For loop to get prime numbers

Question

I'm at the very beginning with Python so my knowledge is quite noob level.

Anyway, I have found this example online:

for num in range(10,20):     #to iterate between 10 to 20
   for i in range(2,num):    #to iterate on the factors of the number
      if num%i == 0:         #to determine the first factor
         j=num/i             #to calculate the second factor
         print(f"{num} equals {i} * {j}")
         break #to move to the next number, the #first FOR
   else:                  # else part of the loop
       print(f"{num} is a prime number")
       break

Here the result from the example:

10 equals 2 * 5
11 is a prime number
12 equals 2 * 6
13 is a prime number
14 equals 2 * 7
15 equals 3 * 5
16 equals 2 * 8
17 is a prime number
18 equals 2 * 9
19 is a prime number

Unfortunately by running that code on Jupyter it returns:

10 equals 2 * 5
11 is a prime number
12 equals 2 * 6
13 is a prime number
14 equals 2 * 7
15 is a prime number  ### THIS IS THE ERROR
16 equals 2 * 8
17 is a prime number
18 equals 2 * 9
19 is a prime number

I can't really find the error here, any advice?

That last `break` is misaligned, which `for` did you intend to indent that under? — Cory Kramer, Jun 01 '21 at 19:48
Now's a good time to learn [how to debug small programs](//ericlippert.com/2014/03/05/how-to-debug-small-programs/) and [to use a debugger](//stackoverflow.com/q/25385173/843953) Step through your code and observe what each line of code does. Identify where your program differs from your expectations by comparing these intermediate results with expected results. Work backwards from there to narrow down the cause of the problem. — Pranav Hosangadi, Jun 01 '21 at 19:49
[Whitespace matters](https://stackoverflow.com/questions/13884499/what-is-python-whitespace-and-how-does-it-work) — Peter Wood, Jun 01 '21 at 19:50

j1-lee · Accepted Answer · 2021-06-01T20:20:42.143

I guess your attempt was

for num in range(10, 20):
    for i in range(2, num):
        if num % i == 0:
            j = num / i
            print(f"{num} equals {i} * {j}")
            break
        else:
            print(f"{num} is a prime number")
            break

whereas the correct code must be like

for num in range(10, 20):
    for i in range(2, num):
        if num % i == 0:
            j = num / i
            print(f"{num} equals {i} * {j}")
            break
    else:
        print(f"{num} is a prime number")

Note (i) different indentation level for else clause, and (ii) no second break. You might want to be more careful about indentation in python.

Here, else belongs to for, not if (and this distinction is realized by the indentation). For this, this documentation might be helpful. In summary, for ... else behaves like this: once the for loop ends, else part is executed, unless the loop was terminated by a break.

I can't believe! I did try so many times and what I had to do was just removing break and move the else: to the same level of the second for. thanks a lot! — m-faccin, Jun 01 '21 at 20:06

For loop to get prime numbers

1 Answers1