Changing lists with a given string

Question

On Python 3 I am trying to write a function find(string_list, search) that takes a list of strings string_list and a single string search as parameters and returns a list of all those strings in string_list that contain the given search string.

So print(find(['she', 'sells', 'sea', 'shells', 'on', 'the', 'sea-shore'], 'he')) would print:

['she', 'shells', 'the']

Here's what I tried so far:

def find(string_list, search):
    letters = set(search)
    for word in string_list:
        if letters & set(word):
            return word
    return (object in string_list) in search

running print(find(['she', 'sells', 'sea', 'shells', 'on', 'the', 'sea-shore'], 'he'))

What I expected = [she, shells, the]

what I got = [she]

Hint: filter. Advanced hint: anything filter can do, list comprehensions can do. — minopret
– minopret, Commented Mar 18, 2013 at 1:22
It seems like you're asking people to write some code for you. While someone might well do that, it's better to try to write the function yourself and then ask a more specific question about any problems you encounter doing so. What have you tried so far? Where did you run into problems? — Martin Atkins
– Martin Atkins, Commented Mar 18, 2013 at 1:24
This is what I have tried def find(string_list, search): letters = set(search) for word in string_list: if letters & set(word): return word return (object in string_list) in search — user2101517
– user2101517, Commented Mar 18, 2013 at 1:28
I edited your question to include your example (the edit is in the queue so it might take a little while to show up for you) but I had to guess how you had it indented. Please edit your question if I got it wrong. It would also be helpful to include an example of how you ran your function and what result it actually produced, contrary to what you expected. — Martin Atkins
– Martin Atkins, Commented Mar 18, 2013 at 1:32
Thanks for taking the time to improve your question. Although David Robinson already gave you a working example, I tried to write a more complete explanation of how you'd get from what you had to the concise list comprehension technique, in the hope that this will be more enlightening for others who come across this question in future. — Martin Atkins
– Martin Atkins, Commented Mar 18, 2013 at 2:02

David Robinson · Accepted Answer · 2013-03-18 01:40:03Z

2

You can do this with:

def find(string_list, search):
    return [s for s in string_list if search in s]

answered Mar 18, 2013 at 1:40

David Robinson

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Comments

Martin Atkins · Accepted Answer · 2013-03-18 02:10:21Z

2

The main problem with your code example is that you can only return from a function once, at which point the function stops executing. This is why your function returns only one value.

If you wish to return multiple values you must return a container object like a list or a set. Here's how your code might look if you use a list:

def find(string_list, search):
    letters = set(search)
    result = [] # create an empty list
    for word in string_list:
        if letters & set(word):
            # append the word to the end of the list
            result.append(word)
    return result

The if test here is actually not doing quite what your problem statement called for. Since a set is an unordered collection, the & operation can test only if the two sets have any elements in common, not that they appear in the same order as the input. For example:

>>> letters = set("hello")
>>> word = set("olleh")
>>> word & letters
set(['h', 'e', 'l', 'o'])

As you can see, the operator is returning a set whose elements are those that are common between the two sets. Since a set is True if it contains any elements at all, this is actually testing whether all of the letters in the search string appear in a given item, not that they appear together in the given order.

A better approach is to test the strings directly using the in operator, which (when applied to strings) tests if one string is a substring of another, in sequence:

def find(string_list, search):
    result = []
    for word in string_list:
        if search in word:
            result.append(word)
    return result

Since this pattern of iterating over every item in a list and doing a test on it is so common, Python provides a shorter way to write this called a list comprehension, which allows you to do this whole thing in one expression:

def find(string_list, search):
    return [word for word in string_list if search in word]

This executes just as the prior example but is more concise.

edited Mar 18, 2013 at 2:10

answered Mar 18, 2013 at 2:00

Martin Atkins

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2 Comments

DSM Over a year ago

It might be worth noting that if set(search) & set(word) is very different from if search in word.

Martin Atkins Over a year ago

DSM, ahh yes, I neglected to call that out in my response. I'll make an edit.

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