# Stepwise logistic regress - GLM - non-callable --\> callable (Non-call expression encountered)

**URL:** <https://discourse.julialang.org/t/stepwise-logistic-regress-glm-non-callable-callable-non-call-expression-encountered/16087>\
**Category:** Optimization (Mathematical)\
**Tags:** regression, glm\
**Created:** [October 9, 2018, 5:58pm UTC](https://discourse.julialang.org/t/stepwise-logistic-regress-glm-non-callable-callable-non-call-expression-encountered/16087 "2018-10-09T17:58:33Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![tobi](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tobi](https://discourse.julialang.org/u/tobi)\
**Post date:** [October 9, 2018, 5:58pm UTC](https://discourse.julialang.org/t/stepwise-logistic-regress-glm-non-callable-callable-non-call-expression-encountered/16087/1 "2018-10-09T17:58:33Z")

</div>

Hello,

I am trying to write a loop for a logistic regession. I have a dataframe with about 20 columns and I would like to use one of the columns after each other and print the coeftable afterwards. In the end I am aiming to make a stepwise logistic regression.

I’ve got this example code which performs well:

```julia
using DataFrames, GLM

srand(42)
df_test = DataFrame(A = rand(Int, 100), B = rand(Int, 100), C = rand(0:1, 100) )

model_test = glm(@formula(C ~ A + B),
 df_test, Binomial(), LogitLink())
println(coeftable(model_test))

```

But I would like to get all the column names (df\_names) and than tell the model to use this names as an input (df\_names[1]):

```julia
df_names = names(df_test)

model_test = glm(@formula(C ~ df_names[1] + df_names[2]),
 df_test, Binomial(), LogitLink())

```

when I execute the code I get this error message:

```julia
Non-call expression encountered
in glm at GLM\src\glmfit.jl:286
in #glm#13 at GLM\src\glmfit.jl:286
in fit at DataFrames\src\statsmodels\statsmodel.jl:52
in #fit#153 at DataFrames\src\statsmodels\statsmodel.jl:52
in at base\<missing>
in #ModelFrame#127 at DataFrames\src\statsmodels\formula.jl:333
in DataFrames.Terms at DataFrames\src\statsmodels\formula.jl:209
in dospecials at DataFrames\src\statsmodels\formula.jl:101
in map at base\abstractarray.jl:1868
in _collect at base\array.jl:488
in next at base\generator.jl:45
in dospecials at DataFrames\src\statsmodels\formula.jl:97

```

so I would like to make df\_names[1] callable. Does anybody has a suggestion how to make it callable?  
And does anybody has an idea how to performe a stepwise logistic regression?

Cheers!  
Tobi

---

<div class="post-metadata">

**Author:** ![tobi](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tobi](https://discourse.julialang.org/u/tobi)\
**Post date:** [October 9, 2018, 7:12pm UTC](https://discourse.julialang.org/t/stepwise-logistic-regress-glm-non-callable-callable-non-call-expression-encountered/16087/3 "2018-10-09T19:12:44Z")

</div>

thank you for your response.  
unfortunately when I do it like you said I get the same error again

```julia
model_test = glm(@formula(C ~ df_test[:df_names[1]] + df_test[:df_names[2]]),
 df_test, Binomial(), LogitLink())

```

when I just execute:

```julia
df_test[:df_names[1]]

```

I get this error:

```julia
MethodError: no method matching getindex(::Symbol, ::Int64)

```

but when I delete the colon than i get the DataArray

```julia
df_test[df_names[1]]

```

but deleting the colon in the model doesn’t help either.

