PHP Exercises: Compute sum of the prime numbers less than 100

Last update on April 19 2025 11:50:41 (UTC/GMT +8 hours)

37. Sum of Primes Below 100

Write a PHP program to compute the sum of the prime numbers less than 100.

Note: There are 25 prime numbers are there in less than 100.
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 and sum of all these numbers is 1060.

Sample Solution:

PHP Code:

<?php
// Array to store prime numbers
$primes = array();
// Flag to check if a number is prime
$is_prime_no = false;

// Loop to find the first 100 prime numbers
for ($i = 2; $i < 100; $i++) {
    $is_prime_no = true; 

    // Nested loop to check if $i is divisible by any number from 2 to $i/2
    for ($j = 2; $j <= ($i/2); $j++) {
        if ($i % $j == 0) {
            // If $i is divisible, it's not a prime number
            $is_prime_no = false;
            break;
        }
    }

    // If $i is a prime number, add it to the $primes array
    if ($is_prime_no) {
        array_push($primes, $i);
    }

    // If 100 prime numbers are found, exit the loop
    if (count($primes) == 100) {
        break;
    }
}

// Output the sum of the first 100 prime numbers
echo array_sum($primes) . "\n";

?>

Explanation:

Initialize Variables:

$primes is an array to store prime numbers.
$is_prime_no is a flag to indicate if a number is prime.

Loop to Find Prime Numbers:

A for loop iterates over numbers starting from 2, aiming to find the first 100 prime numbers.

Check If a Number Is Prime:

For each number $i, $is_prime_no is initially set to true.
An inner loop checks if $i is divisible by any number from 2 to $i/2.
If $i is divisible, $is_prime_no is set to false, indicating $i is not prime, and the inner loop breaks.

Store Prime Numbers:

If $is_prime_no is still true after the inner loop, $i is a prime number and is added to the $primes array.
If the $primes array contains 100 prime numbers, the loop stops.

Calculate and Output the Sum:

array_sum($primes) calculates the sum of the first 100 prime numbers, which is then printed.

Output:

Flowchart:

For more Practice: Solve these Related Problems:

Write a PHP function to generate all prime numbers below 100 using the Sieve of Eratosthenes.
Write a PHP script to calculate the sum of primes under 100 without using iterative loops.
Write a PHP script to recursively compute prime numbers below 100 and sum them.
Write a PHP script to verify the cumulative sum of primes below 100 using array_filter and array_reduce.

PHP Code Editor:

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