Error using plot Vectors must be the same length. Error in Untitled11 (line 40) plot([xn xn],[ymin ymax],'r-'​,'Linewidt​h',2)

function bisection_method()
myfunc = input ('myfunc: ');
A(1)   = input('A: ');
B(1)   = input('B: ');
e(1)   = input('error : '); 
n      = input('loop for n: ');
disp('***********************************************');
x = linspace(A,B,100);
y = myfunc(x);
fig = figure();
set (fig,'color','White')
plot(x,y)
grid on
hold on
ymax = max(y);
ymin = min(y);
plot([A A],[ymin ymax],'k-')
plot([B B],[ymin ymax],'k-')
I=0;
for i=1:n;
    I=1+I; 
  C(i) = (A(i)+B(i))/2;
  fprintf('%d)\t%f\t%f\t%f\t%f\n', I, A(i), B(i), C(i), e(i));
  FC(i) = myfunc(C(i));
  FB(i) = myfunc(B(i));
  if(FC(i)*FB(i)>0)
      B(i+1) = C(i) ;
      A(i+1)=A(i);
  else
      A(i+1) = C(i) ;
      B(i+1)=B(i);
  end
  e(i+1) = (abs((FB(i)-FC(i))/FB(i)))*100;
end  
disp('**********************************************');
xn = B;
yn = myfunc(xn);
while abs(yn)>1e-8
    xn = xn+C(i);
    plot([xn xn],[ymin ymax],'r-','Linewidth',2)
    pause
    ystar  = myfunc(xn);
    if sign(ystar)~= sign(yn)
        C(i) = -C(i);
    end
    yn = myfunc(xn);
    C(i) = C(i)/2;
end
xn
yn 
 function out = myfunc(in)
out = myfunc ;

[Brendan Hamm]

Please format this with the '{ } Code' button.

Answer 1

[Walter Roberson]

You have

x = linspace(A,B,100);
y = myfunc(x);

your myfunc returns its input, so your y is the same size as x, a vector of length 100. Then you have

ymax = max(y);
ymin = min(y);

so your ymin and ymax are each scalars that are the min and max of y respectively.

After that you have for i=1:n and inside that for loop you assign to B(i+1), so your B becomes a vector of length n+1. After the loop you have xn = B; and since your B is a vector of length n+1 your xn becomes a vector of length n+1 before the loop. Inside the loop you have

xn = xn+C(i);
plot([xn xn],[ymin ymax],'r-','Linewidth',2)

The first of those increments the entire vector xn by C(i), resulting in a vector the same length (that is, n+1). Then in the plot [xn xn] is going to be a vector of length 2*(n+1) . Your ymin and ymax are scalar so [ymin ymax] is a vector of length 2. You are trying to plot a vector of 2*(n+1) x values against 2 y values and that is going to fail unless the user entered 0 for n.