cat df1 df2 df3
1 1 NA 1 NA
2 1 NA 2 NA
3 1 NA 3 NA
4 2 1 NA NA
5 2 2 NA NA
6 2 3 NA NA
I want to populate df3 so that when cat = 1, df3 = df2 and when cat = 2, df3 = df1. However I am getting a few different error messages.
My current code looks like this:
df$df3[df$cat == 1] <- df$df2
df$df3[df$cat == 2] <- df$df1

Answer 1

Try this code:
df[df$cat==1,"df3"]<-df[df$cat==1,"df2"]
df[df$cat==2,"df3"]<-df[df$cat==1,"df1"]
The output:
df
cat df1 df2 df3
1 1 1 1 1
2 2 1 2 1
3 3 1 3 NA
4 4 2 NA NA
5 5 2 NA NA
6 5 2 NA NA

Answer 2

You can try
ifelse(df$cat == 1, df$df2, df$df1)
[1] 1 2 3 1 2 3
# saving
df$df3 <- ifelse(df$cat == 1, df$df2, df$df1)
# if there are other values than 1 and 2 you can try a nested ifelse
# that is setting other values to NA
ifelse(df$cat == 1, df$df2, ifelse(df$cat == 2, df$df1, NA))
# or you can try a tidyverse solution.
library(tidyverse)
df %>%
mutate(df3=case_when(cat == 1 ~ df2,
cat == 2 ~ df1))
cat df1 df2 df3
1 1 NA 1 1
2 1 NA 2 2
3 1 NA 3 3
4 2 1 NA 1
5 2 2 NA 2
6 2 3 NA 3
# data
df <- structure(list(cat = c(1L, 1L, 1L, 2L, 2L, 2L), df1 = c(NA, NA,
NA, 1L, 2L, 3L), df2 = c(1L, 2L, 3L, NA, NA, NA), df3 = c(NA,
NA, NA, NA, NA, NA)), .Names = c("cat", "df1", "df2", "df3"), class = "data.frame", row.names = c("1",
"2", "3", "4", "5", "6"))