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ME3255S2017/lecture_23/rk4sys.m
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function [tp,yp] = rk4sys(dydt,tspan,y0,h,varargin) | |
% rk4sys: fourth-order Runge-Kutta for a system of ODEs | |
% [t,y] = rk4sys(dydt,tspan,y0,h,p1,p2,...): integrates | |
% a system of ODEs with fourth-order RK method | |
% input: | |
% dydt = name of the M-file that evaluates the ODEs | |
% tspan = [ti, tf]; initial and final times with output | |
% generated at interval of h, or | |
% = [t0 t1 ... tf]; specific times where solution output | |
% y0 = initial values of dependent variables | |
% h = step size | |
% p1,p2,... = additional parameters used by dydt | |
% output: | |
% tp = vector of independent variable | |
% yp = vector of solution for dependent variables | |
if nargin<4,error('at least 4 input arguments required'), end | |
if any(diff(tspan)<=0),error('tspan not ascending order'), end | |
n = length(tspan); | |
ti = tspan(1);tf = tspan(n); | |
if n == 2 | |
t = (ti:h:tf)'; n = length(t); | |
if t(n)<tf | |
t(n+1) = tf; | |
n = n+1; | |
end | |
else | |
t = tspan; | |
end | |
tt = ti; y(1,:) = y0; | |
np = 1; tp(np) = tt; yp(np,:) = y(1,:); | |
i=1; | |
while(1) | |
tend = t(np+1); | |
hh = t(np+1) - t(np); | |
if hh>h,hh = h;end | |
while(1) | |
if tt+hh>tend,hh = tend-tt;end | |
k1 = dydt(tt,y(i,:),varargin{:})'; | |
ymid = y(i,:) + k1.*hh./2; | |
k2 = dydt(tt+hh/2,ymid,varargin{:})'; | |
ymid = y(i,:) + k2*hh/2; | |
k3 = dydt(tt+hh/2,ymid,varargin{:})'; | |
yend = y(i,:) + k3*hh; | |
k4 = dydt(tt+hh,yend,varargin{:})'; | |
phi = (k1+2*(k2+k3)+k4)/6; | |
y(i+1,:) = y(i,:) + phi*hh; | |
tt = tt+hh; | |
i=i+1; | |
if tt>=tend,break,end | |
end | |
np = np+1; tp(np) = tt; yp(np,:) = y(i,:); | |
if tt>=tf,break,end | |
end |