CODEWITHSUNDEEP
julia
Adam
Adam

Reputation: 1017

Julia: boolean comparisons with arrays

I have a simple question about boolean comparisons in Julia. How do I translate the following Matlab code to Julia?

Matlab:

% create parameters
o = -3;
mat = [65 -4; 65 -3; 65 -2]

% determine which rows of matrix have column 2 less than o AND column 1 equal to 65.
result = (o < mat(:,2) & mat(:,1) == 65)

I've tried the following in Julia:

# create parameters
o = -3
mat = zeros(3,2)
mat[:,1] = 65
mat[1,2] = -4
mat[2,2] = -3
mat[3,2] = -2
mat

# attempt to create desired result
o .< mat[:,2]                                # this part works
mat[:,1] .== 65                              # this part works
test = (o .< mat[:,2] && mat[:,1] .== 65)    # doesn't work
test = (o .< mat[:,2] .& mat[:,1] .== 65)    # doesn't work
test = (o .< mat[:,2] & mat[:,1] .== 65)     # doesn't work

Upvotes: 9

Views: 1553

Answers (2)

mbauman
mbauman

Reputation: 31372

It's a matter of operator precedence. & has a higher precedence in Julia than it does in Matlab. Just shift around your parentheses:

test = (o .< mat[:,2]) .& (mat[:,1] .== 65)

See Noteworthy differences from Matlab in the manual for more details (and it's worth reading through the other differences, too).

Upvotes: 8

David P. Sanders
David P. Sanders

Reputation: 5325

Note that you can use the same array creation syntax in Julia:

julia> mat = [65 -4; 65 -3; 65 -2]
3x2 Array{Int64,2}:
65  -4
65  -3
65  -2

You can also use find to get a list of the resulting indices:

o = -3
test = (o .< mat[:,2]) & (mat[:,1] .== 65)

julia> find(test)
1-element Array{Int64,1}:
3

Upvotes: 1

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