apply a function in data frame with if/else statement in R

Question

I have a data frame looks like the following

data.1 <- data.frame(
  X1 = 6:10,
  X2 = 1:5,
  X3 = c(TRUE,FALSE,TRUE,FALSE,TRUE)
)

  X1 X2    X3
1  6  1  TRUE
2  7  2 FALSE
3  8  3  TRUE
4  9  4 FALSE
5 10  5  TRUE

I want to create a new column X4 with the following logic:

if X3==NULL then X4=NULL
elseif X3==TRUE then X4=X1+X2 
else X4=X1-X2

Thanks in advance

Answer 1

lapply is for when your data is a list, which isn't what you're doing.

Firstly, you won't find a NULL entry in a data.frame . NA , sure, but not NULL , so you should be working around a is.na() . Next, you don't need to test if(x==TRUE) ; R knows how to use if(x) . Okay, so down to business; you were most of the way there with your ifelse . You can assign a vector to the output of an ifelse and it will take care of the vectorisation for you

data.1$X4 <- ifelse(is.na(data.1$X3), 
                    NA, 
                    ifelse(data.1$X3==TRUE,
                           data.1$X1+data.1$X2,
                           data.1$X1-data.1$X2))

data.1
##   X1 X2    X3 X4
## 1  6  1  TRUE  7
## 2  7  2 FALSE  5
## 3  8  3  TRUE 11
## 4  9  4 FALSE  5
## 5 10  5  TRUE 15

That's ugly though. dplyr includes non-standard evaluation, which involves searching the namespace for columns, so you don't need to quote the data name every time you want to reference a column, making this much cleaner. dplyr::mutate changes a column.

library(dplyr)
mutate(data.1, X4 = ifelse(is.na(X3), 
                           NA, 
                           ifelse(X3, 
                                  X1+X2, 
                                  X1-X2)))

data.1
##   X1 X2    X3 X4
## 1  6  1  TRUE  7
## 2  7  2 FALSE  5
## 3  8  3  TRUE 11
## 4  9  4 FALSE  5
## 5 10  5  TRUE 15