PHP Control Flow Statement

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PHP Control Flow Statement

這篇文章將介紹以下四種 Control Flow:

  • If…Else…Elseif
  • Switch
  • While Loops
  • For Loops

If…Else…Elseif

有下列這幾種 if statement:

  • if statement - executes some code if one condition is true
  • if…else statement - executes some code if a condition is true and another code if that condition is false
  • if…elseif….else statement - executes different codes for more than two conditions

Syntax - 1

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if (condition) {
    code to be executed if condition is true;
}
// Example
<?php
$t = 10;
if ($t < 20) {
    echo "Have a good day!";
}
// 輸出結果為： Have a good day!
?>

Syntax - 2

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if (condition) {
    code to be executed if condition is true;
} else {
    code to be executed if condition is false;
}
// Example
<?php
$t = 22;
if ($t < 20) {
    echo "Have a good day!";
} else {
    echo "Have a good night!";
}
// 輸出結果為：Have a good night!
?>

Syntax - 3

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if (condition) {
    code to be executed if this condition is true;
} elseif (condition) {
    code to be executed if this condition is true;
} else {
    code to be executed if all conditions are false;
}
// Example
<?php
$t = 5;
if ($t < 10) {
    echo "Have a good morning!";
} elseif ($t < "20") {
    echo "Have a good day!";
} else {
    echo "Have a good night!";
}
// 輸出結果為：Have a good morning!
?>

Switch

個人認為是用於，已知有哪些條件下時的control flow。
break:是避免往下繼續跑另一區塊的code。
default:是所有case都不符合時會跑的區塊。

Syntax

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switch (n) {
    case label1:
        code to be executed if n=label1;
        break;
    case label2:
        code to be executed if n=label2;
        break;
    case label3:
        code to be executed if n=label3;
        break;
    ...
    default:
        code to be executed if n is different from all labels;
}
// Example
<?php
$favcolor = "red";
switch ($favcolor) {
    case "red":
        echo "Your favorite color is red!";
        break;
    case "blue":
        echo "Your favorite color is blue!";
        break;
    case "green":
        echo "Your favorite color is green!";
        break;
    default:
        echo "Your favorite color is neither red, blue, nor green!";
}
// 輸出結果：Your favorite color is red!
?>

While Loops

當你需要一直重複執行一個code block 且 終止條件不確定在何時會達到下，此時就可以用 While Loops 來達到你的需求。
有以下兩種可以使用:

  • while - 一直重複執行直到條件不成立。
  • do…while - 至少執行一次再繼續執行到條件不成立為止。

Syntax - 1

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while (condition is true) {
    code to be executed;
}
// Example
<?php 
$x = 1; 
while($x <= 5) {
    echo "The number is: $x <br>";
    $x++;
} 
?>
// 輸出結果：
//The number is: 1 
//The number is: 2 
//The number is: 3 
//The number is: 4 
//The number is: 5

Syntax - 2

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do {
    code to be executed;
} while (condition is true);
// Example
<?php 
$x = 6;
do {
    echo "The number is: $x <br>";
    $x++;
} while ($x <= 5);
// 輸出結果：The number is: 6 
?>

For Loops

for loop適用於已知特定次數的情況下，來執行相同的code block。
有以下兩種類型：

  • for loop - 可設置 初始條件、結束條件跟遞增條件。
  • foreach - 專門給 array 使用。

Syntax - 1

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for (init counter; test counter; increment counter) {
    code to be executed;
}
// Example
<?php 
for ($x = 0; $x <= 10; $x++) {
    echo "The number is: $x <br>";
} 
/*
輸出結果：
The number is: 0 
The number is: 1 
The number is: 2 
The number is: 3 
The number is: 4 
The number is: 5 
The number is: 6 
The number is: 7 
The number is: 8 
The number is: 9 
The number is: 10 
*/
?>

Syntax - 2

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foreach ($array as $value) {
    code to be executed;
}
// Example
<?php 
$colors = array("red", "green", "blue", "yellow"); 
foreach ($colors as $value) {
    echo "$value <br>";
}
/*
輸出結果：
red 
green 
blue 
yellow 
*/
?>