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Ilya Nevolin
Ilya Nevolin

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Road to Genius: advanced #46

Each day I solve several coding challenges and puzzles from Codr's ranked mode. The goal is to reach genius rank, along the way I explain how I solve them. You do not need any programming background to get started, and you will learn a ton of new and interesting things as you go.

function POF(num) {
  if (num === 1)
    return true;
  if (num < 4)
    return false;
  if ((num & num - 💰) !== 0)
    return false;
  return (num & 1431655765) === 💎;
}
let A = POF(356);

// 💰 = ? (number)
// 💎 = ? (identifier)
// such that A = false (boolean)
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In today's challenge we need to fix two bugs, in what seems to be quite complex code. Especially if you've never worked with binary operations, but fear not.

The & operator is a binary and, it works like this:

This is purely mathematical (not javascript)
The and-operation returns 1 if both values are 1,
and returns 0 if any of the values is 0.
1 & 1 = 1
1 & 0 = 0
0 & 1 = 0
0 & 0 = 0

Example:
x     = 1010
y     = 1100
x & y = 1000
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We need to ensure that the output from the function POF(356) is false. The first two if-conditions we can ignore because they won't give us any false return value. But the third might:

if ((num & num - 💰) !== 0)
  return false;
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With num as 356, let's have a look at the possible choices for 💰: 0, 1 and 356 itself. If we can use any of these to ensure that the if-condition returns false, then we've solved it, so let's try:

356 in binary is: 101100100

let 💰 = 0
-->  101100100 & 101100100 !== 0

let 💰 = 1
-->  101100100 & 101100011 !== 0

let 💰 = 356
-->  101100100 & 0 == 0
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The answer for 💰 should either be 0 or 1 (but not 356) to ensure the function returns right there. The final bug 💎 can be anything as long as we respect the previous sentence.

coding challenge answer

By solving these challenges you train yourself to be a better programmer. You'll learn newer and better ways of analyzing, debugging and improving code. As a result you'll be more productive and valuable in business. Get started and become a certified Codr today at https://nevolin.be/codr/

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