Trying to add numbers in python loop wrong result

Question

I am new to python. I am trying to write a program that counts a range of values but for multiples of 3 and 4 adds 3 and 4 to them resepectively and when the number is multiple of both adds that number to it. So for instance 1,2,3,4,5,6,7,8,9,10,11,12 should read in the end program as: 1,2,6,8,5,9,7,12,10,11,24

But my code is getting stuck somewhere.

for i in range (1,20):
   if i%3==0 and i%4==0:
       i=i+12
   if i%3==0:
       i=i+3
   if i%4==0:
       i=i+4
   print i

Answer 1

This line has a typo

if i%3==0 and 1%4==0:  # your second condition is 1 % 4 == 0, which is always false

I think you meant

if i%3==0 and i%4==0:

Answer 2

It is better (accurate and cleaner) if you use a different variable.

for i in range (1,20):
   n = i
   if i%3==0 and i%4==0:
       n=i+12
   if i%3==0:
       n=i+3
   if i%4==0:
       n=i+4
   print n

Now you will notice this fixed it for the 9 case, but not the 12 case! We now need to add the use of elif. Also, if a number is a multiple of 3 and 4, then it is also a multiple of their lowest common multiple, in this case 12. So you could re-write your first step to just check for multiples of 12. This gives us:

for i in range (1,20):
   n = i
   if i%12==0
       n=i+12 # or possibly i + i
   elif i%3==0:
       n=i+3
   elif i%4==0:
       n=i+4
   print n

The reason this works is because without the elif the i was getting added to multiple times. For example, with 9, you get to 9%3 ==0, True. Now i is set to 12. Next statement? 12%4 ==0 True. So another 4 is added.

Alternatively, if you would like to do some crazy python stuff:

for i in range(1, 20):
    print i + next((n for n in (12, 4, 3) if i % n == 0), 0)

(Shout out to Jon Clements for this 2-liner answer)

Answer 3

You're allowing multiples cases (if conditions) for each iteration. I think you probably wanted to do exclusive cases.

Maybe something like:

for i in range(1, 20):
    print i, "=>",
    if i % 3 == 0 and i % 4 == 0:
        i += i
    elif i % 3 == 0:
        i += 3
    elif i % 4 == 0:
        i += 4
    print i