go back to a for loop from an if loop in python

Question

The function is supposed to load a list and print the sum of the first and last primes in the list. I have written it so that a new list is made and the idea i that only primes are appended to it - if its not a prime, it should go back to the beginning of that for loop. I want my function to go back to the beginning of the for loop after

if L[i] % j == 0:

as I only want prime numbers to be appended to y, but I don't think 'break' is the right thing to do here as I keep having my entire list being printed when I call the function. I've tried 'break', 'pass', 'continue' and an infinite loop (although I may not have done that correctly). Please help???

def first_plus_last_prime(L):
    y = []
    for i in range(len(L)):

        if L[i] < 2:
            break
        for j in range(2, L[i]):
            if L[i] % j == 0:
                break
        y.append(L[i])

    print(y)
    print(y[0])
    print(y[-1])
    t = y[0] + y[-1]
    return t

Just a suggestion: There is codereview.stackexchange.com. After fixing your code, consider posting it there. Apart from the problem with the implementation itself, there are a bunch of other things that could be improved. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Mar 9, 2019 at 19:42

OSainz · Accepted Answer · 2019-03-09 19:40:16Z

2

You can use the else statement:

def first_plus_last_prime(L):
    y = []
    for i in range(len(L)):

        if L[i] < 2:
            break

        # Look for prime
        for j in range(2, L[i]):
            if L[i] % j == 0:
                break
        else:
            y.append(L[i])

    print(y)
    print(y[0])
    print(y[-1])
    t = y[0] + y[-1]
    return t

Thanks to @UlrichEckhardt for the info. https://docs.python.org/3/tutorial/controlflow.html#break-and-continue-statements-and-else-clauses-on-loops

edited Mar 9, 2019 at 19:40

answered Mar 9, 2019 at 19:19

OSainz

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3 Comments

OSainz Over a year ago

I just answer your question about how to do in case that the number doesn't prime. But why you break the loop when you find a 0 or 1? If you do that you are not going to look the whole list.

Ulrich Eckhardt Over a year ago

Python allows else after a for-loop. Much more elegant and exactly invented for situations like this. docs.python.org/3/tutorial/…

OSainz Over a year ago

@UlrichEckhardt Thank you for the info, I edit the answer to show that way.

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