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copied!<p>first, you should separate the dictionary that was passed in, and the dictionary that you are going to return.</p> <pre><code>def build_person_to_matches(d, name): ret = {} collect = 0 for books in d[name]: for person in d.keys(): if books in d[person]: collect += 1 ret[person] = collect return ret d = { 'Tom': ['fiction', 'documentary', 'science'], 'John': ['computer science', 'math'], 'Jack': ['science', 'computer science', 'math', 'chemistry'] } print build_person_to_matches(d, 'Jack') </code></pre> <p>Second, switch the order of the two for loops, and move the <code>collect = 0</code> line into the first loop.</p> <pre><code>def build_person_to_matches(d, name): ret = {} for person in d.keys(): collect = 0 for books in d[name]: if books in d[person]: collect += 1 ret[person] = collect return ret d = { 'Tom': ['fiction', 'documentary', 'science'], 'John': ['computer science', 'math'], 'Jack': ['science', 'computer science', 'math', 'chemistry'] } print build_person_to_matches(d, 'Jack') </code></pre> <p>Optionally, for the sake of readability, you may as well move the inner loop into its own function, to make it more obvious to the reader what is going on.</p> <pre><code>def how_many_genres_both_people_like(d, person_a, person_b): total = 0 for books in d[person_a]: if books in d[person_b]: total += 1 return total def build_person_to_matches(d, name): ret = {} for person in d.keys(): ret[person] = how_many_genres_both_people_like(d, name, person) return ret d = { 'Tom': ['fiction', 'documentary', 'science'], 'John': ['computer science', 'math'], 'Jack': ['science', 'computer science', 'math', 'chemistry'] } print build_person_to_matches(d, 'Jack') </code></pre> <p>Output:</p> <pre><code>{'John': 2, 'Jack': 4, 'Tom': 1} </code></pre>

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