# Accessing 2D array/matrix values with Booleans
**URL:** https://discourse.julialang.org/t/accessing-2d-array-matrix-values-with-booleans/31384
**Category:** General Usage
**Tags:** question
**Created:** [November 22, 2019, 11:52am UTC](https://discourse.julialang.org/t/accessing-2d-array-matrix-values-with-booleans/31384 "2019-11-22T11:52:29Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@keeler](https://discourse.julialang.org/u/keeler)
#### Post date: [November 22, 2019, 11:52am UTC](https://discourse.julialang.org/t/accessing-2d-array-matrix-values-with-booleans/31384/1 "2019-11-22T11:52:29Z")
Take a matrix A and a vector of Booleans , say, booleRand (whose length is the same as the number of columns or rows in A). I want to be able to access entire rows or columns using the Boolean vector. I am a bit surprised that I couldn’t find an answer to this.
I thought it would be something like this (which is how you do it in MATLAB and NumPy).
```julia
A=[1 2 3;4 5 6;7 8 9]
p=[0.2 0.4 0.3]
booleRand=rand(1,3).>p
A[:, booleRand] # This does NOT work.
p[booleRand] # This does works.
```
The errors:
> ERROR: BoundsError: attempt to access 3×3 Array{Int64,2} at index [Base.Slice(Base.OneTo(3)), Base.LogicalIndex(Bool[true false true])]
Perhaps it has something to do with “true indices”? But what is a true index? Is that using a single index to access a value of a 2/n-D array? ie A[5] which gives 5, just like A[2,2]?
---
### Author:  [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)
#### Post date: [November 22, 2019, 11:55am UTC](https://discourse.julialang.org/t/accessing-2d-array-matrix-values-with-booleans/31384/2 "2019-11-22T11:55:33Z")
Make p and boolRand vectors instead of matrices and it will work.
```julia
julia> p=[0.2, 0.4 , 0.3]
3-element Array{Float64,1}:
0.2
0.4
0.3
julia> booleRand=rand(3).>p
3-element BitArray{1}:
true
true
true
julia> A[:, booleRand]
3×3 Array{Int64,2}:
1 2 3
4 5 6
7 8 9
```
---
### Author:  [@keeler](https://discourse.julialang.org/u/keeler)
#### Post date: [November 22, 2019, 2:00pm UTC](https://discourse.julialang.org/t/accessing-2d-array-matrix-values-with-booleans/31384/3 "2019-11-22T14:00:16Z")
Right. Subtle. Thanks.
So the lesson here is rand(n) is not the same as (or equivalent to) rand(n,1) (or rand(1,n)).
---
### Author:  [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)
#### Post date: [November 22, 2019, 2:03pm UTC](https://discourse.julialang.org/t/accessing-2d-array-matrix-values-with-booleans/31384/4 "2019-11-22T14:03:26Z")
That’s right, Julia differentiates between vectors and matrices, something matlab mostly doesn’t.