What is the difference between “==” and “===” comparison operators in Julia?

Question

What is the difference between == and === comparison operators in Julia?

Answer 1

=== means that it's actually the same object, i.e. the variables point to the same spot in memory. == means that the objects have the same values. For example:

julia> A = rand(5,5) #Make an array
5x5 Array{Float64,2}:
 0.349193  0.408216   0.703084  0.163128   0.815687
 0.211441  0.0185634  0.378299  0.0734293  0.187445
 0.667637  0.139323   0.286794  0.359962   0.229784
 0.476224  0.49812    0.648244  0.831006   0.1787
 0.960756  0.488886   0.195973  0.148958   0.200619

julia> B = A # This sets the pointer of B to the pointer of A
5x5 Array{Float64,2}:
 0.349193  0.408216   0.703084  0.163128   0.815687
 0.211441  0.0185634  0.378299  0.0734293  0.187445
 0.667637  0.139323   0.286794  0.359962   0.229784
 0.476224  0.49812    0.648244  0.831006   0.1787
 0.960756  0.488886   0.195973  0.148958   0.200619

julia> B === A # Same spot in memory
true

julia> B[1,1]=2 #Change a value of B
2

julia> B
5x5 Array{Float64,2}:
 2.0       0.408216   0.703084  0.163128   0.815687
 0.211441  0.0185634  0.378299  0.0734293  0.187445
 0.667637  0.139323   0.286794  0.359962   0.229784
 0.476224  0.49812    0.648244  0.831006   0.1787
 0.960756  0.488886   0.195973  0.148958   0.200619

julia> A #Also changes A since they point to the same spot
5x5 Array{Float64,2}:
 2.0       0.408216   0.703084  0.163128   0.815687
 0.211441  0.0185634  0.378299  0.0734293  0.187445
 0.667637  0.139323   0.286794  0.359962   0.229784
 0.476224  0.49812    0.648244  0.831006   0.1787
 0.960756  0.488886   0.195973  0.148958   0.200619

julia> B = copy(A) #Now make B a copy of A, no longer the same pointer
5x5 Array{Float64,2}:
 2.0       0.408216   0.703084  0.163128   0.815687
 0.211441  0.0185634  0.378299  0.0734293  0.187445
 0.667637  0.139323   0.286794  0.359962   0.229784
 0.476224  0.49812    0.648244  0.831006   0.1787
 0.960756  0.488886   0.195973  0.148958   0.200619

julia> B === A # Now this is false
false

julia> B == A # This is still true
true

julia> B[1,1] = 1 #Changing B
1

julia> B
5x5 Array{Float64,2}:
 1.0       0.408216   0.703084  0.163128   0.815687
 0.211441  0.0185634  0.378299  0.0734293  0.187445
 0.667637  0.139323   0.286794  0.359962   0.229784
 0.476224  0.49812    0.648244  0.831006   0.1787
 0.960756  0.488886   0.195973  0.148958   0.200619

julia> A #Now does not change A
5x5 Array{Float64,2}:
 2.0       0.408216   0.703084  0.163128   0.815687
 0.211441  0.0185634  0.378299  0.0734293  0.187445
 0.667637  0.139323   0.286794  0.359962   0.229784
 0.476224  0.49812    0.648244  0.831006   0.1787
 0.960756  0.488886   0.195973  0.148958   0.200619