This page explains how to randomize an array.
- pick a random item
- swap it with the last element
- now the last item is “shuffled”
- pick a random item from the “unshuffled” items (any except the last 1)
- swap it with the 2nd to last element
- now the 2nd to last item is “shuffled”
- repeat until each item has been swapped with a random remaining item
The process involves swapping values in the array. If you have 2 variables, a and b, what commands would you use to swap their values?
a = b;
b = a;
This does not quite work. Let’s see what happens if we start with a = 5, and b = 10.
a = b means a is now 10. Then we run b = a and we get b = 10 because a was just set to 10. That leaves us with a = 10 and b = 10.
We need to temporarily store the original value of a in another variable. You can think about this as having an apple in one hand and a banana in the other. If you aren’t allow to juggle them, and each hand can only hold 1 item at a time, then you need to set one of the fruits down on a table to free up one of your hands. You can set the apple on the table, put the banana in the apple-hand, then pick up the apple from the table with your banana-hand.
t = a; table gets the apple
a = b; apple-hand gets the banana
b = t; banana-hand gets the apple off the table
This code will work in the playground, except it’s missing the .length key. If you know the number of elements in your array, you can substitute the number. The details are a little tricky because you have to remember that arrays start numbering from 0, and the pickRandom() function (which is a custom function) picks a number starting from 1. That’s why there’s a +1 and a -1 in the rando value.
let deck = [1, 2, 3, 4, 5];
for (let i = deck.length-1; i > 0; i--) {
let temp = deck[i];
let rando = pickRandom(i+1)-1;
deck[i] = deck[rando];
deck[rando] = temp;
}
Edit:
In modern JavaScript, you can swap values without creating your own temp variable.
[deck[i], deck[rando]] = [deck[rando], deck[i]]
You can even do it with more than 2 variables.
let a = 1;
let b = 2;
let c = 3;
let d = 4;
[a,b,c,d] = [d,a,b,c]
This will result in:
a === 4
b === 1
c === 2
d === 3
–Frankie
