Menu
The problem is very simple, I have a function from 'Javascript Allonge' book, and having a hard time in understanding it.
The function is called even, and it's as follows:
var even = function(num) {
return (num === 0) || !(even(num -1));
}
it checks whether the number is even or not, but I do not understand how. It calls itself recursively, and technically, always reaches zero, no? How does that work?
845
Introduction to the JavaScript recursive functionsThe recurse() is a recursive function if it calls itself inside its body, like this: function recurse() { // ... recurse(); // ... } Generally, you use recursive functions to break down a big problem into smaller ones.
In this tutorial, you will learn about recursion in JavaScript with the help of examples. Recursion is a process of calling itself. A function that calls itself is called a recursive function.
You can just use let odd = (num%2 == 0) or return (n%2 == 0) without using the conditional operator.
Recursion has a large amount of overhead as compared to Iteration. It is usually much slower because all function calls must be stored in a stack to allow the return back to the caller functions.
This is based on an inductive definition of numbers being odd or even - a number, n is 'even' when the number before it, n - 1 is odd. This thinking naturally makes sense - 4 is even if 3 is odd.
And so the function even is defined as:
1. even(0) is true - because 0 is even
2. even(n) is the negation of even(n - 1)
Another way to think of it is imagine even(4) being called step by step. By hand, replace the even(4) with result of evaluation it with your function:
even(4)
= !(even(3))
= !(!even(2))
= !(!(!even(1))
= !(!(!(!even(0)))
= !(!(!(!true))
= true
// ...even(4) == true
Well, divide and conquer.
First of all you have two expressions to calculate. The first one is just stopping the recursion at the point when the number is 0, this is easy.
The second expression
!(even(num -1))
is a bit more complicated. It always starts with a call even(num -1) and then negates it.
For the first element !(even(num -1) === true), so now you see that for every second element starting from 1 (1, 3, 5 etc.) it will return false, for others the inverted value.
40
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
