Iterating through collections
Video: Iterating through collectionsArrays are one of the most common things you're find in all programming languages and to get the right perspective here don't think so much of the word array, but think of the wider idea of collections and think of how many different kinds of applications you have in your computer that are really collections of things. Your email program is a collection of emails, your address book is a collection of contacts, your MP3 player containing a collection of albums and playlists, each one being a collection of songs. They're everywhere in programming.
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Finally, the course compares how code is written in several different languages, the libraries and frameworks that have grown around them, and the reasons to choose each one.
- Writing source code
- Understanding compiled and interpreted languages
- Requesting input
- Working with numbers, characters, strings, and operators
- Writing conditional code
- Making the code modular
- Writing loops
- Finding patterns in strings
- Working with arrays and collections
- Adopting a programming style
- Reading and writing to various locations
- Managing memory usage
- Learning about other languages
Iterating through collections
Arrays are one of the most common things you're find in all programming languages and to get the right perspective here don't think so much of the word array, but think of the wider idea of collections and think of how many different kinds of applications you have in your computer that are really collections of things. Your email program is a collection of emails, your address book is a collection of contacts, your MP3 player containing a collection of albums and playlists, each one being a collection of songs. They're everywhere in programming.
And an array is the most fundamental straightforward kind of collection you can have. Now one of the most common things you'll ever need to do is to iterate through or loop through every element in an array and this is actually a very easy thing to do. You don't really need to know much about the array to do it. So let's take a look. So let's say I have a simple array here with five elements in. It doesn't matter if it's 5. It doesn't matter if its 500. I'm going to set up a basic loop to go through this. So I'm going to create a while loop. I could do a for loop as well, but this is fine.
It's got the regular elements. We need to set up the index. Although it's very important when you're working with a basic array you start the index at 0, because our arrays are zero based. Than we do the usual check condition and here's kind of the magic sauce here. When you're doing this, you don't need to know much about the array as long as just asking it what is its length. It can tell you how long it is. We don't need to know this going into it. Sure, I could write while i < 5, but it's much easier to write while i < myArray.length.
And of course, this is a loop, so at the end of the body of the loop we want to increment that index. But know that every time we iterate through or loop through this we can use the index, the value of i to access the current element. Let me show you a little bit about what I mean just in case that isn't clear. If we start to move through our code and we start to execute it, we create this variable i and it's equal to 0. We then move to the next line. We say is i less than myArray.length? We'll go out and look, what is myArray.length? It's going to be 5.
There are five elements in the array. If you remember that .length property gives you how many elements there are. The highest one is of course 4. So this is true. We jump into the body, we start executing any of the code, and then we might say using alert message or something else, we want to access whatever is the first element. So we'll use myArray and whatever the value of i is. Well, i is 0. So we'll get to the first element. Then we'll process further, we'll get to the end of the loop, we will increment the index, i now becomes 1.
We'll jump back and check the while statement again. Is 1 < 5? Yes, it is. That we'll be true and we'll start moving through and we will keep on the looping around, adding one to this index while we go through every element in the array. But at some point we will hit i++. It will change i to 5. Then we'll say is 5 < 5? No, it isn't. It's equal to 5. So this is false and I will just jump out and continue on our merry way. So the important things here, we're starting the index at 0.
When we're checking the condition we're checking against the length of the array and we're doing a less than, not a less than or equal to, and then inside the loop we're using that index to access the current element. Now you might think, well, what so great about this? Well, let me show you a kind of a real example here. So in this simple example here I'm creating an array with five elements, each with the value 500. I want to just add them altogether. Now I'm doing a for loop. So here is a version of the for loop rather then the while loop, but I'm doing exactly the same thing that I showed in these slides.
Start the index of at 0, ask if it's less than the length of the array, and add 1 to it, and then every time we go through this loop I've just got one line of code. I'm going to say the variable total should be set equal to whatever it currently is plus whatever's at the current position. If I'm going on the loop for the first time it's at position 0, second time position 1, third time position 2, and so on. So we should iterate around the for loop five times and then after we are done we'll come down and pop up this alert message.
Well, let's check it. We're adding 500 five times, we're hoping that we don't get an off by one error, but I can hold this in my head correctly, and yes 5 times 500 is 2500. That looks correct. Here's the great thing about programming your array iterations like this. You don't have to know how big the array is. If I come up here and change this and add in a whole bunch of other numbers, I don't have to do anything with the rest of my code, because I wasn't programming it to only go through that array five times.
I'm telling it go through the array however long it is. So I can save this and all I'd change was how many elements we're in the array. I run it again and it still works just fine. So keeping those couple of simple rules in place about knowing where you start and knowing what you're checking when you're iterating around an array can make it very easy to go through all the elements in a collection like this.
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