# Operators Tutorial  wiki

Expressions are evaluated in order to perform calculations which may assign values to variables or pass arguments to functions. Expressions are covered pretty well in section 2.5 of the Reference Manual. Expressions are covered here for completeness and to offer more examples.

We'll use the `=` expression shorthand notation for this page. The values can easily be assigned to a variable, e.g.,

```> x = 7
> print(x)
7
> = 7
7
```

## Arithmetic expressions

Lua has the usual binary arithmetic operators.

```> = 2+3, 5-12, 2*7, 7/8
5       -7      14      0.875
> = 5*(2-8.3)/77.7+99.1
98.694594594595
```
Unary negation:
```> = -(-10), -(10)
10      -10
```
Modulo (division remainder):
```> = 15%7, -4%3, 5.5%1
1       2       0.5
```
Power of:
```> = 7^2, 107^0, 2^8
49      1       256
```

## Relational expressions

Relational operators are supplied which return the boolean values `true` or `false`.

• `==` equal to
• `~=` not equal to
• `<` less than
• `>` greater than
• `<=` less than or equal to
• `>=` greater than or equal to

Examples:

```> = 1 == 1, 1 == 0
true    false
> = 1 ~= 1, 1 ~= 0
false   true
> = 3 < 7, 7 < 7, 8 < 7
true	false	false
> = 3 > 7, 7 > 7, 8 > 7
false	false	true
> = 3 <= 7, 7 <= 7, 8 <= 7
true    true    false
> = 3 >= 7, 7 >= 7, 8 >= 7
false   true    true
```
These also work on strings (alphabetical order) and other types.
```> = "abc" < "def"
true
> = "abc" > "def"
false
> = "abb" < "baa"
true
> = "abc" == "abc"
true
> = "abc" == "a".."bc"
true
```
Objects will not be equal if the types are different or refer to different objects.
```> = {} == "table"
false
> = {} == {}  -- two different tables are created here
false
> t = {}
> t2 = t
> = t == t2   -- we're referencing the same table here
true
```
Coercion does not work here, the types must be converted explicitly. See NumbersTutorial and StringsTutorial for explanation of coercion.
```> = "10" == 10
false
> = tonumber("10") == 10
true
```

## Logical operators

Lua provides the logical operators `and`, `or` and `not`. In Lua both `nil` and the boolean value `false` represent false in a logical expression. Anything that is not false (either `nil` or `false`) is `true`, including 0, which might be surprising coming from some other languages. There are more notes on the implications of this at the end of this page.

```> = false==nil   -- although they are both considered false by logical operators, they're still different values
false
> = true==false, true~=false
false   true
> = 1==0
false
```

### not

The keyword `not` inverts a logical expression value:

```> = true, false, not true, not false
true    false   false   true
> = not nil       -- nil represents false
true
> = not not true  -- true is not not true!
true
> = not "foo"     -- anything not false or nil is true
false
```

### and

The binary operator `and` does not necessarily return a boolean value `true` or `false` to the logical expression x and y. In some languages the `and` operator returns a boolean dependent on the two inputs. Rather in Lua, it returns the first argument (without even executing the second one) if its value is `false` or `nil`, and the second argument if the first argument is not `false` or `nil`. So, a boolean is only returned if the first argument is `false` or the second argument is a boolean. Aka minimal evaluation.

```> = false and true  -- false is returned because it is the first argument
false
> = nil and true    -- as above
nil
> = nil and false
nil
> = nil and "hello", false and "hello"
nil     false
> = false and print("hello") -- the print function isn't evaluated, so "hello" isn't printed
false
```
All of the above expressions return the first argument. All of the following expressions return the second argument, as the first is true.
```> = true and false
false
> = true and true
true
> = 1 and "hello", "hello" and "there"
hello   there
> = true and nil
nil
> = true and print("hello") -- the print function is evaluated here, so "hello" is printed
hello
nil
```
As you can see the logical expressions are still evaluated correctly but we have some interesting behaviour because of the values returned.

### or

The `or` binary operator also does not necessarily return a boolean value (see notes for `and` above). If the first argument is true it is returned, otherwise the second argument is returned. So, a boolean is only returned if the first argument is `true` or the second argument is a boolean.

```> = true or false
true
> = true or nil
true
> = "hello" or "there", 1 or 0
hello   1
> = true or print("hello") -- the print function isn't evaluated, so "hello" isn't printed
true
```
All of the above expressions return the first argument. All of the following expressions return the second argument, as the first is `false` or `nil`.
```> = false or true
true
> = nil or true
true
> = nil or "hello"
hello
> = false or print("hello") -- the print function is evaluated here, so "hello" is printed
hello
```

This can be a very useful property. For example, setting default values in a function:

```> function foo(x)
>>  local value = x or "default"  -- if argument x is false or nil, value becomes "default"
>>  print(value, x)
>> end
>
> foo()       -- no arguments, so x is nil
default nil
> foo(1)
1       1
> foo(true)
true    true
> foo("hello")
hello   hello
```

## Ternary operators

Even though Lua doesn't have a ternary operator (if/else expression), it's possible to create similar behavior with `and` and `or`:

```value = condition and trueval or falseval;
```
If condition is true, trueval is returned, otherwise falseval is returned. To help understand this, remember that `and` has a higher precedence than `or`:
```value = (condition and trueval) or falseval;
```
If condition is true, this causes the `and` to run trueval and return its value. Otherwise, the whole `and` part of the expression will be false, triggering the `or` expression to run falseval. Note that this has the issue that falseval will be run if trueval is `nil` or `false`. This can be worked around by negating the condition if you know falseval is always a "true" value, otherwise you'll just need to use an if-then-else statement.

This does not work in a case of "value = false and true or false".

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Last edited April 28, 2017 3:16 am GMT (diff)