Cosine Similarity

Easy Mathematics

LintCode: https://www.lintcode.com/en/problem/cosine-similarity/

Question

Cosine similarity is a measure of similarity between two vectors of an inner product space that measures the cosine of the angle between them. The cosine of 0° is 1, and it is less than 1 for any other angle.

See wiki: Cosine Similarity

Here is the formula:

Given two vectors A and B with the same size, calculate the cosine similarity.

Return 2.0000 if cosine similarity is invalid (for example A = [0] and B = [0]).

Example
Given A = [1, 2, 3] , B = [2, 3 ,4] .

Return 0.9926 .

Given A = [0] , B = [0] .

Return 2.0000 .

Solution

Java

class Solution {
    /**
     * @param A: An integer array.
     * @param B: An integer array.
     * @return: Cosine similarity.
     */
    public double cosineSimilarity(int[] A, int[] B) {
        if (A.length == 0) { // invalid
            return 2d;
        }
        // write your code here
        int abSum = 0;
        int a2Sum = 0;
        int b2Sum = 0;
        for (int i = 0; i < A.length; i++) {
            abSum += A[i] * B[i];
            a2Sum += A[i] * A[i];
            b2Sum += B[i] * B[i];
        }
        if (a2Sum == 0 || b2Sum == 0) { // invalid
            return 2d;
        }
        return abSum / (Math.sqrt(a2Sum) * Math.sqrt(b2Sum));
    }
}