[MUSIC] All right, in this video we're gonna pick up working with strings, looking at some of the built in functions strings have so that you can learn about strings enough to use them in that first part of the project. So by the end of this video you'll be able to work with the built in String functions in order to manipulate Strings. Now as I mentioned, at the end of the last video, Strings can do a lot of things, and you can see on the slide the URL to the API documentation in the Java docs. And I am not going to put in this video an explanation of every single method in that library that would take forever and it would be very boring. What I am going to do is just highlight five methods that we think will be particularly useful for you when you're doing your project. But, at the same time, keep in mind that whenever you have a doubt you can always go back to those Java docs to learn more and to find more methods that you might wanna use. So we're going to look at these methods in the context of solving problems. So the first problem we're gonna wanna think about solving is, how do we calculate the number of syllables in a piece of text? That's needed to estimate this Flesch score. And in order to do that, what we're going to need to do is, we're gonna have to look at words, character by character. So, let's look at a function that I've written. This method takes in a String word, and a character letter and it just tries to determine does that letter appear anywhere in the word. The way it does this is it uses a loop, so it's going to loop through each character in the word and remember that Strings are represented by a raise of characters so Strings have a length method that will return back to us the number of characters in that String. So this for loop's gonna execute once for every position in the String. Then it's going to check, is the character at that position equal to the letter that I'm looking for? Again, this illustrates the use of another built-in string method, this charAt method. So charAt takes an index, i, in this case, and it returns the character that's at that particular index. The character, remember, is a primitive, so we can compare this with the equals, equals operator. So if it's equal to the letter we're looking for, we'll return true. If the loop finishes and we didn't find the character, we'll go ahead and return false. Okay, so this method works and it illustrates a couple of things in the built in String class. Next I wanna use it to illustrate a kind of for loop that you may not have seen, or if you have seen it, maybe you forgotten about it. It's called a for-each loop and this can be very handy. So, the for-each loop looks kind of like a for loop, only the headers a little bit different. Instead of those three parts, we just basically have two parts. We declare a variable. In this case, a character type variable, C and then we have a colon. And after the colon what we have is some sort of list or array that we want to loop over. And what's gonna happen, each time for each loop is C is going to get the value of each position in that array. So in this case our array, is the array of characters inside the word that we're looking at. So a String class has this built-in method called toCharArray that returns back the array of all the characters in the String. Now, it's not the actual array of the characters in the String, it's a copy of the array of the characters in the String because, remember, Strings are immutable. So we're not allowed to get in there and change the characters in a particular String. So c is going to iterate taking on the values of each element in that array. In this way, c will first get the first character in the String, the next time through the loop it'll have the next character in the String and so on and so forth. So inside the for each loop we just compare c directly to the letter we're looking for. If they're the same we return true, otherwise, if the loop ends we know it's not there and we return false. So at this point, what we're gonna do is, we're gonna give you a couple of end of year quizzes to see how well you really understood what I just said. And then we're gonna pick back up working with this code. All right, now that you're back, we actually have one more challenge for you. It's not gonna be a quiz, but we do want you to pause the video and think about this. What I would like you to do, is I would like you to modify this code, so instead of just returning true or false it returns the index of the first position where it finds this particular character that it's looking for. So one thing we're gonna have to do is we're have to change that method header from boolean to returning an int, but let's just assume that's already done. Just focus on what you have to change inside the code. So go ahead and pause and work on this now. Okay, so we hoped you figured this out, there actually aren't very many changes we have to make to this code. All we have to do is, because we have this variable i that's tracking through the position in the String, is that when we find the character we're looking for, instead of just returning true, we can go ahead and change the code so that it returns the index where we found that position. And that of course is stored in the variable i. If we get to the end, we didn't find it, we wanna return that flag that indicates we didn't find it, which was negative 1, so we just replace that return false with return negative 1. And that's it. So, it turns out that this method that we just implemented is very similar to a built-in method in the string class called index of. Index of takes, rather than a single character, a String and returns the index position in the String that it was called on where it first found the substring that it was passed in. So let's take a look at how this works. Let's assume we have this piece of text that you can see here. Can you hear me? Hello, hello? And I wanna know what is the index of the substring he. Well if you asked Java this using the indexOf method in the String class, it will return eight because he first appears at position 8 in this String. Remember we start counting at 0 so the index of c is 0 and the spaces count. The spaces take up positions in that String array. So then when we count, you'll see that h is the position eight in the array. On the other hand, if we ask for the substring He with a capital h, it won't find it until position 17. Because Java distinguishes between little h's and capital h's. Now, if you're worried about case, Java has a couple of string methods that can help you with that. So and check out the equals ignore case, two lowercase, and two uppercase methods, also in the String class, which you can read about in the Java documentation. If you look for a String that's not there, like help, Java will return negative one, because that String does not appear at all in the text that we're looking at. Okay, now let's switch gears a bit. We've talked about some methods that would be useful for counting the number of syllables in a particular word. Let's now look at how we're gonna count the number of words in an entire document. And in order to count the number of words in an entire document, we need to be able to split the String apart into individual words, so we can count them. Fortunately, Java has a built-in method called split, and what it takes is a pattern that you would like to use to split on. So in this example here, with that same piece of text, I'm calling the split method with a space passed in. So that's a space between those two quotes, a single space. And Java will say, okay, every space is where I wanna split that String up and it will return to me an array of the Strings that are essentially between the spaces. So it will be the words in that String. That's great. But what happens when we, say, add an extra space? I like to put two spaces between my sentences. Well split isn't quite gonna work as we want it to. So it's gonna interpret that space quite literally. And instead of giving me back a nice array of all the words, it's gonna give me back an array of the words with this extra empty String in the middle. And what that empty String is, is it's the text that's between the two spaces that are between the sentences. There's no text there, the spaces are right next to each other, but because it needs to split there, it needs to split among those spaces, it says well, there's this thing here, and there's nothing there, so I'll just put an empty String in my array of Strings that I'll return. Well that's kind of unfortunate, but the fortunate part is that we can fix this problem. So if you look at the Java documentation for the split method, you'll see that it doesn't take a String, per se, what it takes is something called a regular expression. We're gonna talk a lot more about regular expressions in the next video, and they're going to be our key to solving the problem of splitting on not just one space, but any number of spaces in a string.
