# Callback Function Example in DifferentialEquations.jl

**URL:** https://discourse.julialang.org/t/callback-function-example-in-differentialequations-jl/102371
**Category:** New to Julia
**Tags:** diffeq, diffeqcallbacks
**Created:** [August 1, 2023, 7:51pm UTC](https://discourse.julialang.org/t/callback-function-example-in-differentialequations-jl/102371 "2023-08-01T19:51:30Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![yoshi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yoshi/32/38259_2.png) [@yoshi](https://discourse.julialang.org/u/yoshi)
#### Post date: [August 1, 2023, 7:51pm UTC](https://discourse.julialang.org/t/callback-function-example-in-differentialequations-jl/102371/1 "2023-08-01T19:51:30Z")

</div>

I’m trying to understand how to use callback functions with Julia’s solve package. I’m using the code example pasted below and obtaining an error. I’m struggling to understand what I need to fix. Any ideas?

```julia
using DifferentialEquations

# Define your differential equation and initial condition
function f(dx, x, p, t)
    # Define your differential equation here
    dx[1] = -2.0 * x[1] # Example: dx/dt = -2x
end
x0 = [1.0] # Initial condition

# Time span
tspan = (0.0, 2.0) # From t = 0 to t = 2

# Define the problem
prob = ODEProblem(f, x0, tspan)

# Create the solver
solver = Tsit5() # Choose the solver (e.g., Tsit5, RK4, etc.)

# Define a callback function to print the solution at specific time points
function callback_func(sol, t, integrator)
    if t == 0.5 || t == 1.0
        println("Time: ", t, ", Solution: ", sol(t))
    end
    return false
end

# Solve the problem and get the output
sol = solve(prob, solver, callback=callback_func)

