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While loops

From: PHP with MySQL Essential Training

Video: While loops

In the previous chapter we looked at the first type of control structure, logical expressions, which let our code make choices about how to proceed. In this chapter, we're going to be looking at the other main type of control structure, loops. Loops allow us to write code that will execute more than once, without having to write out all of that code over and over again. The code executes once and then it loops back to the start to execute again. And each time the code executes, it doesn't necessarily do the exact same thing. The values of variables may be changed each time it goes through the loop. So that logical expressions inside the loop do different things.

While loops

In the previous chapter we looked at the first type of control structure, logical expressions, which let our code make choices about how to proceed. In this chapter, we're going to be looking at the other main type of control structure, loops. Loops allow us to write code that will execute more than once, without having to write out all of that code over and over again. The code executes once and then it loops back to the start to execute again. And each time the code executes, it doesn't necessarily do the exact same thing. The values of variables may be changed each time it goes through the loop. So that logical expressions inside the loop do different things.

And of course, a critical feature of any loop is knowing when to stop looping. We're going to take a look at three types of loops. While loops, for loops, and foreach loops. We're going to begin with while loops. I think these are the simplest. The syntax is simply while followed by an expression, that's the argument to it and then a statement. The statement is the loop, the thing we're going to do over and over again, while the expression evaluates to true. Now with a single line statement this will work. But most times you're going to have a multi-line statement and I also think it's just a good practice to always use those curly braces.

It makes it very clear what's inside the loop and what's not. Then notice how similar the syntax is to our if statement. In fact if you swapped out the word while for if, it'd be exactly the same. And the expression part works exactly the same way. It's going to be a Boolean expression that's going to evaluate to either true or false. So what happens is it says, while expression is equal to true. Do this block of code. Start at the top and do all of these statements and when you get to the bottom, come back to the top again and reevaluate the expression. Is it still true? If it is, go through and do all the statements again 'til you get to the last curly brace.

Then back to the top. Evaluate that expression again. Once it evaluates to false well, now we don't do it anymore, because we're only doing it while it's true. So now, the code jumps back down to the curly brace, it does not execute that final time, and we move on with the rest of our PHP code from there. Let me give you an example. So let's say that we have while, the variable count is less than or equal to 10. So as long as that's true, as long as count is less than or equal to 10, we're going to echo the count and then add 1 to it.

Then we'll go back to the top, and we'll check again, is count less than or equal to 10. And if it is less than or equal to 10, it will echo the count and then add 1 to the count and go back to the top again. So you can see what's going to happen here. Let's say the count started out being 0 well, the first time through, count is less or equal to 10, so it executes. This time count is equal to one, still less or equal to 19, so the loop executes. It keeps on going until finally count is equal to up to 11. When count is equal to 11, it's no longer less than or equal to 10.

It's false. And at that point it stops executing the loop. Now, two things, about the loop are critical. The loop needs to have, a condition, an expression, that will allow it to execute. Number two something about the condition has to change during the course of the loop. If not you'll end up getting an infinite loop. Let me show you what I mean. Lets say that we didn't have count plus equal 1 in there. We just took that line out. Well now the first time through lets say count is equal to three. Three is less than or equal to 10 so we'll echo three.

Go back to the top of the loop, 3 is less than or equal to 10, so we'll echo 3. It'll just keep going and you'll just keep outputting 3's over and over and over again. But we have to have some relationship between what's happening inside the loop and the condition that's above it. Something has to eventually change so that now the behavior of the loop will be triggered and we won't just be going infinitely through it. Let's try some examples for ourselves. So to start with, I'm going to open basic.html, just some simple html that I've created, and I'll do save as.

And we're going to call this whileloops.php. I'll call it loops, while. So, let's just try our very first loop here. And let's do the example that we just had. Let's do count equals zero, and then while and then our expression. You could have the space here or not. And then while count is less than or equal to 10. And then inside our curly braces, what do we want to do each time? So until count gets to 10 we're going to echo count, I'm going to go ahead and put a comma after it.

Cartenate the comma with a period, and then I'm going to do count and I'm going to use plus plus just to show you. Remember on the slide I did it using count plus equals 1. Well count plus plus does the exact same thing, it's just increments count. Remember that? So, I'll just make a note here Increment by 1 just so we don't forget. Now let's save it and let's try it out. Let's come back over here to Firefox and I'm going to go to local host, and for me, it's going to be Kevin Skoglund. And then in my sandbox, and then I just made this one called whileloops.php.

So, there it is. See what I got back? The value started out as 0. Let's just bring these up together so we can see them at the same time. So, the first time through count was 0. So, it executes, count gets output. Now, count becomes 1. Goes back to the top of the loop. Still meets the condition, still, the expression evaluates to true. So it outputs it again, keeps incrementing. Finally, let's look at the top. When we get to nine, it's, nine is less than or equal to 10, so it outputs, it becomes 10.

