Coercion

Coercion

What is Coercion?

Lua performs automatic conversion of numbers to strings and vice versa where it is appropriate. This is called coercion.

Lua will automatically convert string and number types to the correct format in order to perform calculations. For example, if you try to apply an arithmetic operation to a string, Lua will try to convert that string to a number first, otherwise the operation will not work. If the string cannot be converted to a number an error is raised.

Example

print("This is Lua version " .. 5.1 .. " we are using.")

Output:
This is Lua version 5.1 we are using.

print("Pi = " .. math.pi)

Output:
Pi = 3.1415926535898

print("Pi = " .. 3.1415927)

Output:
Pi = 3.1415927

As shown above, during coercion, we do not have full control over the formatting of the conversion. To format the number as a string as we would like we can use the string.format() function. e.g.:

Example

print(string.format("%.3f", 5.1))

Output:
5.100 -- notice that there are three decimal places.

print("Lua version " .. string.format("%.1f", 5.1))

Output:
Lua version 5.1

This is explicit conversion using a function to convert the number, rather than coercion.

Using Coercion

Example

print(100 + "7")

Output:
107

print("1000" + 234)

Output:
1234

print(234 + "1000")

Output:
1234

print("hello" + 234)

Output:
Fri Oct 17 12:14:00 2008 - Workspace.Script:1: attempt to perform arithmetic on a string value

You can see where a string can be converted to number, the calculation succeeds. The string "hello" cannot be converted to a number and so an error occurs. In statically typed languages (e.g. C) this would cause an error as you cannot assign a value to a variable of an incompatible type. This works in Lua because it is dynamically typed.

A notable exception: comparison operators (== ~= < > <= >=) do not coerce their arguments. The (in)equality operators consider a number to be not equal to its string representation (or any non-number type in fact). Ordered comparison operators throw an error when you feed them different types:

Example

print(100 == "100")

Output:
false

print(100 ~= "hello")

Output:
true

print(100 ~= {})

Output:
true

print(100 == tonumber("100"))

Output:
true

print(100 <= "100")

Output:
Fri Oct 17 12:19:03 2008 - Workspace.Script:1: attempt to compare number with string
Fri Oct 17 12:19:03 2008 - Workspace.Script, line 1
Fri Oct 17 12:19:03 2008 - stack end

See Also

http://lua-users.org/wiki/StringsTutorial