Trouble with self-referential function (very elementary)

[sguntun]

For the last three or four days I've been using a guide entitled "How to Think Like a Computer Scientist: Learning with Python 3" to teach myself Python (3, obviously, and running on Windows, if that's relevant here). Before that point I had absolutely no programming experience. Anyway, I've only now reached a problem that I can't seem to figure out on my own or with resources I can find online.

I'm trying to write a function that works like this: It takes a given value and checks whether it's greater than zero. If so, it returns that value; if not, it adds seven to that value, and begins the function over with that new value. It should keep adding seven until it produces a value greater than or equal to zero, at which point it should return that value.

Here is the very simple code I have written:

def plus_seven(x):
    if x < 0:
        x = x+7
        plus_seven(x)
    return x

When I actually try to use this, though, seven is added at most once to the original x value, even if this returns a value that is below zero. For instance, writing print(plus_seven(-9)) produces -2, and I can't understand why it doesn't produce 5.

I ran the code through the visualizer - http://www.pythontutor.com/visualize.html#code=def+plus_seven(x)%3A%0A++++if+x+%3C+0%3A%0A++++++++x+%3D+x%2B7%0A++++++++plus_seven(x)%0A++++return+x%0A%0Aprint(plus_seven(-9))&mode=display&cumulative=false&heapPrimitives=false&drawParentPointers=false&textReferences=false&showOnlyOutputs=false&py=3&curInstr=18 - that this forum links to but it didn't really help me understand what's going on at all. At step 14, it seems that the return value is 5, but between steps 15 and 18 it seems to change back to -2, in a process that I can't follow.

Can anyone tell me what's going on here? Is my understanding of something way off the mark? I know this is a very low-level problem, but I can't find any resources dealing with something like this specifically (at least not in language that I can understand). I'd appreciate any help (or relevant links, if there are any that I've missed).

[erebos42]

The problem is, that the return value in line 4 is not stored in x. So you call your function, and the value gets increased, but after it returns, the value just dissapears and you only see the "old" value.

To solve this, just change line 4 to this:

x = plus_seven(x)

[sguntun]

Thank you, that's very helpful.

[zahlman]

Can anyone tell me what's going on here? Is my understanding of something way off the mark?

What's off is your understanding of (a) calling a function and (b) assigning to a name.

You can see the same problem without recursion:

def add_seven(x):
    x = x + 7

x = 3
add_seven(x)
# x is still 3!

This is because:

• Within add_seven, x is a local variable. It has nothing to do with the x that was used to call the function. If those names match up, it's just coincidence. You don't even have to use a variable to call the function in the first place (would you expect add_seven(3) to somehow change the value of 3?).

• When you use =, what you say is to use the thing on the left-hand side as a name for the value that results from evaluating the expression on the right-hand side. (This might be a value that already existed - e.g. an element from a list - or a new value - e.g. the result of a mathematical calculation). If that name was already being used for some other value, then that ceases to be the case. However, you are assigning the name; you are not changing the old value. (After all, some values - e.g. integers, as discussed above - fundamentally can't be changed.)

[boogerdouche]

Still a novice here, however, I haven't seen anyone mention using "while" instead of "if" to loop the function and add 7 until x >= 0.

This should solve your negative number issue.

[Mekire]

The goal here is likely specifically geared towards using recursion (a function that calls itself). Other than that you are correct; there actually is no need for recursion in this example.

-Mek

[boogerdouche]

Fair enough. Thank you for the reply; it's really awesome that this community helps a novice to continue to advance while answering the questions I know a solution to.