how to get the plot from function

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

Answer 1

[Star Strider]

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.