R - Data frame manipulation without a for loop - r

Question

I want to read a dataframe read if the first column is T or F and depending on this I will add a new entry to a new column in the matrix using data from the second column.
If z[,1] == true set z[,4] to 2*z[,2]
else set z[,4] to z[,2]
Set if the row in column 1 is true, set the new entry to 2 times the second column, other wise just set it to the value of the second column at that index
Lets create z:
set.seed(4)
z <- data.frame(first=c(T, F, F, T, F), second=sample(-2:2),
third=letters[5:1], stringsAsFactors=FALSE)
z
here is my for loop:
for(i in 1:nrow(z)){
if(z$first == TRUE){
z$newVar2 <- 2*z$second
}
else{
z$newVar2 <- z$second
}
}
Here is without a for loop:
z$newVar<-ifelse(z$first==TRUE, 2*z$second, z$second)
Is there a way to do this with apply? Is there a more efficient way to accomplish this task?

Answer 1

Not what you asked exactly but if working with a matrix data structure, you might as well explore data.table way of going about it:
#Make data.table
setDT(z)
setkey(z)
#Write function to do all the stuff
myfun <- function(first, second){ifelse(first, 2*second, second)}
#Do stuff
z[, newvar2:=myfun(first, second)]
#Printing z
first second third newvar2
1: FALSE -2 d -2
2: FALSE -1 a -1
3: FALSE 1 c 1
4: TRUE 0 e 0
5: TRUE 2 b 4

Answer 2

We can use data.table in a more efficient way still without defining a function, by making use of the fact that TRUE == 1
## use set.seed because we are sampling
set.seed(123)
z <- data.frame(first=c(T, F, F, T, F),
second=sample(-2:2),
third=letters[5:1], stringsAsFactors=FALSE)
library(data.table)
setDT(z)[, newvar2 := (first + 1) * second]
z
# first second third newvar2
# 1: TRUE -1 e -2
# 2: FALSE 1 d 1
# 3: FALSE 2 c 2
# 4: TRUE 0 b 0
# 5: FALSE -2 a -2