Finding a certain item in a list of lists in Python

Question

I am just learning Python and I need some advice regarding searching items in a list. I have a list that contains a list of lists. How can I search for an item in a list and return the value from the same list like the example below.

Example: 
myList = [['Red', 'Tomato'], ['Green', 'Pear'], ['Red', 'Strawberry'], ['Yellow', 'Lemon']]

word = 'Green'
return Pear

Is is possible to find the first instance or the n-th instance in the list as well?

first_instance = 'Red'
return Tomato

last_instance = 'Red'
return Strawberry

will there be two elements in the sub_lists always? or is there a possibility of there being more than two elements? — icyflame
– icyflame, Commented May 6, 2013 at 8:54

Lev Levitsky · Accepted Answer · 2013-05-06 08:52:44Z

3

You can fetch all such elements:

instances = [x[1] for x in myList if x[0] == 'Red']

And process instances[0], instances[-1], etc.

To get the first one only, I'd go with @eumoro's approach using a generator expression.

answered May 6, 2013 at 8:52

Lev Levitsky

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eumiro · Accepted Answer · 2013-05-06 08:52:53Z

2

myList = [['Red', 'Tomato'], ['Green', 'Pear'], ['Red', 'Strawberry'], ['Yellow', 'Lemon']]

# first_instance_red
next(elem[1] for elem in myList if elem[0] == 'Red')

# last_instance_red
next(elem[1] for elem in myList[::-1] if elem[0] == 'Red')

answered May 6, 2013 at 8:52

eumiro

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1 Comment

jamylak Over a year ago

Although OP's use case is tiny, so it won't matter, I would rather use reversed to avoid the shallow copy of [::-1] here since the instance may be found early.

Ashwini Chaudhary · Accepted Answer · 2013-05-06 09:10:19Z

You can use collections.defaultdict:

Using this once the dictionary is created it'll take only O(1) lookup to find any instance of "Red" or any other color.

>>> myList = [['Red', 'Tomato'], ['Green', 'Pear'], ['Red', 'Strawberry'], ['Yellow', 'Lemon']]
>>> from collections import defaultdict
>>> dic = defaultdict(list)
>>> for k,v in myList:
    dic[k].append(v)
...     
>>> dic['Red'][0] #first instance
'Tomato'
>>> dic['Red'][-1] #last instance
'Strawberry'
>>> dic["Yellow"][0] #first instance of Yellow
'Lemon'

Define a simple function here to handle Index errors:

>>> def solve(n, color, dic):
    try:
        return dic[color][n]
    except IndexError:
        return "Error: {0} has only {1} instances".format(color,len(dic[color]))
...     
>>> dic = defaultdict(list)
>>> for k,v in myList:
    dic[k].append(v)
...     
>>> solve(0, "Red", dic)
'Tomato'
>>> solve(-1, "Red", dic)
'Strawberry'
>>> solve(0, "Yellow", dic)
'Lemon'
>>> solve(1, "Yellow", dic)
'Error: Yellow has only 1 instances'

Daren Thomas · Accepted Answer · 2013-05-06 09:03:37Z

0

OK. All of the other answers are probably more pythonic than this. I'm just adding it, because I think when starting out, having some simple loops helps.

def find_first(key, list_of_pairs):
    for pair in list_of_pairs:
        if pair[0] == key:
            return pair[1]
    return None # hey, maybe it isn't in the list at all!

Oh, and you could define find_last as:

find_last = lambda k, lop: find_first(key, reversed(lop))

So. In python you can always go this route. Except you should really look into the list comprehensions mentioned by the other answers.

answered May 6, 2013 at 9:03

Daren Thomas

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Comments

kiriloff · Accepted Answer · 2013-05-07 04:31:17Z

0

Use a dict comprehension, a one liner to make a dict out of a list. Then, query the dict.

>>> d={x[0]:x[1] for x in myList}

d equals {'Green': 'Pear', 'Yellow': 'Lemon', 'Red': 'Strawberry'}

>>> d['Green']
'Pear'

answered May 7, 2013 at 4:31

kiriloff

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