How to write a function for dictionary from 2 lists in python with if condition?

Question

How to write a function that takes a list of strings and a list of characters as arguments and returns a dictionary whose keys are the characters and whose values are lists of the strings that start with that character.

a_func(['apple','orange','banana','berry','corn'],['A','B','C'])

should give me

{'A':['apple'], 'B':['banana','berry'], 'C':['corn]}

I tried this but not sure how to put if condition in this,

a = ['apple','orange','banana','berry','corn']
b = ['A','B','C']
d = {}
for k,v in zip(b,a):
    d.setdefault(k, []).append(v)
    print (d)



---------
#output is
{'A': ['apple']}
{'A': ['apple'], 'B': ['orange']}
{'A': ['apple'], 'C': ['banana'], 'B': ['orange']}

Answer 1

This is what happens when you call zip the way you are trying to use it:

>>> list(zip(['A','B','C'], ['apple','orange','banana','berry','corn']))
[('A', 'apple'), ('B', 'orange'), ('C', 'banana')]

Which, as you can see, clearly does not match up to any helpful result you can use to get to your required output.

A much easier way to do this is to make use of defaultdict. Call your defaultdict with a list, so that the initial value of the dictionary entry will be a list. That way all you have to do is check the first character of each word against the character list using in. From there, just simply append once you find that your first character exists in your character list.

Also, you seem to have lower case words with an uppercase character list, so you should set the casing accordingly.

---

from collections import defaultdict


def a_func(words, chars):
    d = defaultdict(list)
    for word in words:
        upper_char = word[0].upper()
        if upper_char in chars:
            d[upper_char].append(word)
    return d


res = a_func(['apple','orange','banana','berry','corn'],['A','B','C'])

Result:

defaultdict(<class 'list'>, {'A': ['apple'], 'B': ['banana', 'berry'], 'C': ['corn']})

Answer 2

What you basically doing is zipping your list into tuple pairs and since zip() stops at the shortest list, you end up with the output:

[('A', 'apple'), ('B', 'orange'), ('C', 'banana')]

While @Idjaw has provide a explanation of a nice way of achieving your output, a simple one line alternative using dict and list comprehension could also be used:

def group(words, keys): 
    return {key: [val for val in words if val.upper() in key] for key in keys}

Explanation:

• {key: ... for key in keys}: construct a dictionary with the given keys('A', 'B', 'C').
• [val for val in words if val[0].upper() == key]: and for each key in the dictionary give it the value of the words in words if the word begins with the key. This part creates a filtered list.

EDIT: @Idjaw has informed me that you may not be looking for one-liner, in which case you can use this instead:

def group(words, keys):
    d = {key:[] for key in keys}
    for word in words:
        for key in d:
            if word.upper() in key:
                d[key].append(word)
    return d

But if time is you most major concern, you should use @Idjaw's solution instead of mine.

Output:

>>> group(['apple','orange','banana','berry','corn'], ['A','B','C'])
{'B': ['banana', 'berry'], 'A': ['apple'], 'C': ['corn']}

_{Note: if you want your dict to be ordered({'A': ['apple'], 'B': ['banana', 'berry'], 'C': ['corn']}), use OrderedDict()}

Answer 3

Here's a 1-liner:

def f(list1, list2):
    return { x : [y for y in list1 if y.startswith(x.lower()) ] for x in list 2}