How to find prime numbers in an array of arrays [duplicate]

Question

I'm trying to create a function to return the prime numbers on the ages in a multidimensional array. The problem is the function I created is returning only the first occurence and I want it to return all the occurences.

Here it is what I've done:

function findPrimes(){ $people = [ [ 'name' => 'Joseph Richard', 'age' => 23, 'city' => 'Chicago', 'level' => 'Senior', ], [ 'name' => 'John Doe', 'age' => 22, 'city' => 'New York', 'level' => 'Pleno', ], [ 'name' => 'Tess', 'age' => 21, 'city' => 'San Diego', 'level' => 'Junior', ], [ 'name' => 'Steph Morgan', 'age' => 20, 'city' => 'Miami', 'level' => 'Junior', ], [ 'name' => 'Ken Junior Smith', 'age' => 23, 'city' => 'Los Angeles', 'level' => 'Senior', ], [ 'name' => 'Walter Scott', 'age' => 24, 'city' => 'Seattle', 'level' => 'TechLead' ], [ 'name' => 'Diego Maradona', 'age' => 25, 'city' => 'Austin', 'level' => 'TechLead' ], [ 'name' => 'Messi', 'age' => 26, 'city' => 'Portland', 'level' => 'Pleno' ], [ 'name' => 'Hardin Scott', 'age' => 19, 'city' => 'Houston', 'level' => 'Senior' ], ]; $age = array_column($people, 'age'); foreach ($age as $key => $value) { $isPrime = true; for($i = 2; $i <= sqrt($value); $i++) { if($value % $i == 0) { $isPrime = false; } } if ($isPrime == true) { return "$value is prime number"; } else { return "$value is not prime number"; } } } echo findPrimes();

Output: 23

I need it to return all prime numbers in this array. Can anyone help me please?

Humans can only reasonably live so long. Make it easier on yourself and just filter the age column by an array containing all prime numbers under, say, 200. 3v4l.org/4pGTM — mickmackusa
– mickmackusa ♦, Commented Dec 11, 2022 at 21:39
Nice @mickmackusa thanks!! Really simple and effective solution. — Shirley Oliveira
– Shirley Oliveira, Commented Dec 13, 2022 at 18:27

trincot · Accepted Answer · 2022-12-09 20:12:45Z

Your function is executing a return in the first iteration of the foreach loop. And returning a string like "x is prime number" seems not to be what you want as result (as you say you want a list of primes).

So define an array $primes and populate it with those values that are primes, and return that array.

 // ... after the initialisation ... $age = array_column($people, 'age'); $primes = []; foreach ($age as $key => $value) { $primes[] = $value; // first assume it is prime for($i = 2; $i <= sqrt($value); $i++) { if($value % $i == 0) { array_pop($primes); // ...then remove it if it's not break; } } } return $primes; } print_r(findPrimes());

Note that since ages are expected to be in the range of 0 to 140, you can predefine a list of all primes less than 140 (with a sieve). Then all that remains is to check if an age is in that list...

You can also avoid recalculating the square root on every loop iteration by rearranging it like: for($i = 2, $l=sqrt($value); $i <= $l; $i++) { ... }

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