Lua Programming/Expressions

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As explained before, expressions are pieces of code that have a value and that can be evaluated. They cannot be executed directly (with the exception of function calls), and thus, a script that would contain only the following code, which consists of an expression, would be erroneous:

3 + 5
-- The code above is erroneous because all it contains is an expression.
-- The computer cannot execute '3 + 5', since that does not make sense.

Code must be comprised of a sequence of statements. These statements can contain expressions which will be values the statement has to manipulate or use to execute the instruction.

Some code examples in this chapter do not constitute valid code, because they consist of only expressions. In the next chapter, statements will be covered and it will be possible to start writing valid code.

Types[edit]

To evaluate an expression is to compute it to find its value. This is true both in computer science and in mathematics, which also have expressions. In fact, one of the main difference between mathematics and computer science is that programming languages have statements and expressions, while mathematics only have expressions. When expressions are evaluated, they must be evaluated to a value. Values will sometimes be numbers, sometimes be text and sometimes be any of many other data types, which is why they are said to have a type.

In Lua, and in programming in general, expressions will usually consist of one or more values with zero or more operators. Some operators can only be used with some types (it would be illogical, for example, to try to divide text, while it makes sense to divide numbers). There are two kinds of operators: unary operators and binary operators. Unary operators are operators that only take one value. For example, the unary - operator only takes one number as a parameter: -5, -3, -6, etc. It takes one number as a parameter and negates that number. The binary - operator, however, which is not the same operator, takes two values and subtracts the second from the first: 5 - 3, 8 - 6, 4 - 9, etc.

It is possible to obtain a number's type as a string with the type function:

print(type(32425)) --> number

Numbers[edit]

Numbers generally represent quantities, but they can be used for many other things. The number type in Lua works mostly in the same way as real numbers. Numbers can be constructed as integers, decimal numbers, decimal exponents or even in hexadecimal. Here are some valid numbers:

  • 3
  • 3.0
  • 3.1416
  • 314.16e-2
  • 0.31416E1
  • 0xff
  • 0x56

Arithmetic operations[edit]

The operators for numbers in Lua are the following:

Operation Syntax Description Example
Arithmetic negation -a Changes the sign of a and returns the value -3.14159
Addition a + b Returns the sum of a and b 5.2 + 3.6
Subtraction a - b Subtracts b from a and returns the result 6.7 - 1.2
Multiplication a * b Returns the product of a and b 3.2 * 1.5
Exponentiation a ^ b Returns a to the power b, or the exponentiation of a by b 5 ^ 2
Division a / b Divides a by b and returns the result 6.4 / 2
Modulo operation a % b Returns the remainder of the division of a by b 5 % 3

You probably already know all of these operators (they are the same as basic mathematical operators) except the last. The last is called the modulo operator, and simply calculates the remainder of the division of one number by another. 5 % 3, for example, would give 2 as a result because 2 is the remainder of the division of 5 by 3. The modulo operator isn't as common as the other operators, but it can be very useful in certain cases.

Nil[edit]

Nil is the type of the value nil, whose main property is to be different from any other value; it usually represents the absence of a useful value. A function that would return nil, for example, is a function that has nothing useful to return (we'll talk later about functions).

Booleans[edit]

A boolean value can be either true or false, but nothing else. This is literally written in Lua as true or false, which are reserved keywords. The following operators are often used with boolean values, but can also be used with values of any data type:

Operation Syntax Description
Boolean negation not a If a is false or nil, returns true. Otherwise, returns false.
Logical conjunction a and b Returns the first argument if it is false or nil. Otherwise, returns the second argument.
Logical disjunction a or b Returns the first argument if it is neither false nor nil. Otherwise, returns the second argument.

Essentially, the not operator just negates the boolean value (makes it false if it is true and makes it true if it is false), the and operator returns true if both are true and false if not and the or operator returns true if either of arguments is true and false otherwise. This is however not exactly how they work, as the exact way they work is explained in the table above. In Lua, the values false and nil are both considered as false, while everything else is considered as true, and if you do the logic reasoning, you'll realize that the definitions presented in this paragraph correspond with those in the table, although those in the table will not always return a boolean value.

Strings[edit]

Strings are sequences of characters that can be used to represent text. They can be written in Lua by being contained in double quotes, single quotes or long brackets, which were covered before in the section about comments (it should be noted that comments and strings have nothing in common other than the fact they can both be delimited by long brackets, preceded by two hyphens in the case of comments). Strings that aren't contained in long brackets will only continue for one line. Because of this, the only way to make a string that contains many lines without using long brackets is to use escape sequences. This is also the only way to insert single or double quotes in certain cases. Escape sequences consist of two things: an escape character, which will always be a backslash ('\') in Lua, and an identifier that identifies the character to be escaped.

