# Logical operators in R

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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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