Question

I’m in an introductory Python course. I’m taking a practice final and I don’t know exactly what to do for a question. It asks:

Complete the following method that places odd numbers found in the string s in consecutive elements of the list x.

def assign(s):
    length = _____
    x = _____*[0]
    index = 0
    for j in range(_____):
        digit = int(s[j])  #convert to int digit
        if _____: #test for odd number
            x[_____] = digit
            __________
    print("The odd numbers are")
    for j in range(index):
        print(x[j])

I don’t know if it’s the way it’s worded but I have no clue how to go about answering this. My best guess would be:

def assign(s):
    length = len(s)
    x = length*[0]
    index = 0
    for j in range(s):
        digit = int(s[j])  #convert to int digit
        if _____: #test for odd number
            x[_____] = digit
            __________
    print("The odd numbers are")
    for j in range(index):
        print(x[j])

I have no clue how to continue, or if I’m even starting it off right. I can’t change anything that’s already there, I can only fill in the blanks, and they all have to be filled in. Anyone know how to do this?

Answer 1

Here is the solution:

def assign(s):
    length = len(s)
    x = length*[0]
    index = 0
    for j in range(length):
        digit = int(s[j])  #convert to int digit
        if digit % 2: #test for odd number
            x[index] = digit
            index += 1

    print("The odd numbers are")
    for j in range(index):
        print(x[j])

You got the beginning right. An key point is that variable index points to the next odd number that should be added to list x.

---

Note, however, that the template that was provided to you is horrible: it completely goes against the spirit of Python. It looks like C code forced into Python; this is not good practice, as Python offers much more powerful data structures than C. Here is a Pythonic version:

def assign(input_string):
    odd_digits = []
    for char in input_string:
        digit = int(char)  # Convert to integer
        if digit % 2:  # If the digit is odd
            odd_digits.append(digit)
    print("The odd digits are", *odd_digits, sep='\n')  # or: ("The odd digits are", odd_digits)

The main difference is that you have fewer variables: the code is more legible, one has to keep fewer variables in mind when reading the code. The variables that have disappeared are essentially indexes: they are often not needed (indexes are important in C because C is “close to the metal”: it is close to machine language, which requires indexes). As a consequence, the need for keeping track of the length of the input string disappears.

You also have less cruft: in the original version, list x contained many 0 that were essentially useless (slower code, which is also harder to make sense of). Essentially, Python list x was forced to be like a C array: the variable number of odd digits in the string call for a list (variable size), not for an array (fixed size).

Also, the fact that the code is shorter makes is faster to read, and to write!

---

As suggested by Jon, an even more Pythonic version consists in using list comprehensions:

def assign(input_string):
    odd_digits = [char for char in input_string if int(char) % 2]  # or: if char in '13579'
    print("The odd digits are", *odd_digits, sep='\n')  # or: ("The odd digits are", odd_digits)

This is a good example of why Python code is so much faster to write and read than C code.