10 is less than or equal to 10, so it outputs the 10, and then it becomes 11. It goes back to the top. Now count is equal to 11, which is not less than or equal to 10. So, it does not execute this last time, it drops right down to here. And just to make sure that you see this, let's do echo br. And echo, count colon then let's output the value of count here. So, after the loop is completely done, let's take a peak at what count is at that last time.

See, there it is, it's equal to 11 after the loop is done. Now, as I said, going through a loop doesn't have to do the same thing each and every time. I mean obviously the values did change and it went from 0 all the way up to 10, which was nice. We had one little bit of very simple code that actually did something different each time through the loop. We can do even more than that, let's do an If statement. Let's say, if the count is equal to 5, then I'll put my curly braces. And I'm just going to go ahead and do else.

And I'm going to copy this and paste it in here. Alright. So, if it's not equal to five. The else statement is, it's going to do what it was doing before. But for, if it's five, let's have it echo the word, five, with a comma after it. Save it and let's come back here and we'll reload and you'll see that it pops up and it does something different when it gets to 5. This is a very simple example, but if you think for a second, you can see how it would be applicable. How you would say, alright if I'm outputting a set of pictures every fifth picture, I want to put in a different HTML.

Maybe you want to do a line break after something, or you want to finish a UL tage and start a new one. Now, I just want to point out what I was talking about with having a relationship between what happens in the loop and making sure that it will exit. If we were to take this count plus plus, imagine that we moved it up here. Imagine that it was inside that alt statement. Do you see what would happen? It's going to loop infinitely, because what's going to happen, it's going to get to five, and then when it gets to five, it's going to stop incrementing. It's just going to keep looping, and looping, and looping, and looping. So, it's important that we make sure that regardless of what conditional statements get executed, that we still end up with a loop that will eventually break out. Now, if you were to do this and you create an infinite loop, PHP will eventually time out.

The page will say, okay I've tried this for long enough, this seems to be an infinite loop and I'm going to give up. And that's a configurable option in PHP. Is to how long it will wait before it gives up. It is not something you typically need to modify. So, on your own,I want you to try to make a new example. And in your example, I want you to try and have a different behavior. For even numbers and odd numbers, so that as you go through a loop, and it counts up, even numbers and odd numbers are handled differently. I will give you a helpful hint, though. It might be useful for you to use the modulo operator. That's the percent sign.

And that tells you whether a number divides evenly by another number or not, by returning the remainder. So, it returns the remainder once you do division, count Modulo two tells us how many are left over if we divide count by 2. Give that a shot and make sure that you have an understanding of how while loops work. We're going to look at For loops next.

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This video is part of

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PHP with MySQL Essential Training

