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Writing a while statement

From: Foundations of Programming: Fundamentals

Video: Writing a while statement

So I'm going to write some simple iteration code here, but before I do I have a quick question for you and this is not a trick question, it's not a riddle, but I am interested in the first answer, your gut answer that comes to mind. Here is the question. You need to build a straight fence and it must be 50 meters wide and you need fence posts every 10 meters. How many fence posts do you need? Do you have your answer? Good! Let's get into the code.

Writing a while statement

So I'm going to write some simple iteration code here, but before I do I have a quick question for you and this is not a trick question, it's not a riddle, but I am interested in the first answer, your gut answer that comes to mind. Here is the question. You need to build a straight fence and it must be 50 meters wide and you need fence posts every 10 meters. How many fence posts do you need? Do you have your answer? Good! Let's get into the code.

So in my JavaScript here I'm going to create a simple While loop where I'll show you how to do it and I'll show you one of the most common gotchas with iteration. I want to make a variable called amount. Start off at 0 and I want to add the number 100 to it 10 times. So I'm going to create a loop to do that. So when I create a loop as I mentioned there is always three parts to it, so I'll add a few comments here just before we start to build it out. First, we're going to create the index, then we're going to have the loop that checks the condition, and at some point we need to have the ability to increment or add to the index.

So these are just comments. Let's create an index here. It's very common to call your index i. But you can call it whatever you want. So we'll start off with an index of i = 1. Now I'm going to come and do my while loop. Again it takes the same format as ifs. We've got the word while then our opening and closing parentheses and then our opening and closing curly braces that represent the body of the loop. So in while we got the condition that we're going to check. Just like an if statement that needs to be either true or false.

And what I'm going to say is while i < 10 we're going to jump into the loop. What I'm going to do is add 100 to amount, which I could say amount+100. I could have also used the += format here, and then what I need to do inside the body of the loop is here is where I need to increment the index. I can't wait till I leave the loop, because if I do we'll never hit that line. The increment must be inside the loop itself.

I'm just going to say i++. So every time around we're going to add 1 to the variable i, we'll then hit the end of the loop, the closing curly brace, and we'll jump back up to the while statement and check the condition again. Now finally what I'm going to do is use our good old alert box and say "The value is: " and we'll write out the final value. We're only going to hit this line after we're done with the loop and we write out the value of the variable called amount. So I'm going to save that and test it. So we're going through this, we're going to add 100 to amount 10 times which should give us 1000.

But no, we get 900. This is a very common mistake to make and some of you I'm sure have spotted the problem already. I started off with my index set to 1 and then I started the loop. Is i < 10? Sure it is, it's 1. We then go through the loop, we add one to it. It's now 2. Is i < 10? Sure it is, it's 2. We add another hundred, we keep on going. Now at some point i will go from 8 to 9, we'll loop around again, and then it will go from 9 to 10 and we'll loop around again.

We'll check the condition. When i is 10 this will evaluate as false, so the last time we go and run the loop, we run i = 9. So we're only actually adding 100 nine times, not ten times. Now what could have worked here is either saying if i <= 10, or we could have started off with our index set to 0, because that would have looped around once before we even incremented it to 1. But of course the flip side problem here is if I did both and we try and save that and run that page again, we're now going to end up with 1100.

So I want one solution or the other, but certainly not both. Now these are what is known as off-by- one errors and it's really easy to go around either one too many times or one too few if you don't have it exactly straight in your head. And it's not computers causing this issue. The issue is with us. It's an easy mistake to make even without computer is involved. One example of this is known as the fencepost error. This is the question that I asked earlier. You need to build a fence. It needs to be 50 meters wide with fencepost every 10 meters.

How many fenceposts do you need? Now lot of people's gut reaction is 5, some say 7, but the answer of course is 6. And if it's this easy to make off-by- one errors when we're working with really simple concepts like 50 divided by 10 or 100 times 10, how do you think it is when you're dealing with way more complex data with much more abstract start and end positions? And this is just one example. When we start working with collections of data making sure that we're going through it exactly, it's quite easy to get wrong.

Like with the fencepost error, every programmer I've ever known will admit to making off-by-one errors. So a little extra thought with your loops can go a long way.

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This video is part of

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Foundations of Programming: Fundamentals

61 video lessons · 85367 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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