Python if and else print condition

Question

If an integer is divisible by 3, print "Hi"

If it is divisible by 7, print "Bye"

If it is divisible by both 3 and 7, print "HiBye"

As of now I have tried:

for i in range(1,100):
    if i % 3 == 0:
        print "Hi"
    if i % 7 == 0:
        print "Bye"
    if i % 3 == 0 and i % 7 == 0:
        print "HiBye"
    else: 
        print i

But my numbers are repeated. i.e. this is the output I get.

1
2
Hi
3
4
5
Hi
6
Bye
7
8
Hi
9
10
11
Hi
12
13
Bye
14
Hi
15
16
17
Hi
18
19
20
Hi
Bye
HiBye

As you can see, the 3 is repeated again. I think the mistake is in the

else:
    print i

statement

Solution

You need to use elif instead of if, and test for the 3 and 7 case first:

if i % 3 == 0 and i % 7 == 0:
    print "HiBye"
elif i % 3 == 0:
    print "Hi"
elif i % 7 == 0:
    print "Bye"
else: 
    print i

You used independent if statements. Each if statement is tested and their block is executed, regardless of what other if statements your code may execute before or after. elif blocks, however, are attached to their if statement and Python will only ever execute one of the blocks, the first one whose condition is true.

So in the above if..elif..elif..else series of tests, if i % 3 == 0 and i % 7 == 0 is True, none of the other branches will be executed, including the else branch.

Now the output looks like:

>>> for i in range(1, 22):
...     if i % 3 == 0 and i % 7 == 0:
...         print "HiBye"
...     elif i % 3 == 0:
...         print "Hi"
...     elif i % 7 == 0:
...         print "Bye"
...     else:
...         print i
...
1
2
Hi
4
5
Hi
Bye
8
Hi
10
11
Hi
13
Bye
Hi
16
17
Hi
19
20
HiBye

Comments

[Vera Worri]

I changed it to fix the problem

[Martijn Pieters]

Why the last elif? Why not just use else?

[Vera Worri]

Just to make sure the condition is met. In these cases, I find it better to be explicit. It's really the order that python is iterating and passing through the numbers that matters.

[Martijn Pieters]

It's overly verbose and redundant. You are making Python do extra work that you already tested for.

[Vera Worri]

Yeah.. I know. But I found that building in some redundancy when trying to solve a problem helps figure out what python is doing.