For Loop nested within a while loop, run until condition is met

My goal is to run an equation until my convergence criteria is met, basically do itterations until I am within some range of error. My code looks liek this

%% Model Cells
% Boundary conditions is head1 = 3 & head12=1
head = zeros (1,12); % Established a matrix of data
head(1)=3; % Setting boundary conditions
head(12)=1;
depth = [0.00,0.35,0.70,1.05,1.40,1.75,2.10,2.45,2.80,3.15,3.50,3.85];%for future plotting purposes
headold = zeros(1,12);
% Loop iteration & Stop iteration 
x = ones(1,12);
cc = x*.001;
while headold-head<cc % solving for head until convergence criteria is met
    headold = head;
%     head = headold+1;
%     headold+2 = headold+1;
 for i=2:11
      head(i)=(head(i-1)+head(i+1))/2; %solving for head 1 time
 end
end

The result of this is a never ending matlab solution, however when I pause this I get the anser I want .

Answer 1

[darova]

This lines actually means: "loop if condition is met"

while headold-head<cc % solving for head until convergence criteria is met    

Use this

tol = abs(head-headold);
while max(tol)>0.001    % solve if max(tolerance) > 0.001
    %% ...
    tol = abs(head-headold);
end

[Nathan Pavlick]

Perfect, thank you! Why though was mine not correct? I am just trying to understand why its wrong.

[Adam Danz]

Your while-loop is defined as while headold-head<cc. If you execute headold-head<cc you'll find that it produces a vector. The conditional expression should evaluate to a scalar logical value, not a vector. When the expression evaluates to a vector, all of the elements of the vector must be true for the while-loop to continue.

Here's a demo. Test the two x vectors.

x = logical([1 1 1 1 1]);   
% x = logical([1 1 1 1 0]);
while x
    disp('In while-loop')
    x = false; 
end

[darova]

• Why though was mine not correct?

Because headold-head gives negative values only (cc is positive)

Try

while headold-head<cc % solving for head until convergence criteria is met
    headold = head;
%     head = headold+1;
%     headold+2 = headold+1;
 for i=2:11
      head(i)=(head(i-1)+head(i+1))/2; %solving for head 1 time
 end
 plot(headold-head)
 pause(0.1)
end

[Adam Danz]

@dovara, but headold-head<cc will still produce a vector which is the problem I mentioned above.

[darova]

You are right