From Python: tf-idf-cosine: to find document similarity , it is possible to calculate document similarity using tf-idf cosine. Without importing external libraries, are that any ways to calculate cosine similarity between 2 strings?
s1 = "This is a foo bar sentence ." s2 = "This sentence is similar to a foo bar sentence ." s3 = "What is this string ? Totally not related to the other two lines ." cosine_sim(s1, s2) # Should give high cosine similarity cosine_sim(s1, s3) # Shouldn't give high cosine similarity value cosine_sim(s2, s3) # Shouldn't give high cosine similarity value
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A simple pure-Python implementation would be:
import math import re from collections import Counter WORD = re.compile(r"\w+") def get_cosine(vec1, vec2): intersection = set(vec1.keys()) & set(vec2.keys()) numerator = sum([vec1[x] * vec2[x] for x in intersection]) sum1 = sum([vec1[x] ** 2 for x in list(vec1.keys())]) sum2 = sum([vec2[x] ** 2 for x in list(vec2.keys())]) denominator = math.sqrt(sum1) * math.sqrt(sum2) if not denominator: return 0.0 else: return float(numerator) / denominator def text_to_vector(text): words = WORD.findall(text) return Counter(words) text1 = "This is a foo bar sentence ." text2 = "This sentence is similar to a foo bar sentence ." vector1 = text_to_vector(text1) vector2 = text_to_vector(text2) cosine = get_cosine(vector1, vector2) print("Cosine:", cosine)
The cosine formula used here is described here.
This does not include weighting of the words by tf-idf, but in order to use tf-idf, you need to have a reasonably large corpus from which to estimate tfidf weights.
You can also develop it further, by using a more sophisticated way to extract words from a piece of text, stem or lemmatise it, etc.