Matlab, how to know time step size in the plot or how can i get the output of time step size?

Question

dave 2014-7-16

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/142007-matlab-how-to-know-time-step-size-in-the-plot-or-how-can-i-get-the-output-of-time-step-size

评论： Geoff Hayes 2014-7-17

Since i am using matlab ode23s solver, it contains two matlab files . One contain the differential equations and another contains plotting and to run the m-files. The code are working perfectly. Now i want to know the time step size that is using on the plot. How can i get the output of time step size. I would like to know the time step size that is using on the plot too. please help me.

=>

% 3 Nonlinear differential equations after Asymptotic expansion
% with 1-c in dc/dt differential equation
    function xpr= no(t,x)
      %values of parameters
        k_f= 6.7*10.^7;
        k_d= 6.03*10.^8; 
        k_n=2.92*10.^9; 
        k_p=4.94*10.^9;
        %Unknown parameters
        lambda_b= 0.0087;
        % scale parameters
        K_F= k_f * 10.^-9;
        K_D= k_d * 10.^-9; 
        K_N= k_n * 10.^-9; 
        K_P= k_p * 10.^-9;
        LAMBDA_B= lambda_b*10.^-9;
        %Pool Values
        P_C= 3 * 10.^(11);
        P_Q= 2.87 * 10.^(10); 
     % initial conditions
      c_0=x(1);
      s_0=x(2);
      q_0=x(3);
      %Non-linear differential equations.
      % dc_0/dtau=  c_0*(- K_F - K_D - K_N * s_0 - K_P*(1-q_0))
      % ds_0/dtau = Lambda_B * c* P_C *(1-s_0)
      % dq_0/dtau = (1-q_0)* K_P * c_0 *(P_C / P_Q)
    xpr= zeros(3,1);
    xpr(1)= c_0*(- K_F - K_D - K_N * s_0 - K_P*(1-q_0));
    xpr(2)= LAMBDA_B * c_0* P_C *(1-s_0);
    xpr(3)= (1-q_0)* K_P * c_0 *(P_C / P_Q);
    xpr= [xpr(1);xpr(2);xpr(3)];

% TO RUN the 3 nonlinear differential equations after asymptotic expansion. % with 1-c in dc/dt differential equation

       format bank
        close all; 
        clear all; 
        clc; 
        %time interval
        ti=0; 
        tf=0.2; 
        tspan=[ti tf]; 
        x0=[0.25 0.02 0.98]; %initial conditions
        %time interval of [0 2] with initial condition vector [0.25 0.02 0.98] at time 0.
        options= odeset('RelTol',1e-4, 'AbsTol',[1e-4 1e-4 1e-4]);
        [t,x]= ode23s(@no,tspan,x0,options); 
tDiffs = diff(t);
        %Plotting the graphs:
        figure 
        subplot(3,1,1), plot(t,x(:,1),'r'),grid on; 
        title('3 nonlinear differential equations (with 1-c)'),ylabel('c_0'); 
        subplot(3,1,2), plot(t,x(:,2),'b'),grid on; 
        ylabel('s_0'); 
        subplot(3,1,3), plot(t,x(:,3),'g'),grid on; 
        ylabel('q_0');xlabel('Time')

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Answer 1

Geoff Hayes 2014-7-16

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/142007-matlab-how-to-know-time-step-size-in-the-plot-or-how-can-i-get-the-output-of-time-step-size#answer_145310

在 MATLAB Online 中打开

Just after your code executes

[t,x]= ode23s(@no,tspan,x0,options);

add

tDiffs = diff(t);

which will return the difference between adjacent elements in t...which should correspond to the time step sizes used in the plots (as t is used in each plot call).

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dave 2014-7-17

在 MATLAB Online 中打开

yes. the code is in this order

[t,x]= ode23s(@no,tspan,x0,options); 
 tDiffs = diff(t);

Now how can i see the time step size it is using in the plot?

Geoff Hayes 2014-7-17

Look at tDiffs - it will be the difference between adjacent elements, and this difference should be the time step size. Are each of the values in tDiffs identical?

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