PcoWSkbVqDnWTu_dm2ix
We use cookies on this site to enhance your user experience

Jul 03 2018, 9:41 AM PST 5 min

A Generic For loop is a type of for loop that takes advantage of iterator functions. You should understand basic for loops before learning this.

Stateful iterators

First, let’s take a look at iterators. In layman’s terms, an iterator is a function that returns the next set of values each time it is called. Here’s a function that generates a simple iterator:

-- returns an iterator that counts letters between first and last
function letterIterator(first, last)   
    -- store the position that the iterator is at
    local index = first 
    -- return the iterator - a function!
    return function()
        if index <= last then
            -- move to the next character
            index = index + 1  
            -- return the ascii representation of the index plus 95 (a letter)
            -- note that there's only one value being returned
            return string.char(index + 95)
        end
    end
end

And here’s how we can use it:

local iterator = letterIterator(1, 4)
local letter
 
letter = iterator()
print(letter) -- **a**
 
letter = iterator()
print(letter) -- **b**
 
letter = iterator()
print(letter) -- **c**
 
letter = iterator()
print(letter) -- **d**
 
letter = iterator()
print(letter) -- nil
 
letter = iterator()
print(letter) -- nil

As you can see, the iterator returned the first, second, third, and fourth letters, then stopped returning anything. At this point, there’s nothing left to iterate, so you should stop calling the iterator. That’s where the generic for loop comes in. We can write the previous code like this instead:

Explanation

The iterator is the object that keeps track of how many times a for loop is supposed to run. This is different from a the numeric for loop in Articles/Roblox Coding Basics Loops|Loops in that the numeric for loop there is simply an index:

for i = 20, 1, -2 do
	print(i)
end

However, in the generic for loop, we get the values returned by the iterator function. In the iterator returned by letterIterator() above, we saw that string.char(index + 95), not index itself, was returned. Here’s an example using multiple return values:

Standard Library Iterators

Two commonly-used standard library iterator functions are pairs and ipairs. These functions return table keys and their corresponding values. pairs iterates through the entire table, even if the key is non-numerical (such as “Hi”), treating it as a dictionary (see Articles/Table|Tables). ipairs iterates through the table as if it were an array, starting at 1 and counting up consecutive integer indices. Once it hits a nil value, it will stop looping through the table. Also, please note that pairs does not iterate through the table in any particular order. Here’s an example of the difference:

local sampleTable = {
	[1] = "A",
	[2] = 2,
	[3] = "B",
	[5] = 5,
	Hi = "C"
}

Since pairs uses next as its iterator function, some people prefer to call it directly:

Customized Iterators

You too can write your own iterators! Here are two useful examples:

string.gfind

This example returns multiple values:

descendants

This example iterates through all of the Instance/GetChildren|children of an Instance|Instance:

function descendants(obj, depth)
	assert(obj and obj.GetChildren, "object parameter is missing or is not an instance")

	local function yieldtree(obj, level)
		if depth and level > depth then
			return
		end
		for _, o in ipairs(obj:GetChildren()) do
			coroutine.yield(o, level)
			yieldtree(o, level+1)
		end
	end

	return coroutine.wrap(function() yieldtree(obj, 1) end)

For further reference:

Tags:
  • lua
  • coding
  • concept