Accessing a Value in an Array

Regardless of the kind of data structure you work with, 0:00 all data structures are expected to carry out four kinds of operations at minimum. 0:03 We need to be able to access and read values stored in the structure. 0:08 We need to be able to search for an arbitrary value. 0:12 We also need to be able to inset a value at any point into the structure. 0:15 And finally, we need to be able to delete structures. 0:18 Let's look at how these operations are implemented on the array structure in some 0:22 detail starting with Access. 0:27 Elements in an array are identified using a value known as an Index. 0:29 And we use this Index to access and read the value. 0:33 Most programming languages follow a zero-based numbering system 0:37 when it comes to arrays. 0:41 And all this means is that the first index value is equal to zero, not 1. 0:42 Generally speaking, when an array is declared, 0:47 a base amount of contiguous memory is allocated as the array storage. 0:50 Computers refer to memory through the use of an address. 0:55 But instead of keeping a reference to all the memory allocated for 0:58 an array, the array only has to store the address of the first location. 1:01 Because the memory is contiguous, using the base address, the array can calculate 1:06 the address of any value by using the index position of that value as an offset. 1:11 If you want to be more specific, think of it this way. 1:16 Let's say we want to create an array of integers, and then each integer takes up 1:18 a certain amount of space in memory that we'll call M. 1:23 Let's also assume that we know how many elements we're going to create. 1:26 So the size of the array is some number of elements we'll call n. 1:30 The total amount of space that we need to allocate is n times the space per item M. 1:34 If the array keeps track of the location in memory where the first value is held, 1:40 so let's label that M0, 1:45 then it has all the information it needs to find any other element in the list. 1:46 When accessing a value in an array, we use the index. 1:51 So to get the first element in the list, we use the zero index. 1:55 To get the second we use the index value 1, and so on. 1:58 Given that the array knows how much storage is needed for each element, 2:01 it can get the address of any element by starting off with the address for 2:06 the first element. 2:11 And adding to that,the index value times the amount of storage per element. 2:12 For example, to access the second value, we can start with M0. 2:17 And to that, add times M times the index value 1, 2:21 giving us M1 as the location in memory for the second address. 2:24 This is a very simplified model, but that's more or less how it works. 2:28 This is only possible because we know that array memory is contiguous with no gaps. 2:33 Let's switch over to some code. 2:39 As I mentioned earlier, we're going to be using Python in this course. 2:41 If you don't know how to code, or you're interested in this content but 2:45 know a language other than Python, check the note section of this video for 2:49 more information. 2:53 While the code will be in Python, the concepts are universal. 2:54 And more importantly, simple enough that you should have no issue following along 2:58 in your favorite programming language. 3:02 Now, to get started, click on the Launch Workspace button 3:05 on the video page that you are watching right now. 3:08 This should spin up an instance of a treehouse workspace, 3:11 an in-browser coding environment. 3:15 Right now, your workspace should be empty, and that's to be expected. 3:17 So let's add a new file in here. 3:21 Going to go to File > New File. 3:22 And we'll call this arrays.py, py. 3:25 Creating a list in Python is quite simple. 3:30 So we'll call this new_list. 3:33 We use a set of square brackets around a set of values to create a list. 3:36 So 1. 3:40 And we comma separate them. 3:41 So space 2, and space 3. 3:42 This allocates a base amount of memory for the array to use. 3:46 Or when I say array in Python, know that I mean a list. 3:50 Since this is Python, the values aren't stored in memory. 3:53 Instead, the values 1, 2, and 3 are stored elsewhere in memory. 3:57 And the array stores references to each of those objects. 4:02 To access a value, we use a subscript along with an index value. 4:07 So to get the first value, we use the index 0. 4:13 And if we were to assign this to another variable, 4:16 it would say result equal new_list. 4:18 We write out new_list since this is the array that we're accessing the value from. 4:23 And then, a subscript notation, which is a square bracket. 4:27 And then the index value. 4:30 As we saw, since the array has a reference to the base location in memory, 4:32 the position of any element can be determined pretty easily. 4:37 We don't have to iterate over the entire list. 4:42 All we need to do is a simple calculation of an offset from the base memory 4:45 since we're guaranteed that the memory is contiguous. 4:50 For this reason, access is a constant time operation on an array or a Python list. 4:54 This is also why an array crashes if you try to access a value 5:00 using an index that is out of bounds of what the array stores. 5:05 If you've used an array before, you've undoubtedly run into an error or 5:09 a crash where you tried to access a value using an index that was larger than 5:13 the number of elements in the array. 5:17 Since the array calculates the memory address on the fly, 5:20 when you access a value with an out of bounds index, as it's called, 5:23 the memory address returned is not one that's part of the array structure. 5:27 And therefore cannot be read by the array. 5:32 In Python, this is represented by an index error. 5:34 And we can make this happen by using an index we know our array won't contain. 5:37 Now, I'm writing out my code here inside of a text editor, 5:43 which obviously doesn't run the code. 5:46 So let's drag up this console area here. 5:48 And I'm going to write python to bring up the Python interpreter. 5:51 And in here, we can do the same thing. 5:55 So I can say new_list = (1, 2, 3). 5:58 And now, this is an interpreter, so it's actually going to evaluate our code. 6:00 All right, so now, we have a new list. 6:07 If I type out new_list, it gets printed out into the console. 6:09 Okay, I can also do new_list [0], and you'll see that I get the value 1, 6:13 which is the value stored at the zeroth index. 6:18 Now, to highlight that index error, we can do new_list. 6:21 And inside the square brackets, 6:25 we can provide an index that we know our array doesn't contain. 6:27 So here I'll say index 10. 6:31 And if I hit Enter, you'll see it say index error, list index out of range. 6:33 And those are the basics of how we create and read values from an array. 6:39 In the next video, let's take a look at searching. 6:42