---

<div class="post-metadata">

**Author:** ![nalimilan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nalimilan/32/147_2.png) [@nalimilan](https://discourse.julialang.org/u/nalimilan)\
**Post date:** [October 9, 2018, 7:25pm UTC](https://discourse.julialang.org/t/stepwise-logistic-regress-glm-non-callable-callable-non-call-expression-encountered/16087/4 "2018-10-09T19:25:09Z")

</div>

You need to use `@eval` or `eval(Meta.parse(...))` on a string that you build. See [https://juliastats.github.io/StatsModels.jl/latest/formula.html#Constructing-a-formula-programmatically-1](https://juliastats.github.io/StatsModels.jl/latest/formula.html#Constructing-a-formula-programmatically-1).

---

<div class="post-metadata">

**Author:** ![pdeffebach](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pdeffebach/32/10320_2.png) [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)\
**Post date:** [October 9, 2018, 7:35pm UTC](https://discourse.julialang.org/t/stepwise-logistic-regress-glm-non-callable-callable-non-call-expression-encountered/16087/5 "2018-10-09T19:35:34Z")

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This is one of those areas that is ripe for a helper package. I currently do this:

```julia
function expandargs(x)
    :(+$(x...))
end
rhs = [:x1, :x2, :x3]
lhs = :y
@eval @formula($lhs ~ $(expandargs(rhs)))

```

---

<div class="post-metadata">

**Author:** ![alejandromerchan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alejandromerchan/32/10500_2.png) [@alejandromerchan](https://discourse.julialang.org/u/alejandromerchan)\
**Post date:** [October 9, 2018, 7:51pm UTC](https://discourse.julialang.org/t/stepwise-logistic-regress-glm-non-callable-callable-non-call-expression-encountered/16087/6 "2018-10-09T19:51:46Z")

</div>

This is clearly a situation that arises due to the use of the @formula macro. Is it possible to make formulas work directly with vectors? Something like:

```julia
# Following the notation from the question
glm(df_test[3] ~ df_test[1] + df_test[2])

```

For what I understand, you can create pass your own model matrix and your response variable to glm, but obviously that’s more complicated for some users.

---

<div class="post-metadata">

**Author:** ![Nosferican](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nosferican/32/9275_2.png) [@Nosferican](https://discourse.julialang.org/u/Nosferican)\
**Post date:** [October 10, 2018, 3:10am UTC](https://discourse.julialang.org/t/stepwise-logistic-regress-glm-non-callable-callable-non-call-expression-encountered/16087/7 "2018-10-10T03:10:56Z")

</div>

You don’t need a macro for it. Here is a minimal example of one approach you could use

```julia
using DataFrames, GLM, Random

Random.seed!(0)
data = DataFrame(A = rand(0:1, 10), B = rand(10), C = rand(10), D = rand(10))

function step_wise(vars)
    model_formula = @formula(A ~ B + C)
    for var ∈ vars
        model_formula.rhs.args[3] = var
        model = glm(model_formula, data, Binomial(), LogitLink())
        println(coeftable(model))
    end
end
step_wise(names(data)[3:4])

```

---

<div class="post-metadata">

**Author:** ![tobi](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tobi](https://discourse.julialang.org/u/tobi)\
**Post date:** [October 10, 2018, 9:46am UTC](https://discourse.julialang.org/t/stepwise-logistic-regress-glm-non-callable-callable-non-call-expression-encountered/16087/8 "2018-10-10T09:46:29Z")

</div>

thank you for your answers.

@Nosferican your solution works very well.

the hint to make it callable with @eval was also very helpful.

If one makes it callable with @eval don’t forget the bracket after $(…

```julia
model_test = @eval glm(@formula(C ~ $(df_names[1]) + B),
 df_test, Binomial(), LogitLink())

```

---

<div class="post-metadata">

**Author:** ![nalimilan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nalimilan/32/147_2.png) [@nalimilan](https://discourse.julialang.org/u/nalimilan)\
**Post date:** [October 10, 2018, 12:11pm UTC](https://discourse.julialang.org/t/stepwise-logistic-regress-glm-non-callable-callable-non-call-expression-encountered/16087/9 "2018-10-10T12:11:18Z")

</div>

We could provide convenience functions to make this easier, and/or improve docs.