Error plotting (Vector must be the same length)

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Seungryul Lee
Seungryul Lee 2022-4-13
编辑： Davide Masiello 2022-4-14
clear all
clc
f=@(t,y) 4*y.^2;
F = @(t) 1/(4*(1-t));
a=0; b=0.95;
h=0.05;
n = (b-a)/h;
t = a:h:b;
y = zeros(1,length(t));
y(1) = 0.25;
%EULERS METHOD
for i = 2:n+1
y(i) = y(i-1) + h*f(t(i-1),y(i-1));
end
E=y;
fprintf(' Step number_i time_t Approximated solution Exact solution Relative Error\n')
for j = 1:n+1
m=abs(E(j)-F(t(j)));
n = abs(F(t(j)));
kE = m/n;
A(j) = F(t(j));
fprintf('%5d %18f %15f %20f %15f \n',j,t(j),E(j),F(t(j)),kE)
end
plot(t,A,'.-k', t, E, 'o-r')
xlabel('x');
ylabel('y(t)');
grid on
title('Part(a) Plot');
legend('Exact solution','Eulers solution')
**It all works for the table, but I'm getting an error for plotting. It says the vectors should be in the same length, but I don't get the problem.
Please help me.
Thank you.

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回答（1 个）

Davide Masiello
Davide Masiello 2022-4-13
编辑：Davide Masiello 2022-4-14
Ran in:
The vector A has 19 elements, whereas t and E have 20.
clear all
clc
f=@(t,y) 4*y.^2;
F = @(t) 1/(4*(1-t));
a=0; b=0.95;
h=0.05;
n = (b-a)/h;
t = a:h:b;
y = zeros(1,length(t));
y(1) = 0.25;
%EULERS METHOD
for i = 2:n+1
y(i) = y(i-1) + h*f(t(i-1),y(i-1));
end
E=y;
fprintf(' Step number_i time_t Approximated solution Exact solution Relative Error\n')
Step number_i time_t Approximated solution Exact solution Relative Error
for j = 1:n+1
m=abs(E(j)-F(t(j)));
n = abs(F(t(j)));
kE = m/n;
A(j) = F(t(j));
fprintf('%5d %18f %15f %20f %15f \n',j,t(j),E(j),F(t(j)),kE)
end
1 0.000000 0.250000 0.250000 0.000000 2 0.050000 0.262500 0.263158 0.002500 3 0.100000 0.276281 0.277778 0.005387 4 0.150000 0.291548 0.294118 0.008738 5 0.200000 0.308548 0.312500 0.012648 6 0.250000 0.327588 0.333333 0.017237 7 0.300000 0.349051 0.357143 0.022658 8 0.350000 0.373418 0.384615 0.029114 9 0.400000 0.401306 0.416667 0.036866 10 0.450000 0.433515 0.454545 0.046266 11 0.500000 0.471102 0.500000 0.057795 12 0.550000 0.515490 0.555556 0.072118 13 0.600000 0.568636 0.625000 0.090183 14 0.650000 0.633305 0.714286 0.113373 15 0.700000 0.713520 0.833333 0.143776 16 0.750000 0.815343 1.000000 0.184657 17 0.800000 0.948299 1.250000 0.241361 18 0.850000 1.128154 1.666667 0.323108 19 0.900000 1.382700 2.500000 0.446920
plot(t(1:end-1),A,'.-k', t(1:end-1), E(1:end-1), 'o-r')
xlabel('x');
ylabel('y(t)');
grid on
title('Part(a) Plot');
legend('Exact solution','Eulers solution')
  2 个评论
Seungryul Lee
Seungryul Lee 2022-4-13
do you know how to use only first 19 elements of t and E, so that I can plot it?
How can I get rid of the last element of t and E
Davide Masiello
Davide Masiello 2022-4-13
编辑：Davide Masiello 2022-4-14
Yes
plot(t(1:end-1),A,'.-k', t(1:end-1), E(1:end-1), 'o-r')
I updated it in the answer too, so you can see the resulting plot.

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