Python reverse engineering - why the code produced this particular result-CodePudding

I was asked a question about why the following code gives the following result:

def find(j):
    if(j>1):
        j=find(j//10)-(j%10)
        print(j)
    else:
        j=0
    return j
i=122
k=find(i)

RESULT:

-2
-4

Why that particular result? Here is what I have so far:

def find(j) – defying function find with j being an argument if j is greater than 1, the argument j within find function is divided by 10 122 % 10 = 2 print (j) – calls function find to display

From this point on, I am struggling. Why -2 and where is -4 coming from?

CodePudding user response：

The answer is likely due to a Python version difference. Modern versions of Python treat / as float division even if the operands are ints. Python 2 did the equivalent of // (int division) if the operands were ints.

If I modify your code to use int division (which seems like it was probably the intent, since it also uses the % operator) then I get the expected results:

>>> def find(j):
...     if(j>1):
...         j=find(j/10)-(j%10)
...         print(j)
...     else:
...         j=0
...     return j
...
>>> find(122)
-1.22
-3.419999999999999
-5.419999999999999
-5.419999999999999
>>> def find(j):
...     if(j>1):
...         j=find(j//10)-(j%10)
...         print(j)
...     else:
...         j=0
...     return j
...
>>> find(122)
-2
-4
-4

CodePudding user response：

A version of your code modified to explain what it's doing and why follows:

def find(j):
    if(j>1):
        find_result = find(j//10)
        new_j = find_result - (j%10)
        print("j == %r; find(%r) => %r; find(%r) - %r => %r" % (
            j, (j//10), find_result, (j//10), j % 10, new_j))
        j = new_j
    else:
        j=0
    return j

i=122
k=find(i)

...its output is as follows:

j == 12; find(1) => 0; find(1) - 2 => -2
j == 122; find(12) => -2; find(12) - 2 => -4

So: First, you start with j=122. It thus calculates find(12) and subtracts 2 from the result. So before we know what that will do, we need find(12).

find(12) checks if j > 1. Because 1 is not > 1, this is false, so it returns 0. The caller than subtracts 2 from this, getting -2, which it prints.

Then, find(122) takes this result of -2, and subtracts an additional 2 from it, so it gets -4.