Escape sequences in Lua
Escape sequence Description
\n A new line
\" A double quote
\' A single quote (or apostrophe)
\\ A backslash
\t A horizontal tab
\### ### must be a number from 0 to 255. The result will be the corresponding ASCII character.

Escape sequences are used when putting the character directly in the string would cause a problem. For example, if you have a string of text that is enclosed in double quotes and must contain double quotes, then you need to enclose the string in different characters or to escape the double quotes. Escaping characters in strings delimited by long brackets is not necessary, and this is true for all characters. All characters in a string delimited with long brackets will be taken as-is. The % character is used in string patterns to escape magic characters, but the term escaping is then used in another context.

"This is a valid string."
 
'This is also a valid string.'
 
"This is a valid \" string 'that contains unescaped single quotes and escaped double quotes."
 
[[
This is a line that can continue
on more than one line.
 
It can contain single quotes, double quotes and everything else (-- including comments). It ignores everything (including escape characters) except closing long brackets of the same level as the opening long bracket.
]]
 
"This is a valid string that contains tabs \t, double quotes \" and backlashes \\"
 
"This is " not a valid string because there is an unescaped double quote in the middle of it."

For convenience, if an opening long string bracket is immediately followed by a new line, that new line will be ignored. Therefore, the two following strings are equivalent:

[[This is a string
that can continue on many lines.]]
 
[[
This is a string
that can continue on many lines.]]
 
-- Since the opening long bracket of the second string is immediately followed by a new line, that new line is ignored.

It is possible to get the length of a string, as a number, by using the unary length operator ('#'):

print(#("This is a string")) --> 16

Concatenation[edit]

In formal language theory and computer programming, string concatenation is the operation of joining two character strings end-to-end. For example, the concatenation of "snow" and "ball" is "snowball".
—Wikipedia, Concatenation

The string concatenation operator in Lua is denoted by two dots ('..'). Here is an example of concatenation that concatenates "snow" and "ball" and prints the result:

print("snow" .. "ball") --> snowball

This code will concatenate "snow" and "ball" and will print the result.

Other types[edit]

The four basic types in Lua (numbers, booleans, nil and strings) have been described in the previous sections, but four types are missing: functions, tables, userdata and threads. Functions are pieces of code that can be called, receive values and return values back. Tables are data structures that can be used for data manipulation. Userdata are used internally by applications Lua is embedded in to allow Lua to communicate with that program through objects controlled by the application. Finally, threads are used by coroutines, which allow many functions to run at the same time. These will all be described later, so you only need to keep in mind that there are other data types.

Literals[edit]

Literals are notations for representing fixed values in source code. All values can be represented as literals in Lua except threads and userdata. String literals (literals that evaluate to strings), for example, consist of the text that the string must represent enclosed into single quotes, double quotes or long brackets. Number literals, on the other hand, consist the number they represent expressed using decimal notation (ex: 12.43), scientific notation (ex: 3.1416e-2 and 0.31416E1) or hexadecimal notation (ex: 0xff).

Coercion[edit]

Coercion is the conversion of a value of one data type to a value of another data type. Lua provides automatic coercion between string and number values. Any arithmetic operation applied to a string will attempt to convert this string to a number. Conversely, whenever a string is expected and a number is used instead, the number will be converted to a string. This applies both to Lua operators and to default functions (functions that are provided with the language).

print("122" + 1) --> 123
print("The number is " .. 5 .. ".") --> The number is 5.

Coercion of numbers to strings and strings to numbers can also be done manually with the tostring and tonumber functions. The former accepts a number as an argument and converts it to a string, while the second accepts a string as an argument and converts it to a number (a different base than the default decimal one can optionally be given in the second argument).

Operator precedence[edit]

Operator precedence works the same way in Lua as it typically does in mathematics. Certain operators will be evaluated before others, and parentheses can be used to arbitrarily change the order in which operations should be executed. The priority in which operators are evaluated is in the list below, from lower to higher priority. Some of these operators were not discussed yet, but they will all be covered at some point in this book.

  • Boolean or: or
  • Boolean and: and
  • Relational operators: <, >, <=, >=, ~=, ==
  • Concatenation: ..
  • Level 1 mathematical operators: +, -
  • Level 2 mathematical operators: *, /, %
  • Unary operators: not, #, - (the - here is the unary - as in -5, not as in 5 - 3)
  • Exponentiation: ^