# Can't I update velocity AND position of a continuous agent using a custom move\_agent! that is not based on forward euler?

**URL:** <https://discourse.julialang.org/t/cant-i-update-velocity-and-position-of-a-continuous-agent-using-a-custom-move-agent-that-is-not-based-on-forward-euler/125697>\
**Category:** Modelling & Simulations\
**Tags:** question, agents\
**Created:** [February 8, 2025, 8:25pm UTC](https://discourse.julialang.org/t/cant-i-update-velocity-and-position-of-a-continuous-agent-using-a-custom-move-agent-that-is-not-based-on-forward-euler/125697 "2025-02-08T20:25:33Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![raylv17](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/raylv17/32/215121_2.png) [@raylv17](https://discourse.julialang.org/u/raylv17)\
**Post date:** [February 8, 2025, 8:25pm UTC](https://discourse.julialang.org/t/cant-i-update-velocity-and-position-of-a-continuous-agent-using-a-custom-move-agent-that-is-not-based-on-forward-euler/125697/1 "2025-02-08T20:25:34Z")

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I am using the latest Agents.jl (v.6.2.4) package and learning it as well. I want to implement a simple movement of different agents by trying out various differential equation solvers (besides the default forward euler provided by `move_agent!`, as [mentioned in the docs](https://juliadynamics.github.io/Agents.jl/stable/api/#Agents.move_agent!))  
I read in the docs, that for continuous agents in continuous space with periodic boundaries, `move_agent!` is used for `agent_step!` function, but for a more complex `agent_step!`, one can also update the velocities in `agent_step!`.

However, I have noticed, that I can maybe _only_ update velocities, because explicitly updating positions can lead to errors that an agent is beyond the defined extent. (is that true?)

this is my code:

```julia
using Agents, Random
using CairoMakie

Random.seed!(42)
@agent struct Pedestrian(ContinuousAgent{2,Float64})
    desired_velocity::Vector{Float64} # Custom property
end

function agent_step!(agent::Pedestrian, model,dt = 0.1)
    # agent.pos = agent.pos + dt*agent.desired_velocity # may lead to out of extent errors
    agent.vel = agent.vel + dt*agent.desired_velocity
    move_agent!(agent,model,dt)
end

# Add agents with random initial positions/velocities
function initialize(number_of_peds = 4, x_len = 10, y_len = 10)
    spacing = 1/20
    space = ContinuousSpace((x_len, y_len); spacing)
    model = StandardABM(
        Pedestrian,
        space;
        agent_step!,
        scheduler = Schedulers.Randomly()
    )
    for i in 1:number_of_peds
        add_agent!(model;
            pos = rand(2).*10, # Initial position
            vel = [0,0], # Initial velocity
            desired_velocity = (i-1 < (number_of_peds ÷ 2) ? [1,0] : [-1,0])
        )
    end
    return model
end

model = initialize()

for i in 1:10
    step!(model)
    plot, = abmplot(model)
    display(plot)
    sleep(0.1)
end

```

What i wish to implement is something similar to a [Leapfrog\_integration](https://en.wikipedia.org/wiki/Leapfrog_integration) where one can update both positions and velocities at each time step.  
Since I am new to Julia, I understand i might be making some mistakes in the code as well.  
Any suggestion/advice would be greatly appreciated, Thank you! 🙂

---

<div class="post-metadata">

**Author:** ![mastrof](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mastrof/32/17443_2.png) [@mastrof](https://discourse.julialang.org/u/mastrof)\
**Post date:** [February 10, 2025, 11:32am UTC](https://discourse.julialang.org/t/cant-i-update-velocity-and-position-of-a-continuous-agent-using-a-custom-move-agent-that-is-not-based-on-forward-euler/125697/2 "2025-02-10T11:32:45Z")

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You can use `normalize_position` to wrap any evaluated position within your model boundaries, e.g. in the line you commented out in your `agent_step!` function you could write instead

```julia
newpos = normalize_position(agent.pos + dt*agent.desired_velocity, model)
move_agent!(agent, newpos, model)

```

So you can just use whatever integrator you want, just call `normalize_position` before trying to update the agent position.

Alternatively, you can use the `walk!` function instead which respects the model boundary conditions without the need for you to do it explicitly.