# Prime numbers in a spiral

I didn’t have anything better to do, so I thought I would code this Ulam spiral. It’s the prime numbers plotted in red in a counter-clockwise spiral. The starting point (1) is in blue in the center of the screen. It may take awhile to create the 23500 limit prime number table. I have little lines showing the direction if you want to follow the spiral around. You have to turn the iPad in landscape for the correct orientation.

``````
Example of the starting counter clockwise spiral.

16  15  14 13
5   4   3 12
6   1   2 11
7   8   9 10

``````

``````
supportedOrientations(LANDSCAPE_ANY)
displayMode(FULLSCREEN)

function setup()
limit=23500
tm=15
prime={}
for z=1,limit do
table.insert(prime,z)
end
for z=2,math.sqrt(limit) do
for x=z+1,limit do
if prime[x]%z==0 then
prime[x]=0
end
end
end
tabx={1,0,-1,0}
taby={0,1,0,-1}
end

function draw()
background(0)
fill(255)
count=1
pos=1
x=math.floor(WIDTH/2)
y=math.floor(HEIGHT/2)
value=1
for z=1,#prime do
for b=1,2 do
fill(255)
for a=1,count do
if value>#prime then
break
end
if value<limit then
noStroke()
fill(0)
if prime[value]~=0 then
fill(255,0,0)
else
fill(255)
end
if value==1 then
fill(0,0,255)
end
ellipse(x,y,tm)
end
x=x+tabx[pos]*tm
y=y+taby[pos]*tm
if value>1 then
stroke(103, 122, 30, 255)
strokeWidth(2)
line(x,y,hx,hy)
end
hx=x
hy=y
value=value+1
end