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Yellow_

Python recursion/list help?

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Posted · Original PosterOP

I have a program im writing but I don't really know how to get what I want specifically

The function gives a list of combinations of letters based on the parameter 

 

I have this:

 

def comb(string):
    if string == "" or len(string) == 1:
        return list(string)

    elif len(string) > 1:
     
        scperm = []
        for i in range(len(string)):

            spt = string[:i] + string[i+1:]

            for n in comb(spt):

                scperm.append(string[i]+n)

        return scperm

 

Say if the input is "Hi", the output is ["Hi", "iH"], but I'm wanting to get it to be more like 

[ ["H" , "i" ], ["i", "H"] ]

I've tried everything I can think of and havent rolled past anything that remotely worked. Any help is appreciated, thanks!

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16 minutes ago, Yellow_ said:

Say if the input is "Hi", the output is ["Hi", "iH"], but I'm wanting to get it to be more like

Something like this:

def comb(string):
	if len(string) < 1:
		return list(string)
	scperm = []
	scperm2 = []
	for i in range(len(string)):
		scperm.append(string[i])
		scperm2.insert(0, string[i])
	return [scperm, scperm2]

 


Hand, n. A singular instrument worn at the end of the human arm and commonly thrust into somebody’s pocket.

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Posted · Original PosterOP
3 minutes ago, WereCatf said:

Something like this:

 

That kiiiinda gets there?

 

IS there any way to globalize it a little better to take a string of any length? The goal of the project from this book is to use recursion- is there any decent way to do that? 

As I had recursion in my initial code, and it almost got there

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11 minutes ago, Yellow_ said:

is there any decent way to do that?

Decent? Well, no, because it'd be stupid to use recursion to achieve what you wanted.


Hand, n. A singular instrument worn at the end of the human arm and commonly thrust into somebody’s pocket.

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Posted · Original PosterOP
Just now, WereCatf said:

Decent? Well, no, because it'd be stupid to use recursion to achieve what you wanted.

Yeah, thats true. Just trying to test myself. 

 

Is there any way you know that would accomplish that?

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17 minutes ago, Yellow_ said:

Is there any way you know that would accomplish that?

Well, yes. All the ways of doing that are clunky and stupid, but yes. Do you want the complete solution or do you just want me to give you hints on how to do it?


Hand, n. A singular instrument worn at the end of the human arm and commonly thrust into somebody’s pocket.

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Hints: check what type of variable the comb() - function is receiving. For example, if insertyourvariable is str: would check that the variable is a string. Then make your function to behave differently based on that.

 

Full solution:

Spoiler

def comb(string):
		if type(string) is str:
				if len(string) < 1:
					return [[],[],[]]
				return comb([[],[],string])

		if len(string[2]) > 0:
				scperm = string[0]
				scperm2 = string[1]
				scperm.append(string[2][0])
				scperm2.insert(0, string[2][0])
				return comb([scperm, scperm2, string[2][1:]])

		return [string[0], string[1]]

 

 


Hand, n. A singular instrument worn at the end of the human arm and commonly thrust into somebody’s pocket.

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Posted · Original PosterOP
3 minutes ago, WereCatf said:

Well, yes. All the ways of doing that are clunky and stupid, but yes. Do you want the complete solution or do you just want me to give you hints on how to do it?

Hints would be perfect, I really do enjoy figuring these out myself. This question though.. Is recursion used really?

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1 minute ago, Yellow_ said:

This question though.. Is recursion used really?

Recursion is sometimes the only way of doing something, or doing something in an actually practical way, so yes, it is both used and useful. The thing is, most things don't require using recursion. It's not something you're likely to need in any actual real-world usecase for a good while, unless you delve into very heavy maths - problems.


Hand, n. A singular instrument worn at the end of the human arm and commonly thrust into somebody’s pocket.

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2 minutes ago, Yellow_ said:

Is recursion used really?

it's definitely used, but it has a time and place. Another aspect of recursion that makes it more usable is dynamic programming, which is basically "recursion plus memoization", where memoization is just a fancy way of saying "remembering the values you got". that way you are not re-computing values you already solved in the recursive chain.

 

I wouldn't worry about that just yet if you are just learning. Knowing the basics of using recursion is enough.


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Posted · Original PosterOP
3 minutes ago, WereCatf said:

Recursion is sometimes the only way of doing something, or doing something in an actually practical way, so yes, it is both used and useful. The thing is, most things don't require using recursion. It's not something you're likely to need in any actual real-world usecase for a good while, unless you delve into very heavy maths - problems.

I see, well I'll take a swing at what you threw me there. Thanks!

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@Yellow_

 

Why not just write a short helper function?

if __name__ == "__main__":
    def comb(string):
        if string == "" or len(string) == 1:
            return list(string)

        elif len(string) > 1:

            scperm = []
            for i in range(len(string)):

                spt = string[:i] + string[i + 1:]

                for n in comb(spt):
                    scperm.append(string[i] + n)

            return scperm

    def combContainer(string):
        # Get result of comb function
        result = comb(string)
        # Using list comprehension, convert each string permutation to a list of characters
        return [list(item) for item in result]

    print(combContainer("Hi"))

    print(combContainer("abc"))

Output:

 

[['H', 'i'], ['i', 'H']]
[['a', 'b', 'c'], ['a', 'c', 'b'], ['b', 'a', 'c'], ['b', 'c', 'a'], ['c', 'a', 'b'], ['c', 'b', 'a']]

 


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