Using logical 'or' operator correctly with multiple conditions?

Question

I can't really seems to wrap my head around about how I am using the | operator incorrectly. Because my code seems to work correctly without it, I assume I've not understood correctly what it does. But I wasn't able to find an explanation that helps me get to it.

x1 <- c(1,2,3,1,2,3)
x2 <- c(4,5,6,6,5,4)
df <- data.frame(x1,x2)

#simple test: find out which rows have value 1 for x1 and value 4 for x2 at the same time
which(df$x1 == 1 & df$x2 == 4)
[1] 1

#problem: find out which rows have value 1 for x1 and either value 4 or 6 for x2
which(df$x1 == 1 & df$x2 == 4 | 6)
[1] 1 2 3 4 5 6

here should the return be [1] 1 4, but for some reason I just get back all the row indicies...

tobiasegli_te · Accepted Answer · 2016-10-07 08:38:24Z

2

Try

which((df$x1 == 1) & (df$x2 %in% c(4,6)))

or

which((df$x1 == 1) & ((df$x2 == 4)|(df$x2 == 6)))

However the second solution is clearly less elegant, I just added it to show you how to use the logical OR. I would highly recommend to put logical arguments in parentheses.

edited Oct 7, 2016 at 8:38

answered Oct 7, 2016 at 8:35

tobiasegli_te

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2 Comments

Bathsheba Over a year ago

Personally I believe the second way only becomes less elegant with all the superfluous parentheses, but upvoted nonetheless.

Susu Over a year ago

Thanks to both of you! @Bathsheba greatly helped me to understand where it went wrong while you offer a different solution! Thanks, again!

Bathsheba · Accepted Answer · 2016-10-07 08:34:15Z

1

You need to write df$x1 == 1 & (df$x2 == 4 | df$x2 == 6)

Currently your expression is evaluated as

(df$x1 == 1 & df$x2 == 4) | 6

due to operator precedence, which is always true.

answered Oct 7, 2016 at 8:34

Bathsheba

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