for

A way to repeat a block of code when the number of iterations is known.

for loops are helpful for repeating statements a certain number of times. For example, a for loop makes it easy to express the idea "draw five lines" like so:

for x = 10,100,20 do
  line(x, 25, x, 75)
end

The loop's header begins with the keyword for. Loops generally count up or count down as they repeat, or iterate. The statements in parentheses x = 10, 100, 20 tell the loop how it should repeat:

x = 10 tells the loop to start counting at 10 and keep track of iterations using the variable x.
100 tells the loop to count up to 100, inclusive (note: Unlike Lua, both Java and Javascript treat the stop value as exclusive.)
20 tells the loop to count up by 20 at the end of each iteration.

The code following do until end is the loop's body. Statements in the loop body are repeated during each iteration of the loop.

for loops can also count down:

for d = 100,0,-10 do
  circle(50, 50, d)
end

for loops can also contain other loops. The following nested loop draws a grid of points:

-- Loop from left to right.
for x = 10,100,10 do

  -- Loop from top to bottom.
  for y = 10,100,10 do
    point(x, y)
  end

end

for loops are also helpful for iterating through the elements of an array. For example, it's common to iterate through an array that contains information about where or what to draw:

--Create an ordered table array of x-coordinates.
local xCoordinates = {20, 40, 60}

for i = 1,#xCoordinates do
  -- Update the element.
  xCoordinates[i] = xCoordinates[i] + random(-1, 1)

  -- Draw a circle.
  circle(xCoordinates[i], 50, 20)
end

If the array's values aren't modified, the for...of statement can simplify the code. They're similar to for...in loops in JavaScript, for loops in Python and for-each loops in C++ and Java. The following loops have the same effect:

-- Draw circles with a for loop.
local xCoordinates = {20, 40, 60}

for i = 1,#xCoordinates do
  circle(xCoordinates[i], 50, 20);
end

-- Draw circles with a for...in loop.
local xCoordinates = {20, 40, 60}

for i,x in ipairs(xCoordinates) do
  circle(x, 50, 20);
end

In the code snippets above, the variables i and x have different roles.

In the first snippet, i counts from 1 up to 3, which is the length of xCoordinates, which we can get with prepending # to #xCoordinates. i is used to access the element in xCoordinates at index i.

In the second code snippet using Lua's function ipairs(), x isn't keeping track of the loop's progress or an index. During each iteration, x contains the next element of xCoordinates. x starts from the beginning of xCoordinates (20) and updates its value to 40 and then 60 during the next iterations.

Examples

for example 1

function setup()
  size(100, 100)

  describe('Five black vertical lines on a gray background.')
end

function draw()
  background(200)

  -- Draw vertical lines using a loop.
  for x=10,100,20 do
    line(x, 25, x, 75)
  end
end

for example 2

function setup()
  size(100, 100)

  describe('Five white concentric circles drawn on a gray background.')
end

function draw()
  background(200)

  -- Draw concentric circles using a loop.
  for d = 100,0,-20 do
    circle(50, 50, d)
  end
end

for example 3

function setup()
  size(100, 100)

  describe('A grid of black dots on a gray background.')
end

function draw()
  background(200)

  -- Set the spacing for points on the grid.
  local space = 10

  -- Increase the stroke weight.
  strokeWeight(3)

  -- Loop from the left to the right.
  for x=space,100-1,space do
    -- Loop from the top to the bottom.
    for y=space,100-1,space do
      point(x, y)
    end
  end
end

for example 4

-- Declare the variable xCoordinates and assign it an array of numbers.
local xCoordinates = {20, 40, 60, 80}

function setup()
  size(100, 100)

  describe('Four white circles drawn in a horizontal line on a gray background.')
end

function draw()
  background(200)

  -- Access the element at index i and use it to draw a circle.
  for i=1,#xCoordinates do
    circle(xCoordinates[i], 50, 20)
  end
end

for example 5

-- Declare the variable xCoordinates and assign it an array of numbers.
local xCoordinates = {20, 40, 60, 80}

function setup()
  size(100, 100)

  describe('Four white circles drawn in a horizontal line on a gray background.')
end

function draw()
  background(200)

  -- Access each element of the array and use it to draw a circle.
  for _,x in ipairs(xCoordinates) do
    circle(x, 50, 20)
  end
end

for

Examples

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