# Drop array dimension with Statistics.mean etc
**URL:** https://discourse.julialang.org/t/drop-array-dimension-with-statistics-mean-etc/28759
**Category:** General Usage
**Tags:** array
**Created:** [September 14, 2019, 12:35pm UTC](https://discourse.julialang.org/t/drop-array-dimension-with-statistics-mean-etc/28759 "2019-09-14T12:35:38Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@leethargo](https://discourse.julialang.org/u/leethargo)
#### Post date: [September 14, 2019, 12:35pm UTC](https://discourse.julialang.org/t/drop-array-dimension-with-statistics-mean-etc/28759/1 "2019-09-14T12:35:38Z")
I’m using aggregations such as `Statistics.mean` on multi-dimension arrays, working across a specific dimension. The default behavior is to return an Array with the same number of dimensions, but only one entry in the aggregated dimension. That is:
```julia
julia> using Statistics
julia> A = [1 2; 3 4]
2×2 Array{Int64,2}:
1 2
3 4
julia> mean(A)
2.5
julia> mean(A, dims=1)
1×2 Array{Float64,2}:
2.0 3.0
julia> mean(A, dims=1)[1, :]
2-element Array{Float64,1}:
2.0
3.0
```
Since I usually want an array with a reduced number of dimensions, I apply the `[1, :]` to the result. But I find this annoying and somewhat error-prone.
Question 1: What work-around do you know for this use-case?
Question 2: Why would you prefer the current behavior of `mean` over the one I want?
---
### Author:  [@leethargo](https://discourse.julialang.org/u/leethargo)
#### Post date: [September 14, 2019, 12:37pm UTC](https://discourse.julialang.org/t/drop-array-dimension-with-statistics-mean-etc/28759/2 "2019-09-14T12:37:29Z")
For completeness-sake, if I want to take the row-wise means, I’m using
```julia
julia> mean(A, dims=2)[:, 1]
2-element Array{Float64,1}:
1.5
3.5
```
Note that when the keyword argument `dim` is not used, a scalar value is returned, not a `1x1 Array`.
---
### Author:  [@tim.holy](https://discourse.julialang.org/u/tim.holy)
#### Post date: [September 14, 2019, 12:45pm UTC](https://discourse.julialang.org/t/drop-array-dimension-with-statistics-mean-etc/28759/3 "2019-09-14T12:45:16Z")
You can easily do this:
```julia
julia> dropmean(A; dims=:) = dropdims(mean(A; dims=dims); dims=dims)
dropmean (generic function with 1 method)
julia> A = rand(5, 7);
julia> dropmean(A, dims=1)
7-element Array{Float64,1}:
0.37404294862855475
0.26512340497828324
0.4450455597397507
0.5827853682438383
0.7073441513338029
0.5301565274545441
0.4448844069896148
```
To your question 2, how about this?
```julia
julia> Anorm = A ./ sum(A, dims=1)
5×7 Array{Float64,2}:
0.358107 0.643074 0.411948 0.154884 0.26041 0.355657 0.17027
0.0519266 0.180166 0.136706 0.209475 0.0600134 0.20015 0.00529412
0.272589 0.12818 0.126928 0.286857 0.260759 0.035573 0.443182
0.120199 0.0335493 0.29278 0.0304732 0.203559 0.362079 0.0478683
0.197178 0.0150304 0.0316369 0.31831 0.215259 0.0465401 0.333385
julia> sum(Anorm, dims=1)
1×7 Array{Float64,2}:
1.0 1.0 1.0 1.0 1.0 1.0 1.0
```
Keeping the dimensions ensures alignment.
---
### Author:  [@leethargo](https://discourse.julialang.org/u/leethargo)
#### Post date: [September 14, 2019, 12:47pm UTC](https://discourse.julialang.org/t/drop-array-dimension-with-statistics-mean-etc/28759/4 "2019-09-14T12:47:20Z")
Thanks, I didn’t know about `dropdims`, which makes it easy, indeed.
I was worried that `mymean` would need to look at the number of dimensions of the input array to produce the appropriate `[:, :, :, ...]`.