Now we're going to talk about what I think are the most useful functions in Lua's standard library. We'll start with file operations.
You can open files in Lua with the io.open function like this:
> f = io.open('file.txt', 'w')
The first argument is the filename, and then the mode -
just like fopen in C.
w means write mode, and you can use r for read mode.
You can write to a file by using the write method,
giving it the string that you want to write.
> f:write('pizza\n')
I used a colon instead of a period because the write method
treats f as an object being acted upon.
I'll talk more about the colon syntax later.
You can close the file by using the close method.
> f:close()
Reading a file is the same, except that we use the read
method. This method has the same input structure as
io.read. Here's an example:
> f = io.open('file.txt')
> = f:read()
pizza
> f:close()
Next I'll talk about the pieces of the math module that I find most useful.
You can find the square root of a number using the
math.sqrt function.
> = math.sqrt(64)
8
> = math.sqrt(2)
1.4142135623731
The math.random function behaves differently
depending on the number of inputs it receives.
With no inputs, it returns a pseudorandom float
between 0 and 1. It can equal 0, but is always
less than 1.
> = math.random()
0.45865013192345
With one input, it returns a pseudorandom integer between 1 and the input, including both endpoints as possible outputs.
> = math.random(10)
6
With two inputs, it returns a pseudorandom integer between the two inputs; again the endpoints are possible outputs.
> = math.random(5, 8)
5
The math module has common trig functions, which
work in radians. Pi is available as math.pi.
> = math.pi
3.1415926535898
Now for some functions in the string module.
The string.sub function returns the substring of its
first input between two indices given as parameters.
Here's an example:
> str = "pancakes"
> = string.sub(str, 4, 7)
cake
Both endpoints are included. In this example, "c" is the 4th letter and "e" is the 7th.
You can also use string.sub as a method call with
the colon syntax, like this:
> = str:sub(1, 3)
pan
Remember that Lua strings are arbitrary byte sequences.
The string.byte function returns the numeric
value of a byte within a string like this:
> = string.byte(str, 2)
97 -- Editor's note: 97 is the decimal ascii value of 'a'.
Again, you call it as a method call using colon syntax like this:
> = str:byte(2)
97
The string.format function works like the printf function
found in many other languages. Here's an example:
> = string.format("I eat %s %d times a day", str, 3)
I eat pancakes 3 times a day
I won't cover Lua's format specifiers in detail, but they are mostly the same as those in C.
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