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PHP Arrays and Control Structures

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For Loops

5:51 with Alena Holligan

for loops are the most complex loops in PHP because they use 3 expressions instead of just one. But complex doesn't need to mean difficult. Sometimes having something more complex in one place is actually easier to read than looking for all the simple pieces.

Documentation

for loops are the most complex loops in PHP because the contain three expressions. The first expression (expr1) is evaluated (executed) once unconditionally at the beginning of the loop.

In the beginning of each iteration, expr2 is evaluated. If it evaluates to TRUE, the loop continues and the nested statement(s) are executed. If it evaluates to FALSE, the execution of the loop ends.

At the end of each iteration, expr3 is evaluated (executed).

Each of the expressions can be empty or contain multiple expressions separated by commas.

Additional Examples

Single line for loop
for($year = date('Y')-100; ++$year <= date('Y'); print $year."<br />");
Looping through Letters

You can also loop through letters (Very convenient when working with excel columns.)

for ($letter = 'A'; $letter != 'AA'; $letter++) {
   echo $letter."<br />";
}

Take note that you can't use $letter < 'AA'. It only works with !=. If you wanted to use <= you would need to use the actual ordinal value of the characters 

for ($letter = ord("A"); $letter <= ord("Z"); $letter++)
{
    echo chr($letter) . ", ";
}

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    The next loop we'll look at is a for loop. 0:00
    For loops are the most complex loops in PHP, 0:03
    because they use three expressions instead of just one. 0:06
    [SOUND] The first expression is executed once, 0:08
    unconditionally at the beginning of the loop. 0:12
    At the beginning of each iteration expression two is evaluated. 0:15
    This is the conditional part if it evaluates to true the loop continues and 0:20
    the statement block is executed. 0:25
    If it evaluates to false the execution of the loop ends. 0:27
    At the end of each iteration expression three is executed. 0:32
    We actually used all these pieces in our while loop but 0:37
    now we can put them together in one place. 0:40
    Complex doesn't need to mean difficult. 0:43
    Sometimes having something more complex in one place 0:46
    is actually easier to read than looking for all the simple pieces. 0:50
    Let's take a look at how we can write our list of years using a for loop. 0:55
    Let's go back to the loops.php file, and comment out the while loop for 1:01
    our learn array. 1:05
    Then let's go back up to the top of our file, and add our for loop, 1:11
    right below our variables. 1:15
    We start with the keyword for, and 1:18
    then our parentheses followed by the curly braces. 1:21
    Now we can add the expressions, The first expression gives us our starting value. 1:26
    So we'll move the year into our for loop. 1:32
    The second expression is the conditional, so we copy that from our while loop. 1:39
    The third expression is for telling the loop how to update values after each loop. 1:48
    We already have the iterator as part of the conditional, and a for 1:55
    loop will allow us to leave off any of the three expressions. 1:59
    This means that the for loop is perfectly valid now. 2:02
    So I'm gonna leave it like this just to demonstrate. 2:05
    Now we can copy the statement block from our while loop as well. 2:09
    And we're ready to test this out in the browser. 2:15
    We see the years 1917 through 2016 again, 2:21
    because we're iterating before displaying the year. 2:24
    Let's move the iteration to the third expression of our loop. 2:28
    Instead of the ++ being here, we'll add year++. 2:35
    Now when we refresh our browser, we see 1916 through 2016 because 2:43
    the final expression is not executed until the end of the for loop. 2:51
    Let's clean this up a little more. 2:57
    I could move the current year into the for loop, 2:59
    by separating these first two expressions with a comma. 3:02
    That means both these expressions will run before our first loop. 3:09
    Let's refresh the browser, and our page still works. 3:14
    Now instead of the current year I could just use the date function. 3:20
    And instead of subtracting 100, I'll just subtract 99. 3:36
    Now when we refresh the browser we see 1917 through 2016. 3:43
    You'll often see for loops used when looping through numbers such as these. 3:48
    You can also use a for loop with indexed arrays 3:54
    as long as the keys are sequential and no keys have been skipped. 3:58
    This will work well on my learn array if I use the sort to re-index the keys, 4:02
    let's take a look. 4:07
    We'll comment this out, and then we'll move down here. 4:13
    Instead of asort, I want to just use sort, 4:17
    then we'll set up the structure for our for loop, for () { }. 4:22
    Then we can start adding our expressions. 4:30
    First we start with our initial variable. 4:34
    This is going to be an integer, so we can name int or just i for short. 4:36
    Then we can set i = 0 since our keys are zero indexed. 4:42
    Then we can add our conditional. 4:48
    We want to loop while i 4:50
    is less than the number of items in the learn array. 4:55
    So we'll use another handy little function named count, which does just that. 5:00
    It counts the number of items in a given array. 5:07
    So we'll pass at the learn array. 5:10
    Then we add our final expression to iterate through i. 5:13
    Now we're ready to display each of those items. 5:18
    We start with echo $learn and then within square brackets we use i. 5:22
    This holds the key value that we want to use. 5:32
    Finally let's concatenate a line break. 5:36
    Now let's run the script in the console. 5:41
    We see our list of to do items sorted by value again. 5:46
    Great job. 5:50

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