Understanding how a recursive function works: I need help!

Question

Hi everyone! I have a recursive function, DecimalToBinary:
~~~python
def DecimalToBinary(num):
    if num >= 1:
        DecimalToBinary(num // 2)
        print(num % 2, end = '')
~~~
I need help understanding exactly what is happening in line 3. Here's what I think is happening:
~~~python
def DecimalToBinary(num):
    if num >= 1:
    # if the argument is greater than or equal to 1, then:
        DecimalToBinary(num // 2)
        # call the DecimalToBinary Function
        # this is going to divide the num by 2, using floor division
        print(num % 2, end = '')
        # then use a modulo operation to find the remainder of num
        # in the print statement, num now represents the result of DecimalToBinary(num // 2)
       # and print it in one line.
~~~
Here's an example using the integer 3 as an argument:
~~~python
def DecimalToBinary(3):
    if num >= 1:
        DecimalToBinary(3 // 2)
        print(1.5 % 2, end = '')
~~~
.. In this case, the function prints the integer 11.
Am I on the right track here? If not, can anyone help me understand how this function works?

Steven Parker · Accepted Answer

It sounds like you have the basic idea.  For better understanding, you might try making a list of operations (just outputs and calls) performed and maybe start with a slightly larger number so you can see more than 2 levels of recursion.  List the level along with the operation at each step:
Level Operation  
  0   DecimalToBinary(5)  
  1     DecimalToBinary(2)  
  2       DecimalToBinary(1)  
  3         print(1)  
  2       print(0)  
  1     print(1)

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Asher Orr

Asher Orr

Understanding how a recursive function works: I need help!

1 Answer

Steven Parker

Steven Parker

Asher Orr

Asher Orr