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Iterating through collections

From: Foundations of Programming: Fundamentals

Video: 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.

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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This video is part of

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Foundations of Programming: Fundamentals

61 video lessons · 82212 viewers

Simon Allardice
Author

 
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  1. 4m 14s
    1. Welcome
      1m 16s
    2. Making the most of this course
      2m 8s
    3. Using the exercise files
      50s
  2. 22m 11s
    1. What is programming?
      5m 45s
    2. What is a programming language?
      4m 48s
    3. Writing source code
      5m 34s
    4. Compiled and interpreted languages
      6m 4s
  3. 16m 29s
    1. Why JavaScript?
      4m 45s
    2. Creating your first program in JavaScript
      6m 54s
    3. Requesting input
      4m 50s
  4. 31m 38s
    1. Introduction to variables and data types
      5m 16s
    2. Understanding strong, weak, and duck-typed languages
      3m 51s
    3. Working with numbers
      5m 4s
    4. Using characters and strings
      4m 5s
    5. Working with operators
      4m 47s
    6. Properly using white space
      6m 46s
    7. Adding comments to code for human understanding
      1m 49s
  5. 24m 49s
    1. Building with the if statement
      7m 35s
    2. Working with complex conditions
      4m 10s
    3. Setting comparison operators
      6m 59s
    4. Using the switch statement
      6m 5s
  6. 17m 56s
    1. Breaking your code apart
      4m 1s
    2. Creating and calling functions
      2m 57s
    3. Setting parameters and arguments
      6m 7s
    4. Understanding variable scope
      2m 23s
    5. Splitting code into different files
      2m 28s
  7. 13m 32s
    1. Introduction to iteration
      4m 28s
    2. Writing a while statement
      5m 24s
    3. Creating a for loop
      3m 40s
  8. 19m 28s
    1. Cleaning up with string concatenation
      4m 30s
    2. Finding patterns in strings
      8m 3s
    3. Introduction to regular expressions
      6m 55s
  9. 19m 59s
    1. Working with arrays
      5m 47s
    2. Array behavior
      5m 29s
    3. Iterating through collections
      5m 18s
    4. Collections in other languages
      3m 25s
  10. 10m 50s
    1. Programming style
      5m 55s
    2. Writing pseudocode
      4m 55s
  11. 25m 55s
    1. Input/output and persistence
      3m 6s
    2. Reading and writing from the DOM
      8m 11s
    3. Event driven programming
      7m 47s
    4. Introduction to file I/O
      6m 51s
  12. 24m 26s
    1. Introduction to debugging
      5m 57s
    2. Tracing through a section of code
      7m 5s
    3. Understanding error messages
      3m 21s
    4. Using debuggers
      8m 3s
  13. 14m 17s
    1. Introduction to object-oriented languages
      5m 18s
    2. Using classes and objects
      6m 29s
    3. Reviewing object-oriented languages
      2m 30s
  14. 11m 14s
    1. Memory management across languages
      5m 11s
    2. Introduction to algorithms
      4m 2s
    3. Introduction to multithreading
      2m 1s
  15. 29m 20s
    1. Introduction to languages
      1m 42s
    2. C-based languages
      4m 40s
    3. The Java world
      3m 13s
    4. .NET languages: C# and Visual Basic .NET
      6m 17s
    5. Ruby
      3m 4s
    6. Python
      2m 56s
    7. Objective-C
      4m 3s
    8. Libraries and frameworks
      3m 25s
  16. 1m 2s
    1. Where to go from here
      1m 2s

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