131 video lessons · 32885 viewers

Kevin Skoglund
Author

 
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  1. 4m 8s
    1. Welcome
      1m 0s
    2. Using the exercise files
      3m 8s
  2. 15m 6s
    1. What is PHP?
      3m 52s
    2. The history of PHP
      2m 51s
    3. Why choose PHP?
      4m 10s
    4. Installation overview
      4m 13s
  3. 54m 53s
    1. Overview
      2m 33s
    2. Working with Apache Web Server
      6m 56s
    3. Changing the document root
      7m 24s
    4. Enabling PHP
      6m 16s
    5. Upgrading PHP
      3m 30s
    6. Configuring PHP
      10m 3s
    7. Installing MySQL
      5m 46s
    8. Configuring MySQL
      7m 24s
    9. Text editor
      5m 1s
  4. 31m 25s
    1. Overview
      3m 27s
    2. Installing WampServer
      5m 46s
    3. Finding the document root
      2m 24s
    4. Configuring PHP
      8m 12s
    5. Configuring MySQL
      5m 45s
    6. Text editor
      5m 51s
  5. 19m 12s
    1. Embedding PHP code on a page
      6m 43s
    2. Outputting dynamic text
      5m 55s
    3. The operational trail
      2m 27s
    4. Inserting code comments
      4m 7s
  6. 1h 18m
    1. Variables
      7m 50s
    2. Strings
      4m 38s
    3. String functions
      8m 54s
    4. Numbers part one: Integers
      6m 27s
    5. Numbers part two: Floating points
      5m 25s
    6. Arrays
      10m 0s
    7. Associative arrays
      6m 37s
    8. Array functions
      6m 33s
    9. Booleans
      3m 50s
    10. NULL and empty
      5m 15s
    11. Type juggling and casting
      8m 27s
    12. Constants
      4m 43s
  7. 27m 37s
    1. If statements
      6m 0s
    2. Else and elseif statements
      4m 16s
    3. Logical operators
      7m 30s
    4. Switch statements
      9m 51s
  8. 42m 15s
    1. While loops
      8m 41s
    2. For loops
      5m 59s
    3. Foreach loops
      8m 16s
    4. Continue
      8m 28s
    5. Break
      4m 8s
    6. Understanding array pointers
      6m 43s
  9. 37m 25s
    1. Defining functions
      8m 25s
    2. Function arguments
      5m 32s
    3. Returning values from a function
      7m 33s
    4. Multiple return values
      4m 53s
    5. Scope and global variables
      6m 2s
    6. Setting default argument values
      5m 0s
  10. 20m 18s
    1. Common problems
      3m 47s
    2. Warnings and errors
      8m 36s
    3. Debugging and troubleshooting
      7m 55s
  11. 57m 57s
    1. Links and URLs
      5m 33s
    2. Using GET values
      5m 35s
    3. Encoding GET values
      8m 41s
    4. Encoding for HTML
      9m 26s
    5. Including and requiring files
      7m 40s
    6. Modifying headers
      6m 45s
    7. Page redirection
      6m 43s
    8. Output buffering
      7m 34s
  12. 1h 3m
    1. Building forms
      7m 28s
    2. Detecting form submissions
      5m 59s
    3. Single-page form processing
      7m 57s
    4. Validating form values
      10m 40s
    5. Problems with validation logic
      9m 54s
    6. Displaying validation errors
      7m 23s
    7. Custom validation functions
      6m 28s
    8. Single-page form with validations
      7m 25s
  13. 28m 5s
    1. Working with cookies
      2m 49s
    2. Setting cookie values
      5m 55s
    3. Reading cookie values
      6m 1s
    4. Unsetting cookie values
      4m 51s
    5. Working with sessions
      8m 29s
  14. 48m 39s
    1. MySQL introduction
      6m 43s
    2. Creating a database
      7m 41s
    3. Creating a database table
      7m 42s
    4. CRUD in MySQL
      5m 48s
    5. Populating a MySQL database
      7m 32s
    6. Relational database tables
      6m 40s
    7. Populating the relational table
      6m 33s
  15. 56m 4s
    1. Database APIs in PHP
      4m 51s
    2. Connecting to MySQL with PHP
      7m 45s
    3. Retrieving data from MySQL
      8m 47s
    4. Working with retrieved data
      6m 12s
    5. Creating records with PHP
      6m 58s
    6. Updating and deleting records with PHP
      9m 6s
    7. SQL injection
      3m 5s
    8. Escaping strings for MySQL
      6m 45s
    9. Introducing prepared statements
      2m 35s
  16. 35m 58s
    1. Blueprinting the application
      7m 19s
    2. Building the CMS database
      5m 14s
    3. Establishing your work area
      4m 38s
    4. Creating and styling the first page
      4m 22s
    5. Making page assets reusable
      6m 36s
    6. Connecting the application to the database
      7m 49s
  17. 32m 49s
    1. Adding pages to the navigation subjects
      5m 58s
    2. Refactoring the navigation
      6m 7s
    3. Selecting pages from the navigation
      6m 2s
    4. Highlighting the current page
      5m 26s
    5. Moving the navigation to a function
      9m 16s
  18. 1h 45m
    1. Finding a subject in the database
      9m 48s
    2. Refactoring the page selection
      10m 52s
    3. Creating a new subject form
      6m 55s
    4. Processing form values and adding subjects
      11m 20s
    5. Passing data in the session
      9m 16s
    6. Validating form values
      9m 40s
    7. Creating an edit subject form
      8m 30s
    8. Using single-page submission
      7m 44s
    9. Deleting a subject
      9m 44s
    10. Cleaning up
      10m 37s
    11. Assignment: Pages CRUD
      4m 30s
    12. Assignment results: Pages CRUD
      6m 10s
  19. 39m 26s
    1. The public appearance
      8m 52s
    2. Using a context for conditional code
      11m 37s
    3. Adding a default subject behavior
      6m 9s
    4. The public content area
      5m 51s
    5. Protecting page visibility
      6m 57s
  20. 1h 3m
    1. User authentication overview
      4m 3s
    2. Admin CRUD
      8m 41s
    3. Encrypting passwords
      7m 26s
    4. Salting passwords
      5m 42s
    5. Adding password encryption to CMS
      11m 54s
    6. New PHP password functions
      3m 13s
    7. Creating a login system
      11m 28s
    8. Checking for authorization
      5m 48s
    9. Creating a logout page
      5m 40s
  21. 2m 4s
    1. Next steps
      2m 4s

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