How to avoid the warnings on my code

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Left Terry
Left Terry 2024-12-30
编辑： John D'Errico 2024-12-30
Ran in:
Hello. I have the following code and i get a few warnings. How do i fix this problem ? (I used this warning('off','all')
warning but i think it's cheating and i do not prefer it)
%--- Solving the diff. eqn. y'(t) = y(t) - c*y^2(t) with c = 0.5 using
%--- Euler's method, Runge - Kutta method, and predictor-corrector method, and comparing in graph. I have four
%--- initial values for N0
clc, clear all, close all
tmax = 100;
t = [0:tmax];
N0 = [0.1 0.5 1.0 2.0];
f = @(t,N) N - 0.5*N.^2;
h = 0.1;
L = length(t)-1;
N = zeros(1,L);
%--- Euler's method ---
for i = 1:length(N0)
for j = 1:L
N(1) = N0(i);
N(1,j+1) = N(j) + h*f(':',N(j));
end
ylim([0 2.5])
subplot(1,3,1), plot(t,N), str = {'Euler''s method','for the following initial conditions'}; text(40,0.5,str); hold on
title('N'' = N - cN^2, c = 0.5')
xlabel('t'), ylabel('N(t)')
legend('y_0 = 0.1','y_0 = 0.5','y_0 = 1.0','y_0 = 2.0','Location','Best')
end
Warning: Ignoring extra legend entries.
Warning: Ignoring extra legend entries.
Warning: Ignoring extra legend entries.
%--- Runge - Kutta method ---
for i = 1:length(N0)
N(1) = N0(i);
for j = 1:tmax
K1 = f(t(j),N(j));
K2 = f(t(j) + h*0.5, N(j) + h*K1*0.5);
K3 = f(t(j) + h*0.5, N(j) + h*K2*0.5);
K4 = f(t(j) + h, N(j) + h*K3);
N(j+1) = N(j) + h*((K1 + K4)/6 + (K2 + K3)/3);
end
ylim([0 2.5])
subplot(1,3,2), plot(t,N), str = {'Runge - Kutta method','for the following initial conditions'}; text(40,0.5,str); hold on
title('N'' = N - cN^2, c = 0.5')
xlabel('t'), ylabel('N(t)')
legend('y_0 = 0.1','y_0 = 0.5','y_0 = 1.0','y_0 = 2.0','Location','Best')
end
Warning: Ignoring extra legend entries.
Warning: Ignoring extra legend entries.
Warning: Ignoring extra legend entries.
  1 个评论
John D'Errico
John D'Errico 2024-12-30
编辑：John D'Errico 2024-12-30
Lol. You can call it cheating to turn the warnings off. But I'd just call it dangerous in a sense, in that warnings were put there for a reason, to tell you that something bad is probably happening, and you should fix the problem.
Do you really want to turn off the warning light on your car that tells you it is overheating? Or just close your eyes to those warning signs about the bridge being out? ;-) I think you are right. Better to fix the problem.

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Torsten
Torsten 2024-12-30
编辑：Torsten 2024-12-30
Ran in:
%--- Solving the diff. eqn. y'(t) = y(t) - c*y^2(t) with c = 0.5 using
%--- Euler's method, Runge - Kutta method, and predictor-corrector method, and comparing in graph. I have four
%--- initial values for N0
clc, clear all, close all
tmax = 100;
t = [0:tmax];
N0 = [0.1 0.5 1.0 2.0];
f = @(t,N) N - 0.5*N.^2;
h = 0.1;
L = length(t)-1;
N = zeros(1,L);
%--- Euler's method ---
for i = 1:length(N0)
for j = 1:L
N(1) = N0(i);
N(1,j+1) = N(j) + h*f(':',N(j));
end
subplot(1,3,1), plot(t,N), hold on
end
str = {'Euler''s method','for the following initial conditions'}; text(40,0.5,str);
ylim([0 2.5])
title('N'' = N - cN^2, c = 0.5')
xlabel('t'), ylabel('N(t)')
legend({'y_0 = 0.1','y_0 = 0.5','y_0 = 1.0','y_0 = 2.0'},'Location','Best')
%--- Runge - Kutta method ---
for i = 1:length(N0)
N(1) = N0(i);
for j = 1:tmax
K1 = f(t(j),N(j));
K2 = f(t(j) + h*0.5, N(j) + h*K1*0.5);
K3 = f(t(j) + h*0.5, N(j) + h*K2*0.5);
K4 = f(t(j) + h, N(j) + h*K3);
N(j+1) = N(j) + h*((K1 + K4)/6 + (K2 + K3)/3);
end
subplot(1,3,2), plot(t,N), hold on
end
str = {'Runge - Kutta method','for the following initial conditions'}; text(40,0.5,str);
ylim([0 2.5])
legend({'y_0 = 0.1','y_0 = 0.5','y_0 = 1.0','y_0 = 2.0'},'Location','Best')
title('N'' = N - cN^2, c = 0.5')
xlabel('t'), ylabel('N(t)')
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