We've already used loops over indices, when working with a single string or a single list. In this lecture, we're going to use loops over indices to access the items at corresponding positions of two different lists. And the characters at corresponding positions of two different strings. We will begin by implementing this function named sum_items. It takes two parameters, list1 and list2 which are both lists of numbers. And it returns a list of numbers. The job of the function is to return a new list, in which each item is the sum of the items, at the corresponding position of list1 and list2. Let's explore what is meant by the phrase corresponding position. If list 1 refers to a list with 3 elements values 1, 2, and 3, and list 2, refers to a 3 element list with values 2, 4, and 2. Then, there are three elements in each list with indices 0, 1 and 2. When we say that we want to consider the corresponding elements of list1 and list2, we're referring to the elements or the items, which are at the same index. For example, at index 0, there are values 1 and 2, and those are summed to get 3. At index 1, there are values 2 and 4, which are summed to get 6. And at index 2, there are values 3 and 2, which, when summed, yield 5. This makes up the new list, which is returned by this function. Now, let's write this code. We're going to, return a new list, and so we'll create a variable sum list that will initially refer to an empty list. This list will be built up one element at a time, through the course of the function. We saw when we were looking at the example, that we use indices to access the elements of list 1 and list 2, so we want to loop over the indices, for i in range of length of. And now we can pass one of the two lists in here as an argument. List 1 and list 2 will have the same lengths because of the precondition to this function, shown above. Therefore, I can pick either of the two lists, and I'll choose list 1. To choose the length of list 1. That means that range will generate values 0, 1 up to, but not including the length of the list. In the body of the loop, we want to access list one at position I and list 2 at position I, and sum them. Once we've calculated the sum, we want to add that to the sum list. The list depend method, can be used to add that sum to the end of the sum list. That's the only thing we need to do in the body of the for loop. Once the for loop has finished executing and exits, then we can return the sum list. Now let's run the example function call. First, run the module, and then in the shell, call the function. And we can see that it returns a list with values 3, 6, and 5. Let's implement one more function. The name of this function is count_matches, and it involves two strings. It has two string parameters, s1 and s2 and returns an int which is the number of positions in string s1, that became the same character at the corresponding position of s2. For sample, for string ape and string ate, there are two characters in common. At index 0, there are a's, and index 2 there are e's. For the strings head and hard. There are also two characters in common, the h's at index 0 and the d's at index 3. The first step for implementing this function, is to create an accumulator. I'll name it num matches, and it will initially refer to the value 0. That variable will accumulate the number of matches. We saw that when we looked at the example, we were comparing the string characters at the same index. That means, we are gonna write our loop over the indices of the strings. So we'll use range, starting from position 0, or value 0, up to the length of the string s1. And what we'd like to do is compare the character of the the string of s1, so if s1 at index I, is equal to the string s2 at index I. Then we found a match, and we can increment the num_matches variable, adding 1 to it. Once the loop has finished executing and we've passed over every character in the strings, then num_matches is returned. Let's run the example function calls. First the ate and ape example returns 2, and then the head and hard example, also returns 2.
