javascript - How can we increment and then decrement a counter without conditionals? -

javascript - How can we increment and then decrement a counter without conditionals? -

i'm looking clever way increment , decrement counter variable. want counter variable begin @ low value , increment, towards higher value. 1 time reaches higher value, counter decrements until reaches lower value. 1 time reaches lower value, counter increments 1 time again higher value ... think point.

i working on a canvas animation i'd employ cleverness, without using if or other conditional tests:

here conditional logic handles counter variable:

incrementing = true foo = -> length += 1 if incrementing length -= 1 if not incrementing incrementing = false if length > 100 incrementing = true if length < 1

initially, thought utilize modulo. however, modulo partition resets counter lower value -- doesn't decrement counter 1 time reaches top value.

0 % 10 = 0 1 % 10 = 1 2 % 10 = 2 3 % 10 = 3 4 % 10 = 4 5 % 10 = 5 6 % 10 = 6 7 % 10 = 7 8 % 10 = 8 9 % 10 = 9 10 % 10 = 0 11 % 10 = 1 12 % 10 = 2 13 % 10 = 3

i'm sure there has way without using conditional tests. assuming bottom value of 0 , top value of 10, method should output following.

? = 0 ? = 1 ? = 2 ? = 3 ? = 4 ? = 5 ? = 6 ? = 7 ? = 8 ? = 9 ? = 8 ? = 7 ? = 6 ? = 5

first let's @ waves !

hello waves !

can identify 1 matches needs ?

if guessed triangle wave, you're right !

triangle wave oscillates uniform slope until hits min or max, , slope inverted.

two things work out here:

the origin of function start in middle of wave the wave oscillates x -x

we want start output @ 0 , include positive values

function triangle(t, a) { homecoming math.abs(((t + a/2) % a) - a/2); }

let's seek out

for (var i=0; i<20; i++) { console.log(i, triangle(i, 10)); }

output

0 0 1 1 2 2 3 3 4 4 5 5 6 4 7 3 8 2 9 1 10 0 11 1 12 2 13 3 14 4 15 5 16 4 17 3 18 2 19 1

so when phone call triangle(i, 10) 10 "period". tells how many steps want have in our function before repeats.

a period of 6 give 6 values, 0, 1, 2, 3, 2, 1

a period of 4 give 4 values, 0, 1, 2, 1 etc

if want go 0-9, need period of 20

javascript coffeescript logic