# Modifying the input parameters to function in a function while solving differential equations

**URL:** <https://discourse.julialang.org/t/modifying-the-input-parameters-to-function-in-a-function-while-solving-differential-equations/43985>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [July 30, 2020, 3:03pm UTC](https://discourse.julialang.org/t/modifying-the-input-parameters-to-function-in-a-function-while-solving-differential-equations/43985 "2020-07-30T15:03:54Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![aerorohit](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aerorohit/32/22314_2.png) [@aerorohit](https://discourse.julialang.org/u/aerorohit)\
**Post date:** [July 30, 2020, 3:03pm UTC](https://discourse.julialang.org/t/modifying-the-input-parameters-to-function-in-a-function-while-solving-differential-equations/43985/1 "2020-07-30T15:03:54Z")

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I am new to Julia. Some workshops from this year’s JuliaCon really impressed me and I started exploring… This might be quite a basic question.

I am trying to implement a callback while solving an ODE problem. I want the callback to be based on a value of the passed parameter which I want to modify in the function itself. This is my function.

```julia
function df(du, u, (bool_burst, bp, p), t)
    .....
    
    press = #some calculated value
 
    bool_burst = press<bp
    
    
    .....
    
end

function condition(u, t, integrator)
    @show integrator.p[1]
    integrator.p[1]==1
    
end 

function affect!(integrator)
    .....
end

```

In my case, the value of `bool_burst` is not changing when I use @show to print the value.  
Thanks in advance.

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**Author:** ![contradict](https://avatars.discourse-cdn.com/v4/letter/c/ac91a4/32.png) [@contradict](https://discourse.julialang.org/u/contradict)\
**Post date:** [July 30, 2020, 11:15pm UTC](https://discourse.julialang.org/t/modifying-the-input-parameters-to-function-in-a-function-while-solving-differential-equations/43985/2 "2020-07-30T23:15:30Z")

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It is a little hard to tell without seeing more code, but with what you posted, I do not expect the change in `bool_burst` to be visible outside of `du`. I think you need to add `press` to your state vector `u` and then use a continuous callback like

```julia
function continuous_condition(u, t, integrator)
   press = u[some index]
   return press - integrator.p[1]
end

```

Then `affect!` will be called when `continuous_condition` crosses zero.

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**Author:** ![aerorohit](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aerorohit/32/22314_2.png) [@aerorohit](https://discourse.julialang.org/u/aerorohit)\
**Post date:** [August 4, 2020, 3:19pm UTC](https://discourse.julialang.org/t/modifying-the-input-parameters-to-function-in-a-function-while-solving-differential-equations/43985/3 "2020-08-04T15:19:45Z")

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Thank You for your response @contradict.  
Actually `press` is standard atmospheric pressure calculated using a separate function, at given altitude which is tracked in `du`.

```julia
function df(du, u, (bp,p), t)
    x = u[1]
    y = u[2]
    z = u[3]
    vx = u[4]
    vy = u[5]
    vz = u[6]
    Tib = u[7]
    Tf = u[8]
    mg = u[9]
    
    a = atm(floor(y))
    
    ## here press is calculated
    ρ , press, T, ν = ISAdata(y)

    ####
    
    g = 9.81
    
    
    
    rn = 3* mg *p["mol_m_a"]
    rd = 4*π*ρ*p["mol_m_g"]
    r = (rn/rd)^(1.0/3)
    area = π*r^2
    
    Re = ρ*sqrt(vx^2 + vy^2 + vz^2)*2*r/ν
    cib = press/(8314.0 * Tib)
    ρg = cib*p["mol_m_g"]
    
    Sca = ν/(ρ*2*r)
    Scg = Sca
    
    
    acc_y = get_forces_x( p["mp"] + mg, mg, ρ, g, p["cd_up"], vy, area, p["mol_m_g"], p["mol_m_a"])/(mg+ p["mp"])
    dm = get_dm(T, press, 2*r, Re, Scg, Sca, cib, p)
    dTf, dTib = DT(Tf, T, Tib, Re, 2*r, ρg, g, 0.0, press, y, t, mg, p)

    du[1] = 0
    du[2] = vy
    du[3] = 0
    du[4] = 0
    du[5] = acc_y
    du[6] = 0
    du[7] = dTib
    du[8] = dTf
    du[9] = -dm
    
end

function check(u, t, integrator)
   ##Here we have to calculate pressure (press) again to compare, but it is extra calculation for no reason
    press = atm(u[2]).pressure #same as ISAdata used above but this gives only pressure.
    p = press - integrator.p[1]
    p
    
end 

```

I wanted to not calculate `press` again inside `check` .

Your answer will definitely work. If there is any other way where I don’t need to track `press` in `du`. That would be great.

Thank you.

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<div class="post-metadata">

**Author:** ![contradict](https://avatars.discourse-cdn.com/v4/letter/c/ac91a4/32.png) [@contradict](https://discourse.julialang.org/u/contradict)\
**Post date:** [August 4, 2020, 5:49pm UTC](https://discourse.julialang.org/t/modifying-the-input-parameters-to-function-in-a-function-while-solving-differential-equations/43985/4 "2020-08-04T17:49:45Z")

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I see, I think recompute is the way to go here. In the presence of adaptive time stepping, attempting to keep track of the computed value independent of the state vector would be quite difficult. I would not be surprised to learn that the solver might call `check()` at a time for which an interpolated value of `u` was used, so no corresponding call to `df()` existed.

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<div class="post-metadata">

**Author:** ![aerorohit](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aerorohit/32/22314_2.png) [@aerorohit](https://discourse.julialang.org/u/aerorohit)\
**Post date:** [August 4, 2020, 6:30pm UTC](https://discourse.julialang.org/t/modifying-the-input-parameters-to-function-in-a-function-while-solving-differential-equations/43985/5 "2020-08-04T18:30:31Z")

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Thank you @contradict. It was very helpful. I was not aware of the fact that `check()` can be called even without `df()`, which seems quite possible.  
Thank you.