# How to generate a customized \`for\` loop code?

**URL:** <https://discourse.julialang.org/t/how-to-generate-a-customized-for-loop-code/106552>\
**Category:** General Usage\
**Tags:** question, macros, metaprogramming\
**Created:** [November 21, 2023, 11:22pm UTC](https://discourse.julialang.org/t/how-to-generate-a-customized-for-loop-code/106552 "2023-11-21T23:22:11Z")\
**Posts on this page:** 1\
**Showing post:** 3

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**Author:** ![frankwswang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/frankwswang/32/18561_2.png) [@frankwswang](https://discourse.julialang.org/u/frankwswang)\
**Post date:** [November 22, 2023, 7:21pm UTC](https://discourse.julialang.org/t/how-to-generate-a-customized-for-loop-code/106552/3 "2023-11-22T19:21:30Z")

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Thanks for the reply!

I forgot that the `@generated` function does not support closure, so we can’t write an anonymous function inside an `Expr` to be generated when it is first called with a specific `Val(N)`.

However, I don’t understand why we cannot design a macro to automate the process of generating functions like the following without knowing the types of `x` and `y`:

```julia
function f2(x, y)
    @hyperopt for i=1:2, sampler=y, 
        x1 = x[1],
        x2 = x[2]
        model([x1, x2])
    end
end

function f3(x, y)
    @hyperopt for i=1:2, sampler=y, 
        x1 = x[1],
        x2 = x[2],
        x3 = x[3]
        model([x1, x2, x3])
    end
end

```

As far as I understand, we just need to know how to construct a `for` loop expression without explicitly writing it out but using some functions to build the sub-expressions and concatenate them. I found a possibly relevant [thread](https://discourse.julialang.org/t/how-does-julia-merge-several-exprs-to-one-expr/77498/6) about concatenating assignment expressions, but I don’t know how to make them part of the `for` loop expression. I think if I use the `dump` function you mentioned to analyze the expression structure of a for loop, then I might be able to do it.

If you have better ideas, I would much appreciate it! FYI, an example of those variables are:

```julia
varRanges = LinRange.([0:0.01:1, 0:0.02:2, 0:0.03:6]) # the values (ranges) for all the parameters
numOfSamples = 10 # to be assigned to `i` in the for loop
someSample = Hyperopt.RandomSampler() # The sampler provided by Hyperopt.jl

```

We actually don’t need `pars` if we can name the hyperparameters inside the macro before for loop expression. e.g., construct `pars = [Symbol(:a, j) for j=1:N]`, then interpolate its element values as the variable names for the assignments in the expression.

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