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Python

Alex Zhou
Alex Zhou
11,943 Points
Posted by Alex Zhou
Alex Zhou
11,943 Points

How do I make "for i in range(2, a)" iterate through everything before the else statement?

I'm trying to find the sum of all primes below a given number (Ex: Sum of all primes if N = 10 would be 17 because 2+3+5+7=17). However, I messed up somewhere and I don't know where. I did some trial and error and when i changed N from 15 to 16 counter went up by 15 even though it should've stayed the same. I think it's because the else statement triggered first before "for i in range(2, a):" went through everything, but I'm not sure. Can someone help me fix this please? Thanks!

N=16
counter=0
for a in range(2, N):
  for i in range(2, a):
    if a % i == 0:
      break
    else:
      counter+=a
      break
print(counter)

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

Hey Alex Zhou, your code is not behaving as you might think.

I added print statements for N=10 and got the following output:

a= 3 ; i= 2
adding a 3 break
a= 4 ; i= 2
2 divides into 4 break
a= 5 ; i= 2
adding a 5 break
a= 6 ; i= 2
2 divides into 6 break
a= 7 ; i= 2
adding a 7 break
a= 8 ; i= 2
2 divides into 8 break
a= 9 ; i= 2
adding a 9 break
24

Seems i never progresses beyond 2.

If the else is move out such that it aligns with the for loop instead of the if statement, and remove the break in the else block, the results seem correct:

adding a 2 break
a= 3 ; i= 2
adding a 3 break
a= 4 ; i= 2
2 divides into 4 break
a= 5 ; i= 2
a= 5 ; i= 3
a= 5 ; i= 4
adding a 5 break
a= 6 ; i= 2
2 divides into 6 break
a= 7 ; i= 2
a= 7 ; i= 3
a= 7 ; i= 4
a= 7 ; i= 5
a= 7 ; i= 6
adding a 7 break
a= 8 ; i= 2
2 divides into 8 break
a= 9 ; i= 2
a= 9 ; i= 3
3 divides into 9 break
17

with this change, N=15 and N=16 yield the same result: 41

adding a 2 break
a= 3 ; i= 2
adding a 3 break
a= 4 ; i= 2
2 divides into 4 break
a= 5 ; i= 2
a= 5 ; i= 3
a= 5 ; i= 4
adding a 5 break
a= 6 ; i= 2
2 divides into 6 break
a= 7 ; i= 2
a= 7 ; i= 3
a= 7 ; i= 4
a= 7 ; i= 5
a= 7 ; i= 6
adding a 7 break
a= 8 ; i= 2
2 divides into 8 break
a= 9 ; i= 2
a= 9 ; i= 3
3 divides into 9 break
a= 10 ; i= 2
2 divides into 10 break
a= 11 ; i= 2
a= 11 ; i= 3
a= 11 ; i= 4
a= 11 ; i= 5
a= 11 ; i= 6
a= 11 ; i= 7
a= 11 ; i= 8
a= 11 ; i= 9
a= 11 ; i= 10
adding a 11 break
a= 12 ; i= 2
2 divides into 12 break
a= 13 ; i= 2
a= 13 ; i= 3
a= 13 ; i= 4
a= 13 ; i= 5
a= 13 ; i= 6
a= 13 ; i= 7
a= 13 ; i= 8
a= 13 ; i= 9
a= 13 ; i= 10
a= 13 ; i= 11
a= 13 ; i= 12
adding a 13 break
a= 14 ; i= 2
2 divides into 14 break
a= 15 ; i= 2
a= 15 ; i= 3
3 divides into 15 break
41

See [Python docs on else clauses on loops (https://docs.python.org/3/tutorial/controlflow.html#break-and-continue-statements-and-else-clauses-on-loops) for more insight on why this works.

Also, you may want to use for a in range(2, N+1): so that you add N if it happens to be prime.

Post back if you need more help. Good luck!!!