Why?

December 14, 2010

Logical operators in R

Filed under: R — Tags: , , , — csgillespie @ 4:38 pm

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, “|”.

As the commentators point out below, another key difference is for the longer form

“Evaluation proceeds only until the result is determined”

This concept is highlighted in the following example:

> f = function(){cat("My name is f\n");return(TRUE)}
> g = function(){cat("My name is g\n");return(FALSE)}
> f() | g()
My name is f
My name is g
[1] TRUE
> f() || g()
My name is f
[1] TRUE

This has two benefits:

  • Evaluation will be faster. In the above example, the function g isn’t evaluated (thanks to Andrew Robson and NotMe)
  • Also, you can use the double variety to check a property of a data structure before carrying on with your analysis, i.e. all(!is.na(x)) && mean(x) > 0 (thanks to Pat Burns for this tip)

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