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77 lines
2.2 KiB
Markdown
77 lines
2.2 KiB
Markdown
---
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title: For Loop
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---
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## For Loop
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The PHP `for` statement consists of three expressions and a statement:
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`for ((initialization); (condition); (final-expression)) statement`
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### Description
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- initialization
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- Run before the first execution on the loop.
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- This expression is commonly used to create counters.
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- Variables created here are scoped to the loop. Once the loop has finished it is execution they are destroyed.
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- condition
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- Expression that is checked prior to the execution of every iteration.
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- If omitted this expression evaluates to `true`.
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- final-expression
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- Expression that is run after every iteration.
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- Usually used to increment a counter.
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- But it can be used to run any expression.
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- statement
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- Code to be repeated in every loop iteration.
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Any of these three expressions or the statement can be ommited.
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The expressions can contain multiple expressions separated by comma.
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In the (condition) expression, all the comma separated expressions will be evaluated.
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The result is obtained from the last one.
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For loops are commonly used to count a certain number of iterations to repeat a statement.
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### Common Pitfalls
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#### Exceeding the bounds of an array
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When indexing over an array many times it is easy to exceed the bounds of the array (ex. try to reference the 4th element of a 3 element array).
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```php
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// This will cause an error.
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// The bounds of the array will be exceeded.
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$arr = array(1,2,3);
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for ($i = 0; $i <= count($arr); $i++) {
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var_dump($arr[$i]);
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}
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```
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This will output:
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```shell
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int(1) int(2) int(3) NULL
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```
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There are to ways to fix this code.
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Set the condition to either `$i < count($arr)` or `$i <= count($arr) - 1`.
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#### Performance Issues
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The above code can became slow, because the array size is fetched in every iteration.
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In order to fix this problem it is possible to put the array size into a variable.
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```php
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//create the $size variable with a second expression comma separated
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for ($i = 0, $size = count($arr); $i < $size; ++$i) {
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```
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### More Information
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- <a href='https://secure.php.net/manual/en/control-structures.for.php' target='_blank' rel='nofollow'>PHP.net - Control Structures</a>
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