prime numbers in python using for loop and break

Question

I have written a python code to find prime numbers between 2 and 30. But my code is not evaluating for 2 and 3. Can anyone tell me what is wrong in this code?

for i in range(2, 30):
    for j in range(2, i-1):
        if ((i % j) == 0):
            print(i, "is not a prime number")
            break
        else:
            print(i, "is a prime number")
        break

if i is 2 or 3, you won't enter the next for loop – Morb Jun 01 '17 at 09:54 — Morb, Jun 01 '17 at 09:54

score 2 · Answer 1 · answered Jun 01 '17 at 10:01

The logic of your code is wrong. The else clause should be attached to the inner for loop, so it's only executed if the loop is exhausted without finding a divisor.

for i in range(2, 30):
    for j in range(2, i-1):
        if ((i % j) == 0):
            print(i, "is not a prime number")
            break
    else:
        print(i, "is a prime number")

Also note that the outer loop only runs up to 29, since the upper boundary is not included in a range. The inner loop does not include i - 1, but that's totatlly fine, since any non-trivial divisor is less than i - 1.

The inner loop won't be entered at all for 2 and 3, since the range will be empty in these cases. This is fine as well, since the else clause will be immediately entered.

Ok. Got it! I thought else can only be attached to if statement. Thanks. — Lax_Sam, Jun 01 '17 at 10:24
@Laxman See https://stackoverflow.com/q/9979970/279627 for further information on `else` clauses on `for` loops. — Sven Marnach, Jun 01 '17 at 10:48

score 1 · Answer 2 · answered Jun 01 '17 at 09:59

for i in range(2, 30):
    prime = True
    for j in range(2, i-1):
        if ((i % j) == 0):
            prime = False
            # print(i, "is not a prime number")
            break
        # else:
        #  print(i, "is a prime number")
        # break
    if prime:
        print(i, "is a prime number")
    else :
        print(i, "is not a prime number")

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score 0 · Answer 3 · answered Jun 01 '17 at 09:58

It isn't working because the nested for declaration:

for i in range(2, 30):
    for j in range(2, i-1):

Uses range(2, i-1) and until i is 4, range returns no value:

i = 2 --> range(2, 1) # No value
i = 3 --> range(2, 2) # No value
i = 4 --> range(2, 3) # 2

This is because the range function returns values from the first parameter (included) to the second parameter (not included).

prime numbers in python using for loop and break

3 Answers3