Julia: find row in matrix

Question

Using Julia, I'd like to determine if a row is located in a matrix and (if applicable) where in the matrix the row is located. For example, in Matlab this can done with ismember:

a = [1 2 3];
B = [3 1 2; 2 1 3; 1 2 3; 2 3 1]
B =

 3     1     2
 2     1     3
 1     2     3
 2     3     1

ismember(B, a, 'rows')
ans =

 0
 0
 1
 0

From this, we can see a is located in row 3 of B. Is there a similar function to accomplish this in Julia?

Answer 1

You can also make use of array broadcasting by simply testing for equality (.==) without the use of comprehensions:

all(B .== a, dims=2)

Which gives you:

4x1 BitMatrix:
 0
 0
 1
 0

You can then use findall on this array:

findall(all(B .== a, 2))

However, this gives you a vector of CartesianIndex objects:

1-element Vector{CartesianIndex{2}}:
 CartesianIndex(3, 1)

So if you expect to find multiple rows with the value defined in a you can either:

1. simplify this Vector by taking only the row index from each CartesianIndex:

   [cart_idx[1] for cart_idx in findall(all(B .== a, 2))]
   

2. or pass one dimensional BitMatrix to findall (as suggested by Shep Bryan in the comment):

   findall(all(B .== a, dims=2)[:, 1])
   

Either way you get an integer vector of column indices:

1-element Vector{Int64}:
 3

Answer 2

Another pattern is using array comprehension:

julia> Bool[ a == B[i,:] for i=1:size(B,1) ]
4-element Array{Bool,1}:
false
false
true
false

julia> Int[ a == B[i,:] for i=1:size(B,1) ]
4-element Array{Int64,1}:
0
0
1
0

Answer 3

how about:

matchrow(a,B) = findfirst(i->all(j->a[j] == B[i,j],1:size(B,2)),1:size(B,1))

returns 0 when no matching row, or first row number when there is one.

matchrow(a,B)

3

should be as "fast as possible" and pretty simple too.

Answer 4

Though Julia doesn't have a built-in function, its easy enough as a one-liner.

a = [1 2 3];
B = [3 1 2; 2 1 3; 1 2 3; 2 3 1]
ismember(mat, x, dims) = mapslices(elem -> elem == vec(x), mat, dims)
ismember(B, a, 2)  # Returns booleans instead of ints