Learn JavaScript #19 | Find the Missing Number Exercise

Alright guys, I know we haven't actually done anything useful in the webpage for a little bit, and it's going to be a little bit longer because we need to talk about a few more JavaScript things first. So think of JavaScript like a programming language. There are objects, functions, variables, and a lot of other built-ins that we're going to discuss as well. What we're going to in this video, we're going to create a simple function that's going to get a missing number from an array. So, project time. If you guys want to pause this video and attempt that right now, that's probably not going to work, actually. I don't think we've talked about sorting arrays, so that's something we're going to do right now. So, first we need to create an array. So, variable numbers is going to be a list, and we're going to put them in a jumbled up order. So 43, 56, 601, 100, 45, 10, 58 and 34. So, this is a jumbled up list, or array, and we can use the sort method. I'm going to save this and refresh the page, and we're going to do this in an interactive way. We have numbers here. Wou can go numbers.sort, and it's a method so you write it like that. Now, there is a problem with this and this is that it's going to sort as a string. So let me create words, and we're going to say apple, pear, coconut, I probably spelled that wrong, pizza...I guess it's not just going to be fruit, it's going to be foods, hamburger, and banana. Now, this is going to sort it correctly because it's going to do an alphabetical sort. Now if you know how to alphabetize stuff, and I'm going to assume that you do, it first looks at all the first letters, and it arranges them. And when it gets here it's going to see there's two items with the beginning letter P so it's going to move on to the next one, and it's going to say okay, P comes before I, so pair is going to go before pizza, and it continues on. Let's say the first five letters of a word are the same, it's going to move on to the sixth to determine the direction that it should be sorted. So, that's not just on, that doesn't only relate to JavaScript, that relates to alphabetizing in general. I'm going to refresh this and we have words here. So if I type words.sort... and the sort method is available on all arrays, so this is an array method. It's built-in and it is automatically is available. So when I do this it sorts it correctly because it's doing an alphabetized sort. So, when we're using numbers it's trying to do the same thing. So if we see here a hundred is put before thirty-four because one is smaller than three, and so on. So, that as you can see this is doing it wrong so we need to supply a parameter to the method in order to properly sort numbers, and it's a function parameter. So I'm going to go ahead and write numbers.sort and then open up the parentheses. Now in here we're going to write a simple function that takes A and B and we return A minus B, or B minus A...okay, let's just go do it. So, pass in function, this function takes a and b, and then we open up curly braces and we say return a-b; I'm going to... I don't need to save that. I forgot I'm working in in the interactive console here. So when I hit enter it's going to properly sort the numbers. So, that's how you numerically sort an array. It's just a simple line of code here. So, what we're going to do is pause the video now. I want you guys to create a list with a range of numbers, let's say 1 to you know 20, just for simplicity. I don't want to have you guys create like this full-on array of numbers that's like you know a hundred, one hundred indexes or so. So just a list from from 1 to 20, and then make one number that should be in that list not in that list. So let's say the missing number would be 6. So you guys are going to, in your code editor, you're going to create an array with a missing number, and you're going to write a function that's going to determine what the missing number is. Now, the the array should have sequential numbers, so your list including the missing number should look something like this: 1, 2, 3, 4, 7, 8, 9, but I want you to put it in a jumbled up order. So, it would look something like 9, 7, 2, 3, 5, 6, etc. and then you would just make sure that one number was not in there, alright? So, I'm actually going to write down a list that I'm going to be using so that you guys can write the same thing just in case I didn't explain that well enough. Let's remove the words, and let's start writing the list. So... do we have 5 in there, nope. Do we have 2, no. Alright, put a 6 in there, so 16, 18, this is going to be a little more confusing than I thought. Alright, where's that comma? Alright, so, we've got 1, 2, 3, 5, 6, 7, 8, 9, 11, 10, 12, 13, 14, 15, 16, 17, 18, 19, and 20, alright. So, these are all our numbers. Now I'm going to select one at random here, and just to make sure these are correct and that they're sequential numbers, I'm actually going to refresh the page here and I'm going to write numbers.sort, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14...18, 19, and 20. Alright, so, we're missing the number 4, and we have 18 in there twice. I'm going to remove an 18 there and our missing number is 4. Pause the video, write this list out like this it's going to be a lot easier, and then write that function that determines the missing number. And then when you're done proceed on to the next video.