How to check if number is prime?

Question

How do I check if a given number is prime in twig?

Answer 1

Example using build-in functions (sure, not the best ;-) ).

Supposing that number is our number:

{% set result = 0 %}
{% if number / number|round == 1 %}
    {% set result = 1 %}
    {% if number > 3 %}
        {% for i in 2..number-1 if result == 1 %}
            {% if number is divisibleby(i) %}
                {% set result = 2 %}
            {% endif %}
        {% endfor %}
    {% endif %}
{% endif %}

Now, result equals to:

• 0, if number is not integer

• 1, if number is prime

• 2, if number is composite

Answer 2

I think the question of the use-case is vital here. If you want to check whether a large number is prime or not, you will need a plugin and a lot of math.

If your number is guaranteed not too big, I would use twig's in operator:

{% set primes = [2, 3, 5, 6, 11, 13, 17, 19] %}
{% if n > primes|last %}
  too big to tell
{% elseif n in primes %}
  {{n}} is prime
{% else %}
  {{n}} is composite
{% endif %}

Or check for prime factors in a loop:

{% set primes = [2, 3, 5, 6, 11, 13, 17, 19] %}
{% if n > (primes|last) * (primes|last) %}
  too big to tell
{% else %}
  {% set isComposite = false %}
  {% for p in primes %}
    {% if (not isComposite) and  (n is divisibleby (p)) %}
      {{n}} is composite (divisible by {{p}})
      {% set isComposite = true %}
      {# optional break #}
    {% endif %}
  {% endfor %}
  {% if not isComposite %}
    {{n}} is prime
  {% endif %}
{% endif %}

(Twig doesn't have a native break tag, but you can use a plugin)