how to get the plot from function

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Aleksandra Pawlak
Aleksandra Pawlak 2021-4-27
回答： Star Strider 2021-4-27
Hi!
I want to get the three plots on one chart: plot(x1,f(x1), x2,f(x2), x3, f(x3))
I have some errors when i call plot(x1,f(x1)).
f is included inside the function:
f = @(x) 1/(1+(exp(10*(3-x))))-(1/(1+(exp(10*(2-x)))));
clc
close all
clear all
N1=7
u=poisson(N1)
dx1 = (6-0)/(N1-1);
x1 = 0:dx1:6;
figure(1)
plot(x1,u)
% hold on
N2=21
u=poisson(N2)
dx2 = (6-0)/(N2-1);
x2 = 0:dx2:6;
plot(x2,u)
hold on
N3=199
u=poisson(N3)
dx3 = (6-0)/(N3-1);
x3 = 0:dx3:6;
plot(x3,u)
legend(['N1=',num2str(N1)], ['N2=',num2str(N2)], ['N3=',num2str(N3)])
xlabel('x')
ylabel('u')
hold off
function u=poisson(Nx)
A = zeros(Nx,Nx);
b = zeros(Nx,1);
f = @(x) 1/(1+(exp(10*(3-x))))-(1/(1+(exp(10*(2-x)))));
% Macierze A i b
dx = (6-0)/(Nx-1);
x = 0:dx:6;
h=dx;
m=1;
if x>=0 & x<2
m=1;
elseif x>=2 & x<4
m=5;
else x>=4 & x<=6
m=10;
end
for i=1:Nx
if i == 1
A(i,i) = 1;
b(i) = 0;
elseif i == Nx
b(i) = 1;
A(i,i) = 1;
else
A(i,i-1) = m/dx^2;
A(i,i) = -m*(2/dx^2);
A(i,i+1) = m/dx^2;
b(i) = f(x(i));
end
end
u = A\b;
end

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采纳的回答

Star Strider
Star Strider 2021-4-27
Uncomment this hold call —
figure(1)
plot(x1,u)
% hold on
N2=21
u=poisson(N2)
and the code does what you want.
Note that the lines are almost the same, however the legend call demonstrates that they all plot correctly.

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