Control structure (I)

if sentence
while sentence
for sentence
foreach sentence
break sentence

if sentence

if Structure is a lot of languages, including PHP It's one of the most important features , It allows code snippets to execute conditionally .PHP Of if The structure and C Language is similar ：

<?php
if ($a > $b)
  echo "a is bigger than b";

else sentence
It is often necessary to execute a statement when a condition is met , When this condition is not met, other statements are executed , That's exactly what it is. else The function of .else Extended if sentence , Can be in if The value of the expression in the statement is FALSE Execute statement . For example, the following code is in a Greater than a Greater than b Time display a is bigger than b, On the contrary, it shows a is NOT bigger than b：

<?php
if ($a > $b) {
    
  echo "a is greater than b";
} else {
    
  echo "a is NOT greater than b";
}

else if sentence
You can also write elseif, As the name implies , yes if and else The combination of . and else equally , It extends if sentence , Can be in the original if Expression value is FALSE Execute different statements when . But and else The difference is , It's only in elseif The conditional expression value of is TRUE Execute statement . For example, the following codes will be displayed separately according to the conditions a is bigger than b,a equal to b perhaps a is smaller than b：

<?php
if ($a > $b) {
    
    echo "a is bigger than b";
} elseif ($a == $b) {
    
    echo "a is equal to b";
} else {
    
    echo "a is smaller than b";
}

In the same if Statement can have more than one elseif part , The value of the first expression is TRUE（ If any ） Of elseif Part will execute .

elseif The statement is only in the previous if And all the previous elseif The expression value of is FALSE, And the current elseif Expression value is TRUE When the .

Be careful : It must be noted that elseif And else if It is considered identical only if curly braces are used similar to the above example . If you use a colon to define if/elseif Conditions , Then you can't use two words else if, otherwise PHP There will be parsing errors .

<?php

/*  Incorrect use ： */
if($a > $b):
    echo $a." is greater than ".$b;
else if($a == $b): //  Will not compile 
    echo "The above line causes a parse error.";
endif;


/*  Correct usage ： */
if($a > $b):
    echo $a." is greater than ".$b;
elseif($a == $b): //  Pay attention to the use of a word  elseif
    echo $a." equals ".$b;
else:
    echo $a." is neither greater than or equal to ".$b;
endif;

Alternative syntax for process control
PHP Provides some alternative syntax for process control , Include if,while,for,foreach and switch. The basic form of substitution grammar is to put the left curly bracket （{） Change it to a colon （:）, Put the right curly bracket （}） Change to endif;,endwhile;,endfor;,endforeach; as well as endswitch;

<?php if ($a == 5): ?>
A is equal to 5
<?php endif; ?>

Alternative grammar can also be used in else and elseif in . Here's an example that includes elseif and else Of if Examples of structures written in alternative grammatical formats ：

<?php
if ($a == 5):
    echo "a equals 5";
    echo "...";
elseif ($a == 6):
    echo "a equals 6";
    echo "!!!";
else:
    echo "a is neither 5 nor 6";
endif;
?>

Don't mix two grammars in the same control block .

while sentence

while loop yes PHP The simplest type of loop in . It and C In language while Behave the same .while The basic format of a statement is ( The code is in the syntax format , Not a code case , There is no need to hit the code )：

while (expr)
    {
    statement}

while The meaning of the sentence is very simple , It tells PHP as long as while The value of the expression is TRUE Repeat the nested loop statement . The value of the expression is checked every time the loop starts , So even if this value changes in the loop statement , The statement will not stop executing , Until the end of this cycle . Sometimes if while The value of the expression starts with FALSE, Then the loop statement will not be executed once .

and if The sentence is the same , Can be in while Enclose a statement group with curly braces in the loop , Or use alternative grammar ：

while (expr):
    statement
    ...
endwhile;

The following two examples are exactly the same , All display numbers 1 To 10：

<?php
/* example 1 */

$i = 1;
while ($i <= 10) {
    
    echo $i++;  /* the printed value would be $i before the increment (post-increment) */
}

/* example 2 */

$i = 1;
while ($i <= 10):
    print $i;
    $i++;
endwhile;
?>

do-while sentence
do-while Circulation and while The cycle is very similar , The difference is that the value of the expression is checked at the end of each loop, not at the beginning . And general while The main difference between loops is do-while The loop statement of is guaranteed to execute once （ The true value of the expression is checked at the end of each loop ）, However, in general while Not necessarily in the cycle （ The expression truth value is checked at the beginning of the loop , If you start with FALSE Then the whole cycle is terminated immediately ）.

do-while There is only one syntax for loops ：

<?php
$i = 0;
do {
    
   echo $i;
} while ($i > 0);
?>

The above cycle will run exactly once , Because after the first cycle , When checking the true value of an expression , Its value is FALSE（$i No more than 0） And cause the loop to terminate . Senior C Language users may be familiar with a different language do-while Use circularly , Put the sentence in do-while(0) In , Use... Inside the loop break Statement to end the execution loop . The following code snippet demonstrates this approach ：

