Filling in a new column based on a condition in a data frame

Question

I want to add a new column in my data frame so that for each row if LOC == 1 then V is equal to the value given for V1; if LOC==2 then V is equal to the value given for V2. Here is an example:

df <- 
LOC   V1   V2
 1    0.5  0.7
 1    0.5  0.7
 2    0.5  0.7
 1    0.6  0.8

Result should be:

df <-
 LOC   V1   V2   V
 1    0.5  0.7   0.5
 1    0.5  0.7   0.5
 2    0.5  0.7   0.7
 1    0.6  0.8   0.6

I need help on how to do that in R.

score 4 · Answer 1 · answered Nov 23 '15 at 00:58

4

Use matrix indexing:

idx <- cbind(seq_len(nrow(dat)), dat$LOC)
#      row  col
#     [,1] [,2]
#[1,]    1    1
#[2,]    2    1
#[3,]    3    2
#[4,]    4    1

dat[-1][idx]
#[1] 0.5 0.5 0.7 0.6

answered Nov 23 '15 at 00:58

thelatemail

91,185
12
128
188

Thank you. But is there an easier way where I can added it to the data frame that I have? – daragh Nov 23 '15 at 01:08
@daragh - `dat$V <- dat[-1][idx]` and you're done. I'm not sure what is difficult about that. – thelatemail Nov 23 '15 at 01:10

score 4 · Accepted Answer · answered Nov 23 '15 at 01:07

4

If LOC only contains 1 or 2 then this will work

 df$V <- ifelse(df$LOC == 1, df$V1, df$V2)

answered Nov 23 '15 at 01:07

Jacob H

4,317
2
32
39

It's a good solution for 2 groups, but will quickly become complex if you have >2 columns. – thelatemail Nov 23 '15 at 01:44
@thelatemail, agreed. I like your approach, sub-setting with matrices is always fun! – Jacob H Nov 23 '15 at 01:48

Filling in a new column based on a condition in a data frame

2 Answers2

Linked

Related