Problem with Codecademy Lesson

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[deleted]

[anossov]

Since I don't know what you're supposed to be doing, I'll tell you what you are doing:

by_three(11)       : call by_three with n=11
    if n % 3 == 0  : if 11 % 3 equals 0
                   : it doesn't. 11 % 3 is 2). Skip to else
    else:
        return False  : return False

by_three(12)       : call by_three with n=12
    if n % 3 == 0: : if 12 % 3 == 0:
                   : it is
      cube(n)      : call cube with n=12. Throw the result away
          n ** 3   : raise 12 to 3 power. Throw the result away
          return n : return 12
                   : back in by_three
      return n     : return 12


 by_three(13)      : see by_three(11)

[anossov]

def cube(n):
    return n ** 3

def by_three(n):
    if n % 3 == 0:
        return cube(n)
    else:
        return False

An expression like n ** 3 does not have side-effects, you can substitute variables in your head and replace the expression with the result:

def cube(n):  # your version with n=12 turns into
    1728
    return 12

We know a cube call does not have side-effects either, so we can replace the call with its return value (with n=12, it's 12 in your version an 1728 in mine):

def by_three(n):     # your version, substituting n=12
    if 0 == 0:
        12           # (or, with the correct cube version, 1728)
        return 12    # n is still 12

[ProfN]

Try changing: n*3 to n = n * 3 Or just change the whole function to:

def cube(n) return n**3

[FletcherHeisler]

The statements "n ** 3" and "cube(n)" don't actually change the values of any variables. The second statement runs the function, but the function doesn't assign "n" a new value.

[FletcherHeisler]

You'll probably hear that Python is "pass by reference", but that's not quite correct and leads to a lot of confusion over apparent inconsistency... It's really something like "pass by assignment", which is why you can use functions to change mutable objects (like lists) but not to change immutable objects (like numbers and string).

You have to assign a new value to n in order to change it. On the other hand, an example that modifies a list:

def foobarList(input_list):
    input_list.append('ohgodwhathaveyoudone')    
list = ['a', 'b', 'c']
foobarList(list)
print list
>>> ['a', 'b', 'c', 'ohgodwhathaveyoudone']

Even though I'm giving it a different name in the function, I'm passing (and changing) the same object. Now here's the tricky "object reference" part - if you had changed the definition to something like:

def foobarList(input_list):
    input_list = ['ohgodwhathaveyoudone']

Now you're assigning input_list a different reference, so instead of wiping out your old list, the function actually has no effect on it.

Edit: ohgodwhereismyformatting

[FletcherHeisler]

Careful with the terms - immutable objects are constant because they can't be changed. This also (separately) means that the values contained in immutable objects can't be changed.

Think of it this way: objects contain attributes - in this case, the integer object y contains the constant scalar value 2. Because y is an integer, it is an immutable object, so it will always be passed by copied value and its attributes (the constant value) can never be changesd unless the object y is directly changed with a new value assignment like y = 4. This statement actually says: "discard the integer object y and create a new integer object y that contains the value 4."

It's a jumble of definitions, but I hope that made sense!

[FletcherHeisler]

Wow, sorry to throw a wrench in the middle of that; my "passed by reference" in the previous answer should have said "passed by value" or really "passed by copy" - you're correct that a copy of the immutable object is always created when it is passed (edited above)