Change values in a data set in Julia - r

Question

I am converting a function in R to Julia, but I do not know how to convert the following R code:
x[x==0]=4
Basically, x contains rows of numbers, but whenever there is a 0, I need to change it to a 4. The data set x comes from a binomial distribution. Can someone help me define the above code in Julia?

Answer 1

Use the .== (broadcasted ==), ie:
Dot Syntax for Vectorizing Functions
With vector:
julia> x = round.(Int, rand(5)) # notice how round is also broadcasted here
5-element Array{Int64,1}:
0
0
1
0
1
julia> x .== 0
5-element BitArray{1}:
true
true
false
true
false
julia> x[x .== 0] = 4
4
julia> x
5-element Array{Int64,1}:
4
4
1
4
1
With matrix:
julia> y = round.(Int, rand(5, 5))
h5×5 Array{Int64,2}:
0 1 1 0 0
1 0 1 1 1
0 0 0 0 1
1 1 0 0 0
0 1 0 1 1
julia> y[y .== 0] = 4
4
julia> y
5×5 Array{Int64,2}:
4 1 1 4 4
1 4 1 1 1
4 4 4 4 1
1 1 4 4 4
4 1 4 1 1
With dataframe:
julia> using DataFrames
julia> df = DataFrame(x = round.(Int, rand(5)), y = round.(Int, rand(5)))
5×2 DataFrames.DataFrame
│ Row │ x │ y │
├─────┼───┼───┤
│ 1 │ 0 │ 0 │
│ 2 │ 0 │ 1 │
│ 3 │ 0 │ 0 │
│ 4 │ 0 │ 1 │
│ 5 │ 1 │ 0 │
julia> df[:x][df[:x] .== 0] = 4
4
julia> df
5×2 DataFrames.DataFrame
│ Row │ x │ y │
├─────┼───┼───┤
│ 1 │ 4 │ 0 │
│ 2 │ 4 │ 1 │
│ 3 │ 4 │ 0 │
│ 4 │ 4 │ 1 │
│ 5 │ 1 │ 0 │

Answer 2

The simplest solution is to use the replace! function:
replace!(x, 0=>4)
Use replace(x, 0=>4) (without the !) to do the same thing, but creating a copy of the vector.
Note that these functions only exist in version 0.7!

Answer 3

Two small issues two long for a comment are:
In Julia 0.7 you should write x[x .== 0] .= 4 (using a second dot in assignment also)
In general it is faster to use e.g. foreach or a loop than to allocate a vector with x .== 0, e.g.:
julia> using BenchmarkTools
julia> x = rand(1:4, 10^8);
julia> function f1(x)
x[x .== 4] .= 0
end
f1 (generic function with 1 method)
julia> function f2(x)
foreach(i -> x[i] == 0 && (x[i] = 4), eachindex(x))
end
f2 (generic function with 1 method)
julia> #benchmark f1($x)
BenchmarkTools.Trial:
memory estimate: 11.93 MiB
allocs estimate: 10
--------------
minimum time: 137.889 ms (0.00% GC)
median time: 142.335 ms (0.00% GC)
mean time: 143.145 ms (1.08% GC)
maximum time: 160.591 ms (0.00% GC)
--------------
samples: 35
evals/sample: 1
julia> #benchmark f2($x)
BenchmarkTools.Trial:
memory estimate: 0 bytes
allocs estimate: 0
--------------
minimum time: 86.904 ms (0.00% GC)
median time: 87.916 ms (0.00% GC)
mean time: 88.504 ms (0.00% GC)
maximum time: 91.289 ms (0.00% GC)
--------------
samples: 57
evals/sample: 1