recursively duplicate letters in word - 'abcd' -> 'aabbcc'

Question

I just learned what recursion is and I am trying to find a way to duplicate letters in a word recursively. It's just a way to practice. If there is a better way to do it without recursion, I am open to any feedback. However, I'm primarily trying to find a solution in order to illuminate what recursion is before I go back to looking at coursework and lecture slides. It won't print the final letter twice.

My code:

def makeDups(young):
    if len(young) <= 1:
        return young
    elif young[0] != young[1]:
        return 2 * (young[0]) + makeDups(young[1:])
    else:
        return makeDups(young[1:])

young = 'abcdefg'
print(young)
print(makeDups(young))

I found the problem. I changed the first return to this: return 2 * young — yvtfg4, Feb 02 '16 at 12:54
If you're interested in recursion, can I suggest trying to solve the [Towers of Hanoi](https://en.wikipedia.org/wiki/Tower_of_Hanoi) puzzle. — Sobrique, Feb 02 '16 at 13:22

score 0 · Answer 1 · answered Feb 02 '16 at 13:20

0

This doesn't need recursion.

echo "abcd" | perl -pe 's/(.)/$1$1/g'

Output:

aabbccdd

answered Feb 02 '16 at 13:20

Sobrique

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When I put this into my python editor, I get a syntax error. – yvtfg4 Feb 03 '16 at 13:35

score 0 · Answer 2 · answered Feb 02 '16 at 19:53

Your solution also fails to reduplicate a letter that's already doubled. For instance, "aabc" should return "aaaabbcc", right? You specifically avoid that case, returning "aabbcc" instead.

The correction is to eliminate that test, promoting the recursive call to the else clause:

def makeDups(young):
    if len(young) <= 1:
        return 2 * young
    else:
        return 2 * (young[0]) + makeDups(young[1:])

Finally, one last Pythonic update: your code is clean enough to turn this into a single statement:

def makeDups(young):
    return 2 * young if len(young) <= 1 \
        else 2 * (young[0]) + makeDups(young[1:])

woah. You just introduced a case I was avoiding. Thank you so much. — yvtfg4, Feb 03 '16 at 13:16
is this update for python 3? I'm taking a class that uses python 2ish. — yvtfg4, Feb 03 '16 at 13:18

recursively duplicate letters in word - 'abcd' -> 'aabbcc'

2 Answers2