How does php determine whether it is a prime number? Three ways to determine prime numbers (code examples)

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What are prime numbers?

Prime numbers are also called prime numbers. A natural number greater than 1 that cannot be divided by other natural numbers except 1 and itself is called a prime number; otherwise it is called a composite number. (Note: 1 is not a prime number.)

Now we will introduceWhat are the three methods of judging prime numbers in php?

Method 1:

Basic method, counting method.

$num = 7; $n = 0; //用于记录能被整除的个数 -- 计数 for($i = 1;$i <= $num; ++$i){ if($num % $i == 0){ $n++; } } if($n == 2){ echo "$num 是素数"; }else{ echo "$num 不是素数"; }

Method 2:

When a number is equal to the product of two numbers, one of the numbers must be less than half of the number. Use break; as long as one of the numbers can be divided, the loop will end immediately. This reduces the number of loops and speeds up the process.

$num = 5; $flag = true; for($i = 2;$i <= $num/2;++$i){ if($num % $i == 0){ $flag = false; break; } }if($flag){ echo "$num 是素数"; }else{ echo "$num 不是素数"; }

Method 3:

Same as above, when the product of two numbers is equal to a number, then one of the numbers must be less than the square root of the number.

$num = 4; for($i = 2;$i<$num;++$i){ if($num % $i == 0){ echo "$num 不是素数"; break; } if($i >= sqrt($num)){ echo "$num 是素数"; break; } }

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