PHP Loop Learning 3: How to use a for loop statement to traverse an array

Array is a very basic and important data structure. After we create an array, we need to operate it; and one of the most common operations on arrays is loop traversal. PHP supports multiple methods of traversing arrays. Today we will learn how to use for loop statements to traverse arrays. Let’s learn together! !

In the previous article "PHP Loop Learning 2: How to use do-while loop statements", we learned the use of do-while loop statements. The following article will introduce Let's take a look at the for loop statement. Through code examples, we will introduce how to use the for loop statement to traverse an array.

1. Structural analysis of for loop statement

The for loop will pre-define the variable that controls the number of loops in the for statement, so for Loop statements can perform loop operations based on a known number of loops, and are suitable for situations where the number of times the script needs to be run is clearly known.

The syntax format of the for loop is as follows:

for (初始化语句; 循环条件; 变量更新--自增或自减) {
    语句块;   
}

The for loop statement can be disassembled into 4 parts: three of the () Expressions and "statement blocks" in {}, let's analyze them below.

Statement analysis:

• #Initialization statement (expression 1): mainly initializes a variable value, used to set a counter , which is the value at the beginning of the loop; this statement is only executed during the first loop and will not be executed again in the future.

• Loop condition (expression 2): Restriction condition for loop execution, used to control whether to execute the code in the loop body; if the condition is TRUE, the loop continues, if the condition is FALSE , the loop ends and exits the loop immediately.
  

• Variable update (expression 3): an expression with an increment or decrement operation. Every time the loop is executed, the value of the counter is immediately modified so that the loop The conditions gradually become "untenable".
  

• Statement block: Several codes that need to be executed when the condition is judged to be true.
  

Is the above description a bit convoluted? Let’s take a look at the execution flow chart of the for loop statement to understand the execution of the for loop more intuitively. Process:

# Now that we understand the execution process of the for loop, let’s do the actual operation and do a small question to see if we have mastered it!

Training question:

Calculate the sum of adding from 1 to 100

<?php
$sum=0;
for($i=1; $i<=100; $i++){
    $sum+=$i;
}
echo &#39;1 + 2 + 3 +...+ 99 + 100 = &#39;. $sum;
?>

Output:

1 + 2 + 3 +...+ 99 + 100 = 5050

Let’s change it, since You can find the sum of 1 added to 100, or you can find the product of 1 multiplied by 100 (the factorial of 100)

<?php
$sum=1;
for($i=1; $i<=100; $i++){
    $sum*=$i;
}
echo &#39;1 * 2 * 3 *...* 99 * 100 = &#39;. $sum;
?>

Output:

1 * 2 * 3 *...* 99 * 100 = 9.3326215443944E+157

Note: Because nothing is multiplied by 0 It is 0, so the initial value of the $sum variable here cannot be 0, but 1.

2. The for loop statement traverses the array

Let’s first look at how to traverse through code examples, and then analyze code to learn more.

<?php
header("Content-type:text/html;charset=utf-8");
$array= array("香蕉","苹果","梨子","橙子","橘子","榴莲");
for ($i=0; $i < count($array); $i++) { 
    echo $array[$i] . "<br/>";
} 
?>

Output:

香蕉
苹果
梨子
橙子
橘子
榴莲

Code analysis:

• The initialization statement is $ i=0: The array in the example is an index array. The subscript of the array starts from 0 by default, so the value at the beginning of the loop must be 0.

• Loop condition$i <count>: The count() function is used to calculate the length of the array, then the loop condition is<code>$i &lt ; Array length , because when $i = array length , $i has exceeded the index range (the index range is 0 ~ Array length-1).

• Variable update$i : After each loop, the variable $i automatically increases by 1 until $i = array length , because $i exceeds the index range, the loop condition is not established, and then the loop exits.

• ##echo $array[$i] . "
  ": Each time it loops, the array is based on the $i value Subscript to output array elements.

Note: Using a for loop to traverse an array will not change the internal pointer of the array.

Recommended: 《PHP interview questions summary (collection)》《php video tutorial》

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