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Jan 8th, 2007

Nested List Searches

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I was experimenting with the nested list example in thread:
http://www.daniweb.com/techtalkforum...246791-72.html
and was trying to search a nested list like that:

Python Syntax (Toggle Plain Text)
nested_list = [1 ,'Dick', 2, ['Mary', 7, 9, [700, 777, 'Paul']], 13]
 
if 'Paul' in nested_list:
    print 'found Paul'
else:
    print 'Paul not found'
nested_list = [1 ,'Dick', 2, ['Mary', 7, 9, [700, 777, 'Paul']], 13]
 
if 'Paul' in nested_list:
    print 'found Paul'
else:
    print 'Paul not found'

It always tells me that 'Paul' is not found! How come? Is there an error in Python?

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sneekula

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Nearly a Posting Maven

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Jan 9th, 2007

Re: Nested List Searches

if you just want to find "Paul" in your example, one way to do it is to convert that list to string

Python Syntax (Toggle Plain Text)
astring  = ','.join( str(i) for i in nested_list)
if "Paul in astring:
  print "Found"
 
astring  = ','.join( str(i) for i in nested_list)
if "Paul in astring:
  print "Found"

ghostdog74

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Junior Poster

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Jan 9th, 2007

Re: Nested List Searches

Python Syntax (Toggle Plain Text)
if "x" in nested
if "x" in nested

can be written as

Python Syntax (Toggle Plain Text)
for elem in nested: if elem == "x" ...
for elem in nested: if elem == "x" ...

Your list nested_list = [1 ,'Dick', 2, ['Mary', 7, 9, [700, 777, 'Paul']], 13] consists of 5 elemenst:
1
'Dick'
2
['Mary', 7, 9, [700, 777, 'Paul']]
13

As you can see, none of these passes the test "Paul" == elem. In the 4th one (['Mary', 7, 9, [700, 777, 'Paul']]) there is the string Paul, but it is hidden in a list

mawe

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Jan 9th, 2007

Re: Nested List Searches

You can flatten the list, might call this un-nesting and then search ...

python Syntax (Toggle Plain Text)
def xflatten(seq):
    """a generator to flatten a nested list"""
    for x in seq:
        if type(x) is list:
            for y in xflatten(x):
                yield y
        else:
            yield x
nested_list = [1 ,'Dick', 2, ['Mary', 7, 9, [700, 777, 'Paul']], 13]
flat_list = list(xflatten(nested_list))
print flat_list  # [1, 'Dick', 2, 'Mary', 7, 9, 700, 777, 'Paul', 13]
if 'Paul' in flat_list:
    print "Found Paul in flat_list"  # now it works!
def xflatten(seq):
    """a generator to flatten a nested list"""
    for x in seq:
        if type(x) is list:
            for y in xflatten(x):
                yield y
        else:
            yield x
nested_list = [1 ,'Dick', 2, ['Mary', 7, 9, [700, 777, 'Paul']], 13]
flat_list = list(xflatten(nested_list))
print flat_list  # [1, 'Dick', 2, 'Mary', 7, 9, 700, 777, 'Paul', 13]
if 'Paul' in flat_list:
    print "Found Paul in flat_list"  # now it works!

If you are only searching, Ghostdog's solution is most elegant and can be simplified to ...

python Syntax (Toggle Plain Text)
nested_list = [1 ,'Dick', 2, ['Mary', 7, 9, [700, 777, 'Paul']], 13]
nl_string = str(nested_list)
print nl_string  # "[1, 'Dick', 2, ['Mary', 7, 9, [700, 777, 'Paul']], 13]"
if 'Paul' in nl_string:
    print "Found Paul in the string"  # works too!
nested_list = [1 ,'Dick', 2, ['Mary', 7, 9, [700, 777, 'Paul']], 13]
nl_string = str(nested_list)
print nl_string  # "[1, 'Dick', 2, ['Mary', 7, 9, [700, 777, 'Paul']], 13]"
if 'Paul' in nl_string:
    print "Found Paul in the string"  # works too!

vegaseat

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DaniWeb's Hypocrite

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Jan 9th, 2007

Re: Nested List Searches

Thank you fellow Pythonians. I learned from mawe's explanation that in order to find an item or find the maximum item you have to un-nest the list as shown by vegaseat's code.

Converting the list to a string, as Ghostdog74 suggested, works well for the search, but can't find the maximum.

sneekula

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Nearly a Posting Maven

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Jan 9th, 2007

Re: Nested List Searches

Sneekula,
since you are so refreshingly inquisitive, have you tried this ...

python Syntax (Toggle Plain Text)
flat_list = [1, 'Dick', 2, 'Mary', 7, 9, 700, 777, 'Paul', 13]
print max(flat_list)
flat_list = [1, 'Dick', 2, 'Mary', 7, 9, 700, 777, 'Paul', 13]
print max(flat_list)

Would you have predicted the outcome? Can you explain why?

vegaseat

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DaniWeb's Hypocrite

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