Everytime I try to plot, I get a straight line, and not a sine wave:(
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I get a sine curve for the aproximated Euler method, but the exact solution comes out as a straight line with an error message reading "Inner matrix dimensions must agree." SOmething must be going wrong within the exact formula, or with the "i's". I feel so close to the correct solution, but it doesn't graph correctly. Here is my code:
function ystar = Eulermethod202(n)
a=0;
b=5;
h=(b-a)/n;
t=1:h:5;
%Initialize time variable
clear ystar;
%wipe out old variable
ystar(1)=0;
%Initial condition (same for approximation)
clear k1;
%Set up "for" loop
for i=1:length(t)-1;
%Calculate the derivative
k=(-0.5*exp(t(i)/2)*sin(5*t(i)))+(5*exp(t(i)/2)*cos(5*t(i))+ystar(i));
ystar(i+1)=ystar(i)+h*k;%Estimate new value of y*
end
%Exact solution
y=exp(t(i)/2)*sin(5*t(i));
%Plot approximate and exact solutions
plot(t,ystar,'b',t,y,'r');
legend('Approximate','Exact');
title('Euler Approximation');
xlabel('Time');
ylabel('y*(t), y(t)');
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Fangjun Jiang
2011-11-23
For the exact value y, you need to do
y=exp(t/2).*sin(5*t)
t is a vector, t(i) is the individual element depending on the value of i. MATLAB is a MATrix LABoratory. Many operations can be done on a vector or matrix. You need to get familiar with it.
更多回答(1 个)
Walter Roberson
2011-11-23
Your line
y=exp(t(i)/2)*sin(5*t(i));
is after the "for" loop, which should lead you to question what the value of "i" will be when the loop ends.
You can predict, though, that "i" will have a single value (whatever that value is), which is going to lead to the two subexpressions having a single value, which is going to lead to "y" having a single value. And then you try to plot that single value against the array "t".
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