# Automatic differentiation with a for loop in the function definition
**URL:** https://discourse.julialang.org/t/automatic-differentiation-with-a-for-loop-in-the-function-definition/64196
**Category:** General Usage
**Tags:** question
**Created:** [July 7, 2021, 6:45am UTC](https://discourse.julialang.org/t/automatic-differentiation-with-a-for-loop-in-the-function-definition/64196 "2021-07-07T06:45:34Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@aymercan](https://discourse.julialang.org/u/aymercan)
#### Post date: [July 7, 2021, 6:45am UTC](https://discourse.julialang.org/t/automatic-differentiation-with-a-for-loop-in-the-function-definition/64196/1 "2021-07-07T06:45:34Z")
Hi Julia community,
I am using Julia for my economics research and this is my first question posted here. Hopefully it is a simple one that more experienced users can easily answer.
I am trying to use automatic differentiation to compute the derivative of the two functions below.
```julia
function simplefunction_v1(x)
T = 3
temp = x.^(0:T)
return temp
end
function simplefunction_v2(x)
T = 3
temp = zeros(T+1)
temp[1] = 1.0
temp[2] = x
for iT = 3:T+1
temp[iT] = x*temp[iT-1]
end
return temp
end
```
These two functions return equivalent results for a scalar x. I run the following to use the ForwardDiff package to calculate the derivative of each function evaluated at x=2.0.
```julia
using ForwardDiff
ForwardDiff.derivative(simplefunction_v1,2.0)
ForwardDiff.derivative(simplefunction_v2,2.0)
```
While the second line (simplefunction\_v1) above works as expected, I get an error from running the third line (simplefunction\_v2).
It goes without saying that my actual application is quite a bit more involved, where I cannot get rid of the recursive relationship in the function definition. I would greatly appreciate any direction as to how to get the second version of the function working with automatic differentiation.
Thanks a lot.
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [July 7, 2021, 9:50am UTC](https://discourse.julialang.org/t/automatic-differentiation-with-a-for-loop-in-the-function-definition/64196/2 "2021-07-07T09:50:55Z")
> [@aymercan](#):
>
> ```julia
> temp = zeros(T+1)
>
> ```
is not generic. `temp = zeros(eltype(x),T+1)`
---
### Author:  [@aymercan](https://discourse.julialang.org/u/aymercan)
#### Post date: [July 7, 2021, 9:55am UTC](https://discourse.julialang.org/t/automatic-differentiation-with-a-for-loop-in-the-function-definition/64196/4 "2021-07-07T09:55:34Z")
Perfect. Thank you!