Error: list indices must be integer, not Expr

Question

I am trying to get the first N prime numbers in the following way:

@sp.entry_point
    def n_primes(self, num):
        primes = [2]
        p = sp.local("p", 1)
        i = sp.local("i", 3)
        n = sp.local("n", num)
        c = sp.local("c", 0)

        sp.while p.value < n.value:
            sp.while (i.value % primes[c.value] != 0) & (primes[c.value] * primes[c.value] <= i.value):
                c.value += 1

            sp.if (i.value % primes[c.value] != 0):
                primes.append(i.value)
                p.value += 1

            c.value = 0

            i.value += 2

        self.data.last_prime = primes[n.value - 1]

but I get the error: "Error: list indices must be integer, not Expr" on the second while loop (by testing this on smartpy.io/dev). I've tried not using smartpy local variables, but then it doesn't work properly and it gets stuck in an infinite loop for some reason. I have also tried using a for loop instead of a while ("for x in primes") but then I get a "declaration error, x variable escapes its scope", and also couldn't make it work with the conditions that I wanted.

Is there any way of telling the program that the expression used as the list index is an integer?

Note: The equivalent of this program works fine in normal python.

Answer 1

In Michelson you cannot access list items by index, you need to iterate over it.

You should do something like:

sp.for prim in primes:
  sp.if p.value < n.value:
    # stuff...

https://smartpy.io/reference.html#_control_and_syntactic_sugar