PHP Loop Learning 11: How to determine prime numbers, print prime numbers from 1 to 100, and count their numbers

In the previous article "PHP Loop Learning Ten: Determine the Number of Narcissus and Print the Number of All Narcissus", we will introduce to you how to determine the number of narcissus and use a loop to output the number of all narcissus. Method, let’s continue learning about PHP loops and introduce how to determine whether a number is a prime number and output a prime number from 1 to 100. Interested friends can learn about it~

First of all, let’s learn about it What are prime numbers?

Prime number (prime number) refers to a natural number that cannot be divided by other numbers except 1 and itself; if it can be divided by other numbers, it is called a composite number (stipulation) 1 is neither prime nor composite).

For example: the numbers 2, 3, 17, and 19 are all prime numbers. They can only be divided by 1 and themselves.

So if a number num (for example, 17) is given, how do we determine whether this number num is a prime number? Here are several methods to introduce to you:

Method 1: The simplest method is to find based on the definition of prime numbers

Thinking:

• Prime numbers can only be divisible by 1 and itself, that is, they can only be divided by 2 times.

• So we use loops to divide N by using every natural number greater than 1 and less than or equal to N, and count the number of times it is divided. If the number is greater than 2, it is not a prime number; if it is equal to 2, then is a prime number.

Let’s take a look at the implementation code:

<?php
header("content-type:text/html;charset=utf-8");
$num=17;
$count = 1;
for($i = 2; $i <= $num; $i++) {
    if(($num % $i)== 0)
        $count ++;
}
if($count == 2) {
    echo $num . &#39;就是素数<br/>&#39;;
}else{
	echo $num . &#39;不是素数<br/>&#39;;
}
?>

Output result:

## Method 2:

sqrt (this number) respectively. If it can be divided evenly, it means that the number is not a prime number, otherwise it is a prime number.

<?php
header("content-type:text/html;charset=utf-8");
$num=19;
$flag = 1;
for($i = 2; $i*$i <= $num; $i++) //能被2 - sqrt(i)整除的数
{
	if($num % $i == 0)
	{
		$flag = 0;
		break;
	}
}
if($flag == 1) {
    echo $num . &#39;就是素数<br/>&#39;;
}else{
	echo $num . &#39;不是素数<br/>&#39;;
}
?>

How to give a range (such as 1~100) and output all prime numbers?

<?php
header("content-type:text/html;charset=utf-8");
$count = 0;
for($k = 2; $k <= 100; $k++){ //从2开始，遍历到100
	$flag = 1;
	for($i = 2; $i*$i <= $k; $i++) {//能被2 - sqrt(i)整除的数
	
		if($k % $i == 0)
		{
			$flag = 0;
			break;
		}
	}
	if($flag == 1) {
	    echo $k . &#39;是素数<br/>&#39;;
		$count ++;
	}
}
echo &#39;<br/>1~100间共有 &#39;.$count." 个素数";
?>

$count is used. After each prime number is output, it increases by 1, so that we can count how many prime numbers there are between 1 and 100.

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