Alex - 2 months ago 11

R Question

I have three expressions, each involving multiplication with a logical or its negation:

`-2*3*!T + 5*7*T`

5*7*T + -2*3*!T

(-2*3*!T) + 5*7*T

I expect the above to produce the same result. However:

`> -2*3*!T + 5*7*T`

[1] 0 # unexpected!

> 5*7*T + -2*3*!T

[1] 35

> (-2*3*!T) + 5*7*T

[1] 35

I am sure this has something to do with operator precedence and type coercion, but I can't work out how it makes sense to even evaluate

`!T`

`*`

Answer

You're exactly right that this is about operator precedence. As `?base::Syntax`

(which you link above) states, `!`

has lower precedence than all of the arithmetic operators, so the first expression is equivalent to

```
(-2*3)*!(T + 5*7*T)
```

(because the expression containing `!`

has to be evaluated before the final multiplication can be done) or

```
-6*!(36) # T coerced to 1 in numeric operations
```

or

```
-6*FALSE # non-zero numbers coerced to TRUE in logical operations
```

or

```
-6*0 # FALSE coerced to 0 in numeric operations
```