How to add column into a dataframe based on condition? (without ifelse)

Question

I refer to Oscar de Léon's answer to this question: How to add column into a dataframe based on condition?

This is his code:

iris$Regulation <- c("DOWN", "UP")[ (iris$Sepal.Length >= 5) + 1 ]

I understand it assigns values to the column "Regulation" and hence creates it, if it's not there yet. But then? I see it works but I don't understand what it is exactly doing. Could you explain what it does?

The condition returns FALSE/TRUE, coded internally as 0/1. Then add 1 because R vectors are 1-based. The result indexes the vector c("DOWN", "UP"). Try doing what @Sotos said. — Rui Barradas
– Rui Barradas, Commented Nov 20, 2019 at 13:31

AHart · Accepted Answer · 2019-11-20 15:54:32Z

1

The first part effectively creates an object with two entries: the potential outcomes DOWN/UP. The second is a logical argument indexing one of those outcomes.

Might be easier if we create an object for the outcomes:

outcome <- c('DOWN','UP')

Square brackets index the object:

> outcome[ 2 ]
[1] "UP"

Inside the brackets is a logical statement returning FALSE/TRUE. Focusing only on the first 5 observations:

> iris[1:5,'Sepal.Length'] >= 5
[1]  TRUE FALSE FALSE FALSE  TRUE

Which store numerically as 0/1:

> as.numeric(iris[1:5,'Sepal.Length'] >= 5)
[1] 1 0 0 0 1

Addding 1 to the above returns 1s and 2s, which index either DOWN/UP in the outcomes object:

> condition[ (iris[1:5,'Sepal.Length'] >= 5) + 1 ]
[1] "UP"   "DOWN" "DOWN" "DOWN" "UP"

edited Nov 20, 2019 at 15:54

answered Nov 20, 2019 at 13:54

AHart

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