Julia throws undefined error when defining variable

First, note that if you're using Julia v1.5+, then you don't need to do i(the current stable v1.5.4 example below) globally:

julia> i = 1
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julia> while i < 5
           println(i)
           i += 1     # <----- this works in Julia v1.5+
       end
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However, it seems you are using old Julia V1.4, in which case I think Logan Kilpatrick gave you the answer: you need to do ia global scope from the loop inside . As Logan mentioned, try adding the increment's location , like this example from the function's documentation :whileglobaliwhile

julia> i = 1 ;

julia> while i < 5
           println(i)
           global i += 1     # <------------ Try this!
       end
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Also note that you don't need to specify that it's a global if your whileloop is inside a function like

julia> function foo(istart)
           i = istart
           while i < 5
               println(i)
               i += 1     # <-- 'global' not needed inside a function!
           end
       end
foo (generic function with 1 method)

julia> foo(1)
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