```

```julia
ERROR: MethodError: no method matching CallbackSet(::typeof(callback_func))
Closest candidates are:
  CallbackSet() at ~/.julia/packages/SciMLBase/BC0Kw/src/callbacks.jl:339
  CallbackSet(::SciMLBase.AbstractContinuousCallback) at ~/.julia/packages/SciMLBase/BC0Kw/src/callbacks.jl:338
  CallbackSet(::SciMLBase.AbstractDiscreteCallback) at ~/.julia/packages/SciMLBase/BC0Kw/src/callbacks.jl:337
  ...
Stacktrace:
 [1] __init(prob::ODEProblem{Vector{Float64}, Tuple{Float64, Float64}, true, SciMLBase.NullParameters, ODEFunction{true, typeof(f), LinearAlgebra.UniformScaling{Bool}, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, typeof(SciMLBase.DEFAULT_OBSERVED), Nothing}, Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}, SciMLBase.StandardODEProblem}, alg::Tsit5{typeof(OrdinaryDiffEq.trivial_limiter!), typeof(OrdinaryDiffEq.trivial_limiter!), Static.False}, timeseries_init::Tuple{}, ts_init::Tuple{}, ks_init::Tuple{}, recompile::Type{Val{true}}; saveat::Tuple{}, tstops::Tuple{}, d_discontinuities::Tuple{}, save_idxs::Nothing, save_everystep::Bool, save_on::Bool, save_start::Bool, save_end::Nothing, callback::typeof(callback_func), dense::Bool, calck::Bool, dt::Float64, dtmin::Nothing, dtmax::Float64, force_dtmin::Bool, adaptive::Bool, gamma::Rational{Int64}, abstol::Nothing, reltol::Nothing, qmin::Rational{Int64}, qmax::Int64, qsteady_min::Int64, qsteady_max::Int64, beta1::Nothing, beta2::Nothing, qoldinit::Rational{Int64}, controller::Nothing, fullnormalize::Bool, failfactor::Int64, maxiters::Int64, internalnorm::typeof(DiffEqBase.ODE_DEFAULT_NORM), internalopnorm::typeof(LinearAlgebra.opnorm), isoutofdomain::typeof(DiffEqBase.ODE_DEFAULT_ISOUTOFDOMAIN), unstable_check::typeof(DiffEqBase.ODE_DEFAULT_UNSTABLE_CHECK), verbose::Bool, timeseries_errors::Bool, dense_errors::Bool, advance_to_tstop::Bool, stop_at_next_tstop::Bool, initialize_save::Bool, progress::Bool, progress_steps::Int64, progress_name::String, progress_message::typeof(DiffEqBase.ODE_DEFAULT_PROG_MESSAGE), userdata::Nothing, allow_extrapolation::Bool, initialize_integrator::Bool, alias_u0::Bool, alias_du0::Bool, initializealg::OrdinaryDiffEq.DefaultInit, kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ OrdinaryDiffEq ~/.julia/packages/OrdinaryDiffEq/eqC5v/src/solve.jl:208
 [2] __solve(::ODEProblem{Vector{Float64}, Tuple{Float64, Float64}, true, SciMLBase.NullParameters, ODEFunction{true, typeof(f), LinearAlgebra.UniformScaling{Bool}, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, typeof(SciMLBase.DEFAULT_OBSERVED), Nothing}, Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}, SciMLBase.StandardODEProblem}, ::Tsit5{typeof(OrdinaryDiffEq.trivial_limiter!), typeof(OrdinaryDiffEq.trivial_limiter!), Static.False}; kwargs::Base.Pairs{Symbol, typeof(callback_func), Tuple{Symbol}, NamedTuple{(:callback,), Tuple{typeof(callback_func)}}})
   @ OrdinaryDiffEq ~/.julia/packages/OrdinaryDiffEq/eqC5v/src/solve.jl:4
 [3] solve_call(_prob::ODEProblem{Vector{Float64}, Tuple{Float64, Float64}, true, SciMLBase.NullParameters, ODEFunction{true, typeof(f), LinearAlgebra.UniformScaling{Bool}, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, typeof(SciMLBase.DEFAULT_OBSERVED), Nothing}, Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}, SciMLBase.StandardODEProblem}, args::Tsit5{typeof(OrdinaryDiffEq.trivial_limiter!), typeof(OrdinaryDiffEq.trivial_limiter!), Static.False}; merge_callbacks::Bool, kwargshandle::KeywordArgError, kwargs::Base.Pairs{Symbol, typeof(callback_func), Tuple{Symbol}, NamedTuple{(:callback,), Tuple{typeof(callback_func)}}})
   @ DiffEqBase ~/.julia/packages/DiffEqBase/KouNZ/src/solve.jl:437
 [4] solve_up(prob::ODEProblem{Vector{Float64}, Tuple{Float64, Float64}, true, SciMLBase.NullParameters, ODEFunction{true, typeof(f), LinearAlgebra.UniformScaling{Bool}, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, typeof(SciMLBase.DEFAULT_OBSERVED), Nothing}, Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}, SciMLBase.StandardODEProblem}, sensealg::Nothing, u0::Vector{Float64}, p::SciMLBase.NullParameters, args::Tsit5{typeof(OrdinaryDiffEq.trivial_limiter!), typeof(OrdinaryDiffEq.trivial_limiter!), Static.False}; kwargs::Base.Pairs{Symbol, typeof(callback_func), Tuple{Symbol}, NamedTuple{(:callback,), Tuple{typeof(callback_func)}}})
   @ DiffEqBase ~/.julia/packages/DiffEqBase/KouNZ/src/solve.jl:780
 [5] solve(prob::ODEProblem{Vector{Float64}, Tuple{Float64, Float64}, true, SciMLBase.NullParameters, ODEFunction{true, typeof(f), LinearAlgebra.UniformScaling{Bool}, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, typeof(SciMLBase.DEFAULT_OBSERVED), Nothing}, Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}, SciMLBase.StandardODEProblem}, args::Tsit5{typeof(OrdinaryDiffEq.trivial_limiter!), typeof(OrdinaryDiffEq.trivial_limiter!), Static.False}; sensealg::Nothing, u0::Nothing, p::Nothing, kwargs::Base.Pairs{Symbol, typeof(callback_func), Tuple{Symbol}, NamedTuple{(:callback,), Tuple{typeof(callback_func)}}})
   @ DiffEqBase ~/.julia/packages/DiffEqBase/KouNZ/src/solve.jl:760
 [6] top-level scope
   @ REPL[8]:2

```

---

<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: [August 1, 2023, 9:12pm UTC](https://discourse.julialang.org/t/callback-function-example-in-differentialequations-jl/102371/2 "2023-08-01T21:12:44Z")

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There is no such thing as just using a function like that. Did you mean to define a DiscreteCallback? See the documentation:

[https://docs.sciml.ai/DiffEqDocs/stable/features/callback\_functions/](https://docs.sciml.ai/DiffEqDocs/stable/features/callback_functions/)

---

<div class="post-metadata">

### Author: ![yoshi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yoshi/32/38259_2.png) [@yoshi](https://discourse.julialang.org/u/yoshi)
#### Post date: [August 1, 2023, 9:45pm UTC](https://discourse.julialang.org/t/callback-function-example-in-differentialequations-jl/102371/3 "2023-08-01T21:45:54Z")

</div>

Yes – I think I did, I see there’s a code example too here. thanks!