iterating through nested lists - flattening

[pavlos03]

Hi guys,

does anyone know a fast and efficient way to iterate through a nested list within a nested list withing a nested list etc etc which you don't know the overall length?

I've put something like this together to check what is each location in a list (list, tuple, string, int)

PYTHON Code:

arrPts = []
arrPts.append([2,3,4,5])
arrPts.append("hello")
arrPts.append(6)
arrPts.append([7,[8,9],10])
 
 
for i in arrPts:
        if type(i) == type(list()):
            print "nested LIST found"
            for j in i :
                if type(j) == type(list()):
                    print "nested LIST found"
                    for k in j :
                        if type(k) == type(list()):
                            print "nested LIST found"
                        else:
                            print k
                else:
                    print j
                   
        elif type(i) == type(tuple()):
            print "TUPLE"
            for j in i:
                print j
        elif type(i) == type(str()):
            print i
        elif type(i) == type(int()):
            print i

yet as u can see going from 'i' to 'j' to 'k' and son on can be endless, especially if you don't know the depth of the list.

Any suggestions? I think it has something to do with flattening the list, with the yield command?


thanks

Pav

[Steve Baer]

you could use a recursive function for this. Just jotting down a function here without testing so it may have a typo

Python Code:

arrPts = []
arrPts.append([2,3,4,5])
arrPts.append("hello")
arrPts.append(6)
arrPts.append([7,[8,9],10])
 
def WalkList( thelist ):
    for item in thelist:
        if item is list:
            print "nested LIST found"
            WalkList(item)
        elif item is tuple:
            print "TUPLE"
            WalkList(item)
        else:
            print item
 
WalkList(arrPts)

[pavlos03]

Hi Steve and thanks,

what you wrote will iterate through every level of sub_list on a basis of "all the possible sublists at position 0", then "all the possible sublists at the position 1" etc.

what if i want to iterate through :
all elements in root lists, then through all elements in level 1 sublists , then through level 2 sublists etc? have you got anything to suggest for that? I hope i'm explaining this good enough.

thanks again

Pav

[Steve Baer]

I'm sure I could answer this if you gave me a sample of input and what the output order you want is

[Goswin]

Hi Pavlos03,
does this work for you ?

Python Code:

def WalkList2(thelist):       
    def WalkListRecursive(thelist):       
        # first split into iterable and non iterable objects
        collections = []   
        for item  in thelist:
            if hasattr(item,"__iter__"): # returns true for list , tuples, .net-arrays and other collections but not Strings
                collections.append(item) #save for iterating later
            else:
                result.append(item) #first collect all items of this level nesting
 
        #second go to next level of nesting
        for collection  in collections:
            WalkListRecursive(collection)
   
    result = []
    WalkListRecursive(thelist)
    return result
   
arrPts = []
arrPts.append([2,3,4,5])
arrPts.append("hello")
arrPts.append(6)
arrPts.append([7,["vf","xx"],10])
 
unnested = WalkList2(arrPts)
print unnested

you can also look here for probably more efficient implementations:
http://stackoverflow.com/questions/2...ists-in-python

[Goswin]

if the order of the resulting list is not relevant you can also use this:

Python Code:

def WalkList3(thelist):       
    def WalkListRecursive(thelist):
        for item  in thelist:
            if hasattr(item,"__iter__"): # returns true for list , tuples, .net-arrays and other collections but not Strings
                WalkListRecursive(item) # recursive call
            else:
                result.append(item)
 
    result = []
    WalkListRecursive(thelist)
    return result 
       
       
arrPts = []
arrPts.append([2,3,4,5])
arrPts.append("hello")
arrPts.append(6)
arrPts.append([7,["vf","xx"],10])
 
unnested = WalkList3(arrPts)
print unnested

[Goswin]

Hi Steve, in regard to your first code posting:

Python Code:

a = [1,2,3]
print a is list # returns False for me, not True
 

[AHD]

This will work though:

Python Code:

a = [1,2,3]
print type(a) is list

Ps. How do you guys post source code with syntax highlighting and all that jazz?

[Goswin]

AHD, the jazz is here:
http://python.rhino3d.com/threads/7-...x-Highlighting

[AHD]

Sweet, that was easy. Thanks Goswin.