Original subtitles

‫I really like the problem that we're going to work on in this video, it's the fact that digit some

‫problem and the reason why I like it is because I think it's a perfect one to practice a lot of the

‫things that we've learned in the course.

‫So after we read this, definitely try to pause the video and try to build it yourself before we go

‫through the solution, because I think that you'll see that pretty much everything that we've gone through

‫in the course, there's nothing new in it.

‫So you have all the knowledge you need in order to build this.

‫Don't worry if you can't do it, because it's still very challenging, but I definitely recommend it

‫because it'll help you learn it even better.

‫Even if you fail doing it, you'll remember the solution better.

‫So what is a factorial factorial is they give you the algebraic answer up here.

‫Here's the example.

‫Factorial of ten is ten times nine times, eight times seven all the way down.

‫Factorial of ten is massive just by itself.

‫It's three million six hundred twenty eight thousand eight hundred.

‫So as you can see in pectorals grow very, very quickly and the sum of the digits in ten factorial is

‫27.

‫And so the way that you can count that is it's just literally the sum of the digits, three six to eight

‫zero and zero.

‫So that equals twenty seven.

‫So what we need to do is find the sum of the digits in the number 100 factorial.

‫So as you can see, we have to find the factorial, then we have to split it up, get all the digits,

‫atom together and that's how we'll get the sum.

‫So let's start this off by creating a factorial method.

‫So factorial value some generator.

‫That's a nice long name.

‫And we're going to pass in factorial as the argument, OK, the first thing we're going to do is to

‫set up an array.

‫And so in this array, we want it to range from one to the value of the argument passed into the function.

‫So in this case, I'm going to do one dot dot factorial and then want to cast this and convert it into

‫an array.

‫And now we because if we're matching what we saw over here, this needs to be in reverse.

‫So how do we reverse this?

‫We just called the reverse method on.

‫And so when we call the reverse method on it, if this is 10 factorial, then what it's going to do

‫is pass in ten and then multiply it ten times eventually when we have this part in, but is going to

‫do it backwards.

‫So if we pass in ten is factorial, it's immediately going to create an array that looks like of this.

‫But that won't work when we do our factorial method on it.

‫So we need to split this up.

‫So our array looks like this.

‫Does that make sense?

‫That's that's a little bit of just a tricky way of being able to get at you swap out or you reverse

‫the order so that you can be able to call the factorial on it.

‫So now that we have that where it's an array so we can call each on it and iterate over, I'm going

‫to create an iterator variable called I and pass End of the Block.

‫And now I'm going to call I'm going to bring the factorial method or argument in, because here, remember,

‫we want to grab this some.

‫So I'm going to increment it by this value.

‫And so this value is going to be factorial times, ie, minus one.

‫OK, now don't worry, if you don't get that, let's just hold on one second and go through it again.

‫So let's pick up where we left off right here.

‫So I got one through 10.

‫We reversed it.

‫So it's going to be assuming factorial.

‫The argument is 10, we reversed it.

‫So it's going to be 10 through one.

‫And now we're iterating over that new array.

‫So let's just for visual sake, say that we have our red dot all the way to one.

‫OK, so we're iterating over it.

‫And so what we're doing is we're taking the value of factorial right here.

‫And so we're going to say this is what starts off with.

‫So in this case, a B, 10, and then we're going to add the product of factorial, which is also ten

‫times IE minus one.

‫So in this case, I is going to be 10 minus one, which is going to be nine.

‫So this is going to be ten there.

‫That's going to be the sum of ten plus ten times nine, nine.

‫So it's going to iterate through that all the way through.

‫So hopefully that makes sense.

‫I think it's a good way of writing the factorial method.

‫So now that we have that, that's just a factorial now we actually have to create the the sum of the

‫digits.

‫So I'm going to say factorial to string and then split this.

‫And so what this is going to do is simply split it up so that each one of the digits is separated.

‫And now I'm going to call the map method.

‫So on the map method, we're going to pass it in a block and call the two integer method on it.

‫And then we're simply going to call inject so that we can add all of this up and that is it.

‫So let's now call our method pass in one 100, because that's what we need for this one.

‫And let's see if this works.

‫Ruby Project Guler, this is problem 20.

‫And 648 is the solution.

‫So great job, this is a this is one that I thought was perfect for this course because it makes you

‫use a lot of the things we've learned, like being able to work with blocks, being able to call methods

‫on arrays, chain methods together, use methods like inject and map in conjunction with each other

‫to be able to build a pretty, pretty simple, elegant and simplified solution to a complex problem.

‫So nice work.