Earlier we saw that each character of a string has an index associated with it. And we learned how to access each character using its index. In this lecture we will use something called a for loop to access the characters of a string in turn, one at a time. To begin I'm going to make an example string, hi there. And we'll use this string in our first for loop where we're going to print each character of it one at a time. Char, that variable C-H-A-R, is short for character. And it's just a variable name. The value of char will change as the loop executes. In the first iteration of the loop, char gets the value of s at position zero, which is h, and then h is printed. Through the next iteration of the loop char's value changes, so char refers now to the character acquisition 1 of s, which is i. And i is printed. Through the third iteration of the loop, char gets the value at position two of s, which is a space, and so that space is printed. And this pattern continues until the end of the string is reached. So each character of s is printed one at a time. Char is a variable name. We could have easily chosen something else. For example, this is equivalent. For banana in s, print banana. Of course, it's best to pick a meaningful variable name that indicates what it is that the variable represents. Here's another for loop. In this loop, there are two print statements. So the variable char, is printed two times. Initially, to the first iteration of the loop, char gets the value h. And h is printed twice, then char gets i and i is printed twice and so on until the end of the string s is reached. We will now use our for loop to solve a problem. The first problem that we'll tackle is to count the number of vowels in a string. I'm going to start by writing a couple of example function calls. I'm going to name the function count_vowels, and it will take a single string argument. The first example uses Happy Anniversary as the argument. And in that case, count vowels should return five. [SOUND] The next string, xyz, doesn't contain any vowels, so this function should return zero. Let's add the the type contract, there's one string parameter, and the function returns an int. And now let's write the function header. The name of the function is count_vowels. And it has one parameter. I'm going to name that parameter s. And the job of this function is to return the number of vowels in s. You may have noticed the example function calls that we are not including y, so I need to add that to my description. Do not treat the letter y as a vowel. Now we can write the body of the function. Before we write this code I want to start by thinking about how we determined what the response of the example function call should be. What I did was examine each character of the string one at a time beginning with the h. So then I looked at the h and decided it was not a vowel but the a is, so I kept track of that. Then I looked at ppy space and encountered another vowel, nni is another one, ve, and rs then another a, that's five. RY exclamation mark. So looking character by character at the string we see that there are five vowels. We will model our program after the approach that we just took. So we want to keep track of the number of vowels that we've seen so far. And initially, we've seen zero vowels. So I'm going to use nun_vowels to keep track. We pass over the string examining one character at a time. And we can do this using a for loop. So for each character in s we want to check to see whether that character is a vowel or not. I can use an if statement to check that condition. So if a character appears in the string aeiou lowercase and AEIOU uppercase then it is a vowel. And in that case we want to increment the value of nun_vowels. So nun_vowels will get it's current value plus 1. After this loop has finished executing, and we've passed over each character of the string, nun_vowels will contain the total number of vowels in s, and that is the total value that we will return. Let's stop for a moment and think about this assignment statement. Notice on the right-hand side of this assignment statement the expression involves the variable nun_vowels. Imagine that the nun_vowels statement that occurred outside the loop was not there. When we hit this assignment statement and look up the value of nun_vowels a name error would occur, so that assignment statement that we used initially to initialize nun_vowels to zero is crucially important. Now let's run this code, checking our example calls. So we will first run the module, and then in the shell call count vowels. With Happy Anniversary we got five, and with xyz we get zero. The variable num_vowels is an accumulator, because it accumulates information. It has an initial value of zero and it's value goes up by one whenever a vowel is found. The loop ends after every letter in x has been examined and all vowels have been counted. Accumulators are not necessarily numeric as we'll see in the next problem. Our next task is to return a string containing all of the vowels in a given string. I'm going to start with example calls again. And we can use the same examples as we did in our previous function. So, this function's name will be collect_vowels because we are going to accumulate them in a string. And given the string, Happy Anniversary, this should return a string with the letters lowercase a, uppercase a, I, E, lowercase a again. In the case where there are no vowels in the string, it should return the empty string. So it's just an open quotation mark, closed quotation mark. The type contract, this takes a string parameter and returns a string. And the name of the function as we've seen is collect_vowels. The name of the parameter, again, I will use the name s. And the job of this function is to return the vowels from s. Y isn't treated as a vowel in this case either. Like we did with count_vowels I want to start by considering how we solved this problem when we were determining what the example function call should return. I examined each character of the string one at a time, so I'm looking first the H, and then the a, and whenever I encountered a vowel I kept track of that so I have an a P-P-Y space and then another a. So I want to keep track that I've got those two. N-N-I and so on. So I look through the string character by character and when I find a vowel I add it to the set of characters that I've seen so far. Let's implement this function now. Like before we need to accumulate some information. Before it was an integer, this time we want to accumulate some information in a string. So I'll use the variable vowels to refer to a string. And that string will initially be empty. I haven't see any vowels so far. We're going to examine the string ask character by character using for loop, and we want to check to see whether that character is a vowel using the exact same approach that we did the first time. If the character is a vowel, I want to add it to the set of vowels that I've seen so far. So my vowels variable will refer to its original value plus the character that we've just seen. I'm using this concatenation operator to add the character to the current string of vowels. At the end of the function, once the for loop has finished executing and each character from the string has been examined, vowels will refer to a string containing each of the vowels from s and that is what will be returned. We need to test the function so we will run the module, and then call the example function calls in the shell. Collect vowels returns a string that we expect and this call with a different argument also returns the string that we expect. Like the variable nun_vowels, the variable vowels is also an accumulator. As it is accumulating a string over the course of the function's execution. Like nun_vowels that started at zero and grew, vowel starts out as empty, and also grows.
