index out of bounds

%clc
clear
%parameters
dt = 1e-6; 
B1 = 10;
phi = 0;
gamma = 267.513*10^6; %rad/(s*T)
Bzi = -5; %in HZ
Bzj = 5; %in HZ
a = 1000000;
G = 42.576; % Gyromagnetic ratio  (MHz/T)   MHZ=1*10^6   
B_zi = Bzi/(a*G); %in tesla       
B_zj = Bzj/(a*G); %in tesla
% mag_initial = [0 0 1; 0 0 1];

%functions 
% equation magnetizaion = [m_x1 m_y1 m_z1; m_x2 m_y2 m_z2];
mag = zeros(2,3);
B = zeros(size(mag));

% line up
mag(:,3) = 1;

B(1,3) = B_zi;
B(2,3) = B_zj;

%time evolution
for nn = (1:10000)
  % calculate B
  M = mean(mag); %average of mag
  m_x = M(1);
  m_y = M(2);
  euler = complex(cos(phi),sin(phi));

    m = complex(m_x,m_y);
    B0 = euler*m;
    B_x = real(B0);
    B_y = imag(B0);
    B = [B_x B_y B_zi; B_x B_y B_zj];  % equation  B = [B_x+B1 B_y B_zi; B_x+B1 B_y B_zj];
   [tt, mm] = ode45('bloch',[0:dt/2:dt],mag,[],gamma,B);
    mag = mm(:,3);
end

 my function is 
  function dmdt = bloch(tspan,mag,spot,gamma,B)  %outputs and inputs 
  mag = reshape(mag,2,3); 
  dmdt = gamma*cross(mag,B);
  dmdt = dmdt(:);
return

the error I got

Attempted to access M(2); index out of bounds because numel(M)=1.

Error in main5 (line 32) m_y = M(2);

Error in run (line 57) evalin('caller', [s ';']);

Answer 1

[Image Analyst]

Looks like it should work. In fact it does for me. So I think your mean is not what you think it is. After you call mean(), put these lines:

which -all mean
whos M

What do you see?

[fang-chu]

    Name      Size            Bytes  Class     Attributes

    M         1x3                24  double              

    Name      Size            Bytes  Class     Attributes

    M         1x1                 8  double              

Attempted to access M(2); index out of bounds because numel(M)=1.

Error in main5 (line 34)
    m_y = M(2);

this is what show after I run it

Do I have to add "if" comment? what do you mean my mean is not what I think it is?