Invalid iteration specification, what does it mean?

New to Julia
1

I would like to write a function in a .jl file, that can recursively produce polynomials for me. However I got the following error message: invalid iteration specification.

> function P(n)
    Pn(x)=1  ##if n==0 Pn(x)=1
    global  i=0 ##declare i for the for loop
    if n!=0
        Pn_1(x)=1
        Pn(x)=x
        seged(x)=x      ##declare the function for the for loop
        for global i in 1:n-1
            seged(x)=(((2*i+1)*x*Pn(x)-(i)*Pn_1(x))/(i+1)) ##the formula for the recursive function
            Pn_1(x)= P(x)
            Pn(x)=seged(x)
        end
    end
    return Pn(x)
end

Also I have come from Octave and this code run without errors back there.

2

Afaik this is not valid syntax in Julia. Just remove the global (why would you need it?)

2 Likes
3

The error is thrown when the text fails to parse to a valid Julia expression.

julia> :(for global i in iter end)
ERROR: syntax: invalid iteration specification
Stacktrace:
 [1] top-level scope
   @ none:1

It’s also redundant, you already had the global i declaration earlier. A for loop (or let block) header introduces a new local variable into their scope by default, so if you want to reassign a global variable, you could 1) rename the iteration variable e.g. for i2 in 1:n-1 and assign i = i2 in the body, or 2) specify for outer i in 1:n-1 to borrow i from the outer scope.

Mostly unrelated to the error, but expect more syntax and design differences in totally unrelated languages, this isn’t like Octave cloning MATLAB.

4

The closest syntax we have to this is the outer keyword. Julia is not a MATLAB or Octave emulator, so you should not expect syntax that works there to just work in Julia.

julia> i = 0
0

julia> function f()
           global i = 0
           for outer i in 1:10
           end
           return i
       end
f (generic function with 1 method)

julia> f()
10

julia> i
10

Our analysis of your code is that you probably do not need the make i global or use outer. If that is the case, then making it global will result in unnecessarily slow code.

1 Like