Python recursive list comprehension to iterative approach

Question

I'm trying to understand how to think a recursive method iteratively. For example, I have the following backtracking method:

def bitStr(n, s):
    if n == 1:
        return s
    return [digit + bits for digit in bitStr(1, s) for bits in bitStr(n - 1, s)]

I'm practicing how to do accomplish a similar iteratively or explicitly using double for-loop.

I started something like this which I understand is incorrect; however, unable to fix it:

def bitStr2(n, s):
    if n == 1:
        return [c for c in s]
    for bits in bitStr2(n - 1, s):
        for digit in bitStr2(1, s):
            return digit + bits

Thank You

Answer 1

There are two issues in your code.

First, as pointed out by @MisterMiyagi, you switched the loops. In a list comprehension, loops are read from left to right. You should write the regular loops like this:

for digit in bitStr2(1, s):
    for bits in bitStr2(n - 1, s):
        ...

Second, a list comprehension produces... a list. You have to store the elements in a list:

...
result = []
for digit in bitStr2(1, s):
    for bits in bitStr2(n - 1, s):
        result.append(digit + bits)
return result

(Conversely: never use a list comprehension if you don't want to produce a list. ) And you don't have to handle differently the n = 1 case. Full code:

def bitStr2(n, s):
    if n == 1:
        return s
    result = []
    for digit in bitStr2(1, s):
        for bits in bitStr2(n - 1, s):
            result.append(digit + bits)
    return result

---

Note that for digit in bitStr(1, s) is equivalent to for digit in s . I don't see why you call the method bitStr in this case, since you already know the result.