Your function assumes that all composite numbers above 10 are divisible by a number smaller than 10; not true.
Here's an article explaining some of the intricacies that go into finding prime numbers
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Keep in mind that the first method mentioned could be optimized more by only checking numbers smaller than sqrt(num) instead of num / 2.
Here's an article explaining some of the intricacies that go into finding prime numbers
. Keep in mind that the first method mentioned could be optimized more by only checking numbers smaller than sqrt(num) instead of num / 2.