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

if you want you can nest the ifelses:

df$size2 <- ifelse(df$type == "A", 3*df$size,
                       ifelse(df$type == "B", 1*df$size,
                           ifelse(df$type == "C", 2*df$size, NA)))


#    > df
#  type size size2
#1    A    1     3
#2    B    3     3
#3    A    4    12
#4    C    2     4
#5    C    5    10
#6    A    4    12
#7    B   32    32
#8    C    3     6

Answer 2

This might be a might more R-ish (and I called my dataframe 'dat' instead of 'df' since df is a commonly used function.

> facs <- c(3,1,2)
> dat$size2= dat$size* facs[ match( dat$type, c("A","B","C") ) ]
> dat
  type size size2
1    A    1     3
2    B    3     3
3    A    4    12
4    C    2     4
5    C    5    10
6    A    4    12
7    B   32    32
8    C    3     6

The match function is used to construct indexes to supply to the extract function [.

Answer 3

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