[Tutor] Valid Username Program

[Alan Gauld]

On 08/10/2020 12:29, Aliyan Navaid wrote:
>    def valid_username(username):
>        if len(username) < 3:
>            print("Username must be atleast 3 characters long")
>        elif len(username) > 15:
>            print("Username is too long")
>        else:
>            print("Valid Username")

>    I wanted to ask that instead of elif, I could’ve written another If
>    statement in this program right ?

Not quite. Consider the else part. How would you have written that?
You would need a third if statement that checked the inverse of
both the previous conditions:

if 3 <= len(username) <= 15:
   print("Valid Username")

So you have to decide if the extra comparison is better or worse
than the elif structure. In this case it doesn't make much difference,
but what if you had several elif statements your "else" test would need
to incorporate all of those tests negated.

Also you have to be very careful not to double-process values. You are
OK in this example but it can get more complex. consider the following:

size = int(input("size"))
if size > 50: print("Too big")
if size > 30: print("That's a big one!")
if size > 20: print("Normal")
if size > 10: print("That's small!")
is size <= 10: print("Too small")

Now what happens if size is 60?
The first 4 tests all print!
But if we used elifs instead then only the first condition
would be triggered.

>    And can you please also explain that when to use elif statement and when
>    to use multiple if statements.

You can use multiple if statements
a) if the tests are completely independent(and will always stay that
way) or
b) if, in a case like the example above, you actually want multiple
cases to trigger.

But those are the exception rather than the rule, in most scenarios
elif is the safer option.

The third scenario to use multiple ifs is inside a function
where each 'if' triggers a 'return' statement that will exit
the function. In that case the subsequent 'if' is only
reached if the predecessors were not triggered.
That will only make sense if you have studied functions already.
If not just ignore it for now!


-- 
Alan G
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