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PHP Exercises: Print the number of combinations

PHP: Exercise-66 with Solution

Write a PHP program that accept an even number (n should be greater than or equal to 4 and less than or equal to 50000, Goldbach number) from the user and create a combinations that express the given number as a sum of two prime numbers. Print the number of combinations.
Goldbach number: A Goldbach number is a positive even integer that can be expressed as the sum of two odd primes.[4] Since four is the only even number greater than two that requires the even prime 2 in order to be written as the sum of two primes, another form of the statement of Goldbach's conjecture is that all even integers greater than 4 are Goldbach numbers.
The expression of a given even number as a sum of two primes is called a Goldbach partition of that number. The following are examples of Goldbach partitions for some even numbers:
6 = 3 + 3
8 = 3 + 5
10 = 3 + 7 = 5 + 5
12 = 7 + 5
...
100 = 3 + 97 = 11 + 89 = 17 + 83 = 29 + 71 = 41 + 59 = 47 + 53.

Input: n ( n ≤ 10000). Input 0 to exit the program.

Sample Solution: -

PHP Code:

<?php
define('MAX', 50001);
define('COUNT', 5133);
$table = new SplFixedArray(MAX);
$table[0] = $table[1] = false;
$primes = new SplFixedArray(COUNT);
$p_i = 0;
$i = 2;
while ($i < MAX) {
    $primes[$p_i++] = $i;
    $table[$i] = true;
    for ($j = $i * 2; $j < MAX; $j += $i) {
        $table[$j] = false;
    }
    while (++$i < MAX && $table[$i] === false);
} 
while (true) {
    fscanf(STDIN, '%d', $n);
    if ($n === 0) {
        break;
    }
    $c = 0;
    for ($i = 0; $i < COUNT; $i++) {
        if ($primes[$i] > $n / 2) {
            break;
        }
        if ($table[$n - $primes[$i]]) {
            $c++;
        }
    }
    echo "Number of combinations: ";
    echo $c . PHP_EOL;
}

?>

Sample Input:
100
0

Sample Output: 

Number of combinations: 6

Flowchart:

Flowchart: Print the number of combinations.

PHP Code Editor:

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Previous: Write a PHP program to compute the sum of first n given prime numbers.
Next: Write a PHP program to create maximum number of regions obtained by drawing n given straight lines.

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PHP: Tips of the Day

How to Sort Multi-dimensional Array by Value?

Try a usort, If you are still on PHP 5.2 or earlier, you'll have to define a sorting function first:

Example:

function sortByOrder($a, $b) {
    return $a['order'] - $b['order'];
}

usort($myArray, 'sortByOrder');

Starting in PHP 5.3, you can use an anonymous function:

usort($myArray, function($a, $b) {
    return $a['order'] - $b['order'];
});

And finally with PHP 7 you can use the spaceship operator:

usort($myArray, function($a, $b) {
    return $a['order'] <=> $b['order'];
});

To extend this to multi-dimensional sorting, reference the second/third sorting elements if the first is zero - best explained below. You can also use this for sorting on sub-elements.

usort($myArray, function($a, $b) {
    $retval = $a['order'] <=> $b['order'];
    if ($retval == 0) {
        $retval = $a['suborder'] <=> $b['suborder'];
        if ($retval == 0) {
            $retval = $a['details']['subsuborder'] <=> $b['details']['subsuborder'];
        }
    }
    return $retval;
});

If you need to retain key associations, use uasort() - see comparison of array sorting functions in the manual

Ref : https://bit.ly/3i77vCC