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Introduction to iteration

From: Foundations of Programming: Fundamentals

Video: Introduction to iteration

So let's talk about iteration. This is the programming buzzword for a loop. And here's the idea. You start writing some code. A few statements that deal with, say, looking in all the MP3 files in a folder or changing the position of an image on the screen. So we write some statements to do it once. Then we figure out we need to do it again. So we write them again and again and again. All these can get pretty tedious and rather than write it out several times or even several thousand times, I'd like to be able to write my statements once and then just say "Repeat this." Repeat it 5 times or repeat it 5,000 times, or keep repeating until told to stop, or go through all the fields on a web page or go through every MP3 file in a folder.

Introduction to iteration

So let's talk about iteration. This is the programming buzzword for a loop. And here's the idea. You start writing some code. A few statements that deal with, say, looking in all the MP3 files in a folder or changing the position of an image on the screen. So we write some statements to do it once. Then we figure out we need to do it again. So we write them again and again and again. All these can get pretty tedious and rather than write it out several times or even several thousand times, I'd like to be able to write my statements once and then just say "Repeat this." Repeat it 5 times or repeat it 5,000 times, or keep repeating until told to stop, or go through all the fields on a web page or go through every MP3 file in a folder.

So we might repeat two statements, we might repeat a dozen, we might repeat a hundred. And this is what we do by creating loops and you'll also hear this called iteration. Now creating loops is actually easy. The main issue with any loop is not when to loop. It's when to stop. If we need to loop 5,000 times, who's keeping track? If we need to loop until told to stop, well, where's the code to figure out how to stop? So all our loops will actually have conditions that control how long they loop.

So how do you do this? Well, we've seen an if statement. If the condition is true, do whatever is in the block. In this case here I have got a variable a = 1. If a < 10 we pop up an alert box. Classic if statement here. Well, simply replace the if with the word while and we have a loop. Not if this condition is true, but while this condition is true, execute the body of this code.

It could be one line in this block or a hundred lines. And whatever is in the body of the loop gets executed for each iteration, meaning every time the loop goes around. So here is what happens. We set off setting the variable a = 1 and we are storing that value in memory. We then jump down to the while and we check the condition just like with an if statement. Here, it evaluates as true. Yes, a is less than 10. So we jump into the body of the loop inside the curly braces and we execute whatever code is there, in this case just popping up an alert.

Then we get to the end of the loop. The end of the loop is marked by the closing curly brace of the while statement. What will happen here is we jump back to the start of the loop and we check the condition again. And this happens automatically. Is a < 10? Well, yes it is. So we move on and we go on again. Now here's the problem. If nothing in this block changes the value of a, and a is what we are checking, we will now have an infinite loop.

And an infinite loop is a very bad thing. We would now be stuck in this loop forever. Or at least until we turn our computer off or force our program to terminate. So in this loop and in most loops we are needing to keep track of something. In this case, we need to add a line to increment the variable a. So if I add an a++, and that's inside the body of the loop, then when we are going around this one again we will check is a < 10, yes, it is, we pop up the alert, we do whatever other code is here, and right before the body of the loop closes, we say a++.

Meaning we add one to the value of a. a is now 2. We come back up and we will check it again. Is a < 10? Yes, it is. We'll jump back into the body loop and we'll keep going, but we are incrementing a. And at some point a will be 10. Then we ask it again, is a < 10? No, it isn't. It's 10. That's not less than 10. That is 10. So we count as false. Then we will jump out of the loop or we'll jump right to the end without executing any of that code and we'll move on and we'll execute whatever code comes after.

So this while loop is the most basic loop in JavaScript and it's common across many, many language and takes this general form. It's almost identical in lot of other languages. But there are other kinds of loops as we'll see in a moment.

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This video is part of

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Foundations of Programming: Fundamentals

61 video lessons · 82157 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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