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Asher Orr
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Asher Orr
Python Development Techdegree Graduate 9,408 Points

Understanding how a recursive function works: I need help!

Hi everyone! I have a recursive function, DecimalToBinary:

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:

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:

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?

1 Answer

Steven Parker
Steven Parker
229,608 Points

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)