Help me Understand this code

JavaScript
1

I just browsing some examples code and see this code, it looks cool but, it confused me.

function chickenOptions(n) {
  var res = [0];
  for(var i = 1; i <= n; i++) {
    if (!(i % 3) || !(i % 10))
      res.push(i);
    else
      for(var k = i-10; k>=0; k-=10)
        if(!(k % 3)) {
          res.push(i);
          break;
        }
  }
  return res;
}

Can someone help me understand it?
Help me understand how it works and why it works.
Thanks.

2

Step through the code with http://pythontutor.com/javascript.html#mode=edit

3

Pseudocoded:

Taken n as a parameter.
declare res as 0-length array. //result array
for (i from 1 to n){
  if (3 divides i evenly OR 10 divides i){
    push i on to res[]
  } else {
    for (k = i-10 to 0, counting down by 10s){
      if (3 divides i){
      push i on to res[]
      resume outer for loop
      }
    }
  }
  return result array
}

Effect : finds every number between 1 and n that is divisible by 3 or 10, AND every number ending in a 3, 6, or 9, regardless of the first digit (if we’re talking about decimal numbers).

Whoops: Forgot one edge case. The numbers ending in 3, 6, and 9 that are less than 9 smaller than n are not included unless they are divisible by 3 themselves.

4

As to why you would want these numbers, I have no idea. As to why it works, the issue is about boolean logic and modulo arithmetic. The % operator returns a number between 0 and m, as in:
r = x % m
Essentially, the output of the modulo is the remainder if you divided by m. If m divides x perfectly, there will be no remainder, and the output of the modulo operator is 0. Since 0 is a “falsy” value in Javascript (and many other C-type languages), negating it with the ! operator means that !(x%m) returns true when m divides x perfectly. The || or boolean OR operator is evaluated left-to-right, and exits when the first condition is true, since if ANY of the tests are true, the whole expression is. So, in the first conditional, even if 3 doesn’t divide i, but 10 does, the expression is true and the number is added to the results array.

5

Thanks for the help.

1 Like
6

What was this code for? And where did it come from?