What is a Dot Product?

The dot product is a way of multiplying two vectors together. It is calculated by multiplying the corresponding elements of the two vectors and then adding the products together. The result of the dot product is a single number.

Interpreting the Dot Product

The result of the dot product is a single number. This number represents the amount of overlap between the two vectors. In this case, the two vectors have a lot of overlap, so the dot product is a large number.

|a| indicates the magnitude (length) of vector a, |b| the magnitude of vector b, and theta the angle between the two vectors. The magnitudes of the vectors and the cosine of the angle between them are multiplied to get the dot product.

$$\vec{a} \cdot \vec{b} = |a| |b| \cos \theta$$

The dot product is calculated by multiplying the respective vector components and adding the results.

$$\vec{a} \cdot \vec{b} = a_x b_x + a_y b_y + a_z b_z + \ldots$$

If the two vectors are perpendicular to each other, the dot product will be zero. This is because the product of any two perpendicular vectors will always be zero.

If the two vectors are in opposite directions, the dot product will be negative. This is because the product of any two vectors in opposite directions will always be negative.

The dot product is a mathematical operation that takes two vectors and produces a scalar.

A vector represents a quantity characterized by both its magnitude and direction.

The dot product is usefull in tasks such as text classification and recommendation systems, where it helps in quantifying the similarity between vectors. Additionally, it can assist in measuring the distance between vectors, enabling applications such as clustering and dimensionality reduction.

Practical Examples

Suppose we are developing a movie recommendation system in which movies are described on a scale of 0 to 10 based on two features: action and romance.

We have three films:

  • Movie A: Action = 8, Romance = 3
  • Movie B: Action = 2, Romance = 7
  • Movie C: Action = 5, Romance = 5

A user watches Movie A and gives it a high rating. This indicates that the user prefers movies with a high level of action and a moderate level of romance. We may describe the user’s preference as a vector, which is the same as the Movie A vector in this case:

user = np.array([8, 3])

Now we’d like to recommend another film to this user. The dot product can assist us in determining which movie best matches the user’s preferences.

  • The dot product of user and movieB is 29, indicating a moderate overlap.
  • The dot product of user and movieC is 55, suggesting a higher degree of overlap than between user and movieB.

Based on these dot products, we would recommend Movie C to this user because it has a higher dot product, indicating that it is more in accordance with the customer’s preferences.

Visualizing the Dot Product

The following visualization will help you grasp the concepts more intuitively and deepen your comprehension of this essential mathematical operation.

NoteBook: Dot Product


  • Limited to Euclidean Space: The standard dot product is applicable only in Euclidean space.
  • Dependent on Magnitude: The dot product is influenced not only by the angle between two vectors, but also by their magnitude. As a result, two vector pairs with the same orientation but different lengths will have different dot products. This can be a disadvantage in some situations, such as when you’re only concerned with the vectors’ direction.
  • Limited Contextual Information: In text analysis, two documents may have a high dot product because they use similar words, but those words might be used in entirely different contexts, leading to potential misinterpretation of the similarity.
  • No Cross-Dimensional Information: In 3D space, the dot product won’t tell you if two vectors are twisting around each other. This information can only be obtained using a different operation known as the cross product.