Lesson goal: Prime number test

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A prime number is a number that is divisible only by 1 and itself. For example, 13 is prime since only 1 and 13 divide evenly into 13; 14 however is not prime since 1, 2, 7, and 14 divide evenly into it.

In this lesson, we'll write a function called prime to test if a number is prime. The function will return true if the number if prime, and false if the number is not prime. To keep things simple, we'll implement something related to the "naive" test described on Wikipedia (link).

We'll use these conditions to test for primality of a number, held in variable $n$:

  • Know that $2$ is prime (and the only even prime number).
  • If $n$ is even, then two will divide evenly into it, and we'll conclude that the number is not prime.
  • We'll only test if odd numbers divide evenly into $n$, since even numbers will only divide evenly into $n$ is $n$ is also even, and we have the first bullet point already.
  • We'll only test numbers from 3 up to $\sqrt{n}$ since if $n$ has other factors, one must be $< \sqrt{n}$.

Now you try. Fix the if statement and the for statement.

Type your code here:


See your results here:

This code will not run. In the prime function, you need to fix the following:
  • What will you fill into the first if statement to see if $2$ divides evenly into $n$, causing the function to return false?
  • What will you fill into the for statement, to start at $3$ and count up to $\sqrt{n}$, going next to 5, then 7, then 9, etc. (i.e. only odd numbers)? Dismiss.
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