# Question: how to get dot products from size(x, n) matrix and length(n) vector
**URL:** https://discourse.julialang.org/t/question-how-to-get-dot-products-from-size-x-n-matrix-and-length-n-vector/83257
**Category:** Statistics
**Created:** [June 23, 2022, 3:36pm UTC](https://discourse.julialang.org/t/question-how-to-get-dot-products-from-size-x-n-matrix-and-length-n-vector/83257 "2022-06-23T15:36:58Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@bwanab](https://discourse.julialang.org/u/bwanab)
#### Post date: [June 23, 2022, 3:36pm UTC](https://discourse.julialang.org/t/question-how-to-get-dot-products-from-size-x-n-matrix-and-length-n-vector/83257/1 "2022-06-23T15:36:58Z")
Matrix: X2, Vector: w2
What I’m trying to do is get the vector of dot products from each row of the matrix with the vector. The best I’ve come up with so far is:
```julia
sum(X2 .* w2', dims=1)
```
This works, but I have this nagging feeling that there’s a way to do it that makes it more obvious I’m doing dot products.
I benchmarked:
```julia
[dot(X2[i,:], w2) for i in 1:size(X2)[1]]
```
But, that’s an order of magnitude slower.
---
### Author:  [@nilshg](https://discourse.julialang.org/u/nilshg)
#### Post date: [June 23, 2022, 3:55pm UTC](https://discourse.julialang.org/t/question-how-to-get-dot-products-from-size-x-n-matrix-and-length-n-vector/83257/2 "2022-06-23T15:55:52Z")
Could you clarify what output you are expecting? For
```julia
julia> X2 = reshape(1:6, 3, 2)
3×2 reshape(::UnitRange{Int64}, 3, 2) with eltype Int64:
1 4
2 5
3 6
julia> w2 = [7, 8]
2-element Vector{Int64}:
7
8
```
your two proposed code snippets give:
```julia
julia> sum(X2 .* w2', dims=1)
1×2 Matrix{Int64}:
42 120
julia> [dot(X2[i,:], w2) for i in 1:size(X2)[1]]
3-element Vector{Int64}:
39
54
69
```
The second looks more in line with your question (three rows and three elements in `w2` yielding three dot products). If that’s indeed the output you’re after I’d suggest:
```julia
julia> X2 * w2
3-element Vector{Int64}:
39
54
69
```
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [June 23, 2022, 3:58pm UTC](https://discourse.julialang.org/t/question-how-to-get-dot-products-from-size-x-n-matrix-and-length-n-vector/83257/3 "2022-06-23T15:58:44Z")
shouldn’t
```julia
X2 * w2
julia> X2 = rand(1:3, 4, 3)
4×3 Matrix{Int64}:
3 1 1
1 2 3
3 1 1
2 3 3
julia> x2 = [1,2,3]
3-element Vector{Int64}:
1
2
3
julia> X2 * x2
4-element Vector{Int64}:
8
14
8
17
```
just work?
---
### Author:  [@bwanab](https://discourse.julialang.org/u/bwanab)
#### Post date: [June 23, 2022, 6:50pm UTC](https://discourse.julialang.org/t/question-how-to-get-dot-products-from-size-x-n-matrix-and-length-n-vector/83257/4 "2022-06-23T18:50:11Z")
You’re right. I’d been trying a bunch of different things in the repl, but I posted the wrong thing when I posted. It should have been:
```julia
sum(X2' .* w2, dims=1)
```
or
```julia
sum(X2 .* w2', dims=2)
```
But, I think the winner is X2 \* w2. Stupid of me for not thinking of that, but thanks to you guys for looking at it and setting me on the right path!