Creating new variable in R conditional on other columns within a data frame

Question

I have data frame with two column say "a" and "b" now I want to create another column "c" which will take value 1 if the following condition satisfied:

a>x and within this subset b>y. x and y are arbitrary constant. The condition is somewhat nested. so if a>x returns 25 observation then b>y will search within that 25 observation NOT within entire data frame.

Please see stackoverflow.com/questions/5963269/… for how to help us help you answer your question. Also, I don't see how this is nested at all. What's wrong with a>x & b>y ? — Ari B. Friedman
– Ari B. Friedman, Commented Apr 24, 2012 at 20:00

Joshua Ulrich · Accepted Answer · 2012-04-24 20:09:15Z

2

I really suspect you could have figured this out had you tried something...

R> x <- data.frame(a=1:10,b=10:1)
R> (x$c <- x$a > 3 & x$b > 4)
    a  b     c
1   1 10 FALSE
2   2  9 FALSE
3   3  8 FALSE
4   4  7  TRUE
5   5  6  TRUE
6   6  5  TRUE
7   7  4 FALSE
8   8  3 FALSE
9   9  2 FALSE
10 10  1 FALSE

answered Apr 24, 2012 at 20:09

Joshua Ulrich

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6 Comments

João Daniel Over a year ago

When you do this are you avoiding R to evaluate b>y for the rows where a>x is FALSE?

Joshua Ulrich Over a year ago

@JoãoDaniel: x$b > 4 is evaluated for every element in the vector. Run the commands individually to see what they each return ((xa <- x$a > 3); (xb <- x$b > 4); (xc <- xa & xb)). Elements in xc are only TRUE if the corresponding elements of xa and xb are both TRUE.

João Daniel Over a year ago

So actually it doesn't do what the questions asks for. The question says "so if a>x returns 25 observation then b>y will search within that 25 observation NOT within entire data frame.", and it doesn't do that, right?

Joshua Ulrich Over a year ago

@JoãoDaniel: you're getting stuck on semantics. Look at my answer. x$c is only TRUE IFF both conditions are TRUE. It doesn't matter that I evaluated unnecessary elements for x$b > 4.

João Daniel Over a year ago

It's true when they say that sometimes what we view is not what we see. I could only think that the matter was to avoid evaluating all the elements of b.

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