<?php
do {
    
    if ($i < 5) {
    
        echo "i is not big enough";
        break;
    }
    $i *= $factor;
    if ($i < $minimum_limit) {
    
        break;
    }
    echo "i is ok";

    /* process i */

} while(0);
?>

If you can't understand it immediately, don't worry . Even without this “ characteristic ” You can also write powerful code to . since PHP 5.3.0 rise , You can also use goto To jump out of the loop .

for sentence

for loop yes PHP The most complex loop structure in the world . Its behavior and C Language similarity . for The syntax of circulation is ( The code is in the syntax format , Not a code case , There is no need to hit the code )：

for (expr1; expr2; expr3)
    {
    statement}

The first expression （expr1） Evaluate unconditionally before the loop begins （ And implement ） once .

expr2 Evaluate before each cycle begins . If the value is TRUE, Then continue the cycle , Execute nested loop statements . If the value is FALSE, Then stop the cycle .

expr3 Evaluated after each loop （ And implement ）.

Each expression can be empty or include multiple expressions separated by commas . expression expr2 in , All expressions separated by commas are evaluated , But only take the last result .expr2 Being empty means that it will loop indefinitely （ and C equally ,PHP Secretly think its value is TRUE）. This may not be as useless as expected , Because I often want to use conditional break Statement to end the loop instead of for True value judgment of expression .

Consider the following example , They all display numbers 1 To 10：

<?php
/* example 1 */

for ($i = 1; $i <= 10; $i++) {
    
    echo $i;
}

/* example 2 */

for ($i = 1; ; $i++) {
    
    if ($i > 10) {
    
        break;
    }
    echo $i;
}

/* example 3 */

$i = 1;
for (;;) {
    
    if ($i > 10) {
    
        break;
    }
    echo $i;
    $i++;
}

/* example 4 */

for ($i = 1, $j = 0; $i <= 10; $j += $i, print $i, $i++);
?>

Of course , The first example looks the most concise （ Or someone thinks it's the fourth ）, But users may find that in for Using empty expressions in a loop is convenient in many cases . PHP Colons are also supported for Alternative syntax for loops ( The code is in the syntax format , Not a code case , There is no need to hit the code ).

for (expr1; expr2; expr3):
    statement;
    ...
endfor;

Sometimes it is often necessary to traverse the array like the following example ：

<?php
/* *  This array will change the values of some of its cells during traversal  */
$people = Array(
        Array('name' => 'Kalle', 'salt' => 856412),
        Array('name' => 'Pierre', 'salt' => 215863)
        );

for($i = 0; $i < sizeof($people); ++$i)
{
    
    $people[$i]['salt'] = rand(000000, 999999);
}
?>

The above code may be slow to execute , Because we have to calculate the length of the array every time we loop . Because the length of the array is always the same , You can use an intermediate variable to store the array length to optimize instead of calling count()：

<?php
$people = Array(
        Array('name' => 'Kalle', 'salt' => 856412),
        Array('name' => 'Pierre', 'salt' => 215863)
        );

for($i = 0, $size = sizeof($people); $i < $size; ++$i)
{
    
    $people[$i]['salt'] = rand(000000, 999999);
}
?>

foreach sentence

foreach Syntax structure provides a simple way to traverse an array .foreach Can only be applied to arrays and objects , If you try to apply variables of other data types , Or uninitialized variables will send an error message . There are two kinds of grammar

foreach (array as $value)
    statement
foreach (array as $key => $value)
    statement

The first format traverses a given array Array . In each cycle , The value of the current cell is assigned to $value And the pointer inside the array moves one step forward .

The second format does the same thing , Only the key name of the current cell is assigned to the variable in each loop $key.

Change the value of an element
stay foreach There are two ways to change the value of an element

stay $value And before & To modify the elements of an array .
adopt $key Reassign .
edit /home/project/foreach.php

<?php
$arr1 = $arr2 = [1, 2, 3, 4];

foreach ($arr1 as &$value) {
    
    $value = $value * 2;
}

var_dump($arr1, $value);

foreach ($arr2 as $key => $value) {
    
    $arr2[$key] = $value * 2;
}
var_dump($arr2, $value);

perform

php foreach.php

It can be seen from the results , Of the last element of the array $value Reference in foreach It will remain after the cycle . It is recommended to use unset() To destroy it .

break sentence

break End the present for,foreach,while,do-while perhaps switch The execution of the structure . break You can accept an optional numeric parameter to decide how many cycles to jump out of .

edit /home/project/break.php

<?php
$i = 0;
while (++$i) {
    
    for ($j = 0;$j < 10;$j++) {
    
        switch ($i) {
    
        case 1:
            echo "At 1".PHP_EOL;
            break 2;
        case 5:
            echo "At 5".PHP_EOL;
            break 2;
        case 10:
            echo "At 10; quitting".PHP_EOL;
            break 3;
        default:
            break;
        }
    }
}

perform

php break.php

As you can see from the results ,break With parameters, you can specify how many cycles to jump out .