Why do they tell us to make range x-1 in is_prime


For step 2 it says - "For each number n from 2 to x - 1, test if x is evenly divisible by n."
I presume this is intended for find the prime numbers dived it by every number except itself and by 1 but range(2,x) is already one below x. I am having trouble explaining this but what I am trying to show is that say x is 20 - range (2, 20) is already 2 - 19. Why subtract 1 from x?


you don't. range is already doing this substraction for you. You need the values 2 till x - 1, but range already doesn't include x, so the x-1 step is done for you by range


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