# Child of Urban (System Dynamics)

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;; System dynamics model globals globals [ ;; stock values y1 y2 y3 ;; size of each step, see SYSTEM-DYNAMICS-GO dt ;; Additional global variable sigma r b ] ;; Initializes the system dynamics model. ;; Call this in your model's SETUP procedure. to system-dynamics-setup reset-ticks set dt 0.001 ;; initialize stock values set y1 random-float 0.0 set y2 random-float 1.0 set y3 random-float 0.0 ;; initialize sigma set sigma 10.0 ;; initialize r set r 28.0 ;; initialize b set b 2.6667 end ;; Step through the system dynamics model by performing next iteration of Euler's method. ;; Call this in your model's GO procedure. to system-dynamics-go ;; compute variable and flow values once per step let local-y1' y1' let local-y2' y2' let local-y3' y3' ;; update stock values ;; use temporary variables so order of computation doesn't affect result. let new-y1 ( y1 + local-y1' ) let new-y2 ( y2 + local-y2' ) let new-y3 ( y3 + local-y3' ) set y1 new-y1 set y2 new-y2 set y3 new-y3 tick-advance dt end ;; Report value of flow to-report y1' report ( sigma * (y2 - y1) ) * dt end ;; Report value of flow to-report y2' report ( y1 * (r - y3) - y2 ) * dt end ;; Report value of flow to-report y3' report ( y1 * y2 - b * y3 ) * dt end ;; Plot the current state of the system dynamics model's stocks ;; Call this procedure in your plot's update commands. to system-dynamics-do-plot if plot-pen-exists? "y1" [ set-current-plot-pen "y1" plotxy ticks y1 ] if plot-pen-exists? "y2" [ set-current-plot-pen "y2" plotxy ticks y2 ] if plot-pen-exists? "y3" [ set-current-plot-pen "y3" plotxy ticks y3 ] end

There is only one version of this model, created 7 months ago by Javinel Servano.

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**Parent:** Urban (System Dynamics)

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