# How to define state-dependent delay in delayed differential equations solver in Julia?

**URL:** <https://discourse.julialang.org/t/how-to-define-state-dependent-delay-in-delayed-differential-equations-solver-in-julia/18007>\
**Category:** Modelling & Simulations\
**Created:** [November 26, 2018, 9:06am UTC](https://discourse.julialang.org/t/how-to-define-state-dependent-delay-in-delayed-differential-equations-solver-in-julia/18007 "2018-11-26T09:06:50Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![PeterXG](https://avatars.discourse-cdn.com/v4/letter/p/9de0a6/32.png) [@PeterXG](https://discourse.julialang.org/u/PeterXG)\
**Post date:** [November 26, 2018, 9:06am UTC](https://discourse.julialang.org/t/how-to-define-state-dependent-delay-in-delayed-differential-equations-solver-in-julia/18007/1 "2018-11-26T09:06:50Z")

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Dear All,

I’m transferring from Matlab to Julia to solve delayed differential equations.

I read documentation and notice it says define state-dependent delay by

```julia
prob = DDEProblem(bc_model,u0,h,tspan; dependent_lags = ((u,p,t) -> tau,))

```

but how can I define tau with time t?

for example: lag = sin(t)?

Thank you very much!

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<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [November 27, 2018, 7:34am UTC](https://discourse.julialang.org/t/how-to-define-state-dependent-delay-in-delayed-differential-equations-solver-in-julia/18007/2 "2018-11-27T07:34:20Z")

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> [@PeterXG](#):
>
> for example: lag = sin(t)?

```julia
prob = DDEProblem(bc_model,u0,h,tspan; dependent_lags = ((u,p,t) -> sin(t),))

```

Or you don’t declare the lags. The MATLAB ddesd algorithm is equivalent to undeclared lags with `MethodOfSteps(RK4())`. Usually not declaring lags is bad for performance and accuracy though, so we don’t recommend it.