I do not understand this error: MethodError: no method matching iterate(::DataFrame)

Your example is not an MWE (see Please read: make it easier to help you), but it looks like you’re calling compose(c, ...) in your loop without first defining c.

It might be useful to have a look at the Julia language docs and maybe the documentation for the packages you’re using to get the hang of the basic usage, as step-by-step debugging via Discourse might be pretty time consuming…

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