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PHP Exercises: Print the number of prime numbers which are less than or equal to a given integer

PHP: Exercise-52 with Solution

Write a PHP program to print the number of prime numbers which are less than or equal to a given integer.

Input:
n (1 ≤ n ≤ 999,999) .

Pictorial Presentation:

PHP: Print the number of prime numbers which are less than or equal to a given integer.

Sample Solution: -

PHP Code:

<?php

$a = array_fill(0, 1000000, true);
$a[0] = $a[1] = false;
 
for($i = 2; $i * $i < 1000000; $i++){
    if(!$a[$i]) continue;
    for($j = $i * $i; $j < 1000000; $j += $i){
        $a[$j] = false;
    }
}
 
$sum = array_fill(0, 1000000, 0);
 
for($i = 1; $i < 1000000; $i++){
    $sum[$i] += $sum[$i - 1];
    if($a[$i]) $sum[$i]++;
}
 
while(fscanf(STDIN, "%d", $n)){
    echo "Number of prime numbers which are less than or equal to n: ";
    echo $sum[$n] . "\n";
}
?>

Sample Output: 

Sample Input: 50
Number of prime numbers which are less than or equal to n: 15

Flowchart:

Flowchart: Print the number of prime numbers which are less than or equal to a given integer.

PHP Code Editor:

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Previous: Write a PHP program to compute the amount of the debt in n months.
Next: Write a PHP program to compute the radius and the central coordinate (x, y) of a circle which is constructed by three given points on the plane surface.

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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