"Not defined variable" in a 'while loop' in Julia - while-loop

Question

I am trying to do a sensitivity analysis in Julia using JuMP. Here is my code:
using JuMP, Plots, Gurobi
m=Model(with_optimizer(Gurobi.Optimizer))
#variable(m, x>=0)
#variable(m, y>=0)
#variable(m, k>=0)
k = 0
while k<=1
φ(x,y,k)=3*x+k*y
#objective(m, Max, φ(x,y,k))
#constraint(m, 2*x-4>=0)
#constraint(m, y-0.5*x>=0)
pl=optimize!(m)
k=k+0.2
end
The problem is that I get an error:
UndefVarError: k not defined
What am I missing?

Answer 1

julia> k =0
0
julia> while k<10
k=k+1
end
ERROR: UndefVarError: k not defined
Stacktrace:
[1] top-level scope at ./REPL[11]:2
In julia if we are operating with loops the variables we initialise outside our loop can not be directly accessed within a loop on default. To do that we have to set those variable on to global use as on default they are considered to be local
julia> while k<10
global k=k+1
end
Now this works fine

Answer 2

Disclaimer: This is an alternative solution, that it was suggested by a member of Julia Discorse
In a discussion at Julia Discourse, it is suggested to wrap the code in a function in order to increase speed and to avoid the global issue:
function run_code()
model = Model(with_optimizer(Gurobi.Optimizer))
#variable(model, x >= 0)
#variable(model, y >= 0)
#constraint(model, 2x - 4 >= 0)
k = 0
while k <= 1
#objective(model, Max, 3x + k * y)
optimize!(model)
k = k + 0.2
end
end
run_code()