Filter and filter! give different results

General Usage
1

I am wondering why filter and filter! give different results in the example below. Case 1 with filter! includes an extra element whereas Case 2 with filter works as expected

mutable struct M
    x::Int
    y::Int
end

Case 1:

m1 = [M(i,i+1) for i in 1:4]
filter!(x->x == m1[2], m1)

Results:

println(m1)
M[M(2, 3), M(3, 4)]

Case 2:

m2 = [M(i,i+1) for i in 1:4]
m3 = filter(x->x == m2[2], m2)

Results:

julia> println(m3)
M[M(2, 3)]
2

I would guess in Case 1 what m1[2] is changes as you mutate m1 with filter!; in other words, each time you evaluate that anonymous function inside filter! you could be getting a different object because the filter is changing the array.

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3

Thanks. That was a silly mistake on my part. Using z = m1[2] fixes the problem.

I am trying to navigate several issues with tests of equality on mutable structs. Evidently, there is a long standing issue. Is there a recommended solution for testing that the fields in two structs are equal? Should I iterate over the fields and use getfield to test the values?

1 Like