R - How to apply different functions to certain rows in a column

Question

I am trying to apply different functions to different rows based on the value of a string in an adjacent column. My dataframe looks like this:

type    size
A            


        

Answer 1

This could do it like this, creating separate logical vectors for each type:

As <- df$type == 'A'
Bs <- df$type == 'B'
Cs <- df$type == 'C'
df$size2[As] <- 3*df$size[As]
df$size2[Bs] <- df$size[Bs]
df$size2[Cs] <- 2*df$size[Cs]

but a more direct approach would be to create a separate lookup table like this:

df$size2 <- c(A=3,B=1,C=2)[as.character(df$type)] * df$size