# Logical operators in R

December 14, 2010
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(This article was first published on Why? » R, and kindly contributed to R-bloggers)

A logical operator

In R, the operators “|” and “&” indicate the logical operations OR and AND. For example, to test if `x` equals 1 and `y` equals 2 we do the following:

``` > x = 1; y = 2 > (x == 1) & (y == 2) [1] TRUE```

However, if you are used to programming in C you may be tempted to write
``` #Gives the same answer as above (in this example...) > (x == 1) && (y == 2) [1] TRUE ```
At this point you could be lulled into a false sense of security and believe that they could be used interchangeably. Big mistake.

Let’s consider another example, this time a vector comparison:
``` > z = 1:6 > (z > 2) & (z < 5) [1] FALSE FALSE TRUE TRUE FALSE FALSE > z[(z>2) & (z<5)] [1] 3 4 ```
but the double “&&” gives
``` > (z > 2) && (z < 5) [1] FALSE > z[(z > 2) && (z < 5)] integer(0)#Probably not what you want ```
It’s all gone a bit pear shaped! In fact it could have been worse:
``` > (z > 2) && (z < 5) [1] TRUE > z[(z > 0) && (z < 5)] [1] 1 2 3 4 5 6 ```
Now you’ve the wrong answer and something that would be very tricky to spot. This is because R recylces the `TRUE` variable.

## What’s the difference?

Well from the R help page:

“The longer form evaluates left to right examining only the first element of each vector”

where the longer form refers to “&&”.  So
``` > (z > 2) && (z < 5) [1] FALSE ```
is equivalent to:
``` > (z[1] > 2) & (z[1] < 5) [1] FALSE ```
The same concept applies to the OR operator, “|”.

## What do you use the double operator for?

To be honest, I’m not sure. I can think of a few contrived situations, but nothing really useful. The R help page isn’t that enlightening either. If anyone has suggestions please feel free to leave a comment and I’ll update this section.

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