CODEWITHSUNDEEP
pythonpython-3.xlistrecursionpermutation
userj
userj

Reputation: 158

How to get a list of all binary numbers of length n?

I am trying to write some recursive python code which prints every binary number of length n. The input of the function should only be n. This is my code so far:

def printAll(n):
    result=[]
    stringSoFar=''
    def printAllrec(stringSoFar,n,result):
        if n ==0:
            result.append(stringSoFar)
            if len(result) == (2):
                return result
        else:
            printAllrec((stringSoFar+"0"),n-1,result)
            printAllrec((stringSoFar+"1"),n-1,result)

    return printAllrec(stringSoFar,n,result)
print(printAll(2))

However, this code keeps returning 'None'. I don't understand why it won't work. Any tips would be greatly appreciated.

Upvotes: 1

Views: 1740

Answers (2)

Alain T.
Alain T.

Reputation: 42139

Using a recursive generator could make the code much more compact. You just need to take every result from n-1 and append a "0" and a "1".

def allBits(n):
    if n: yield from ( bits+bit for bits in allBits(n-1) for bit in ("0","1") )
    else: yield ""

for bits in allBits(3):print(bits)

000
001
010
011
100
101
110
111

Upvotes: 0

ShadowRanger
ShadowRanger

Reputation: 155604

Your code doesn't need more returns, it needs to return the right thing. printAllrec doesn't use return values here, it justs modifies a list provided to the top level printAllrec call, which is delegated to the child calls. But the top level function, printAll, needs to return that list, not the garbage return from the internal recursive function. Just change:

def printAll(n):
    ...

    return printAllrec(stringSoFar,n,result)

to:

def printAll(n):
    ...
    printAllrec(stringSoFar,n,result)
    return result  # returns the actual list

Try it online!

and it works as expected.

Upvotes: 2

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