MATLAB: Unable to perform assignment because the indices on the left side are not compatible with the size of the right side

for loopindexingintegralMATLABmatrixnumerical integrationtrapzvector

Can anyone solve the problem in my code that returns the error by ? 
close all
clear all
clc
%%

mu0 = 4*pi*1e-7; % Vs/Am
M0 = 1e3; % A/m
maxnum = 31;
rho1_min = 0;
rho1_max = 0.25;
xlim = [-1, 1];
ylim = xlim;
zlim = xlim;
x = linspace(min(xlim), max(xlim), maxnum);
y = linspace(min(ylim), max(ylim), maxnum);
z = linspace(min(zlim), max(zlim), maxnum);
[Xg, Yg, Zg] = ndgrid(x, y, z);
rho = sqrt(Xg.^2 + Yg.^2 + Zg.^2);
phi = angle(Xg + 1i*Yg);
theta = angle(Zg + 1i*sqrt(Xg.^2 + Yg.^2));
%%
RHO = sqrt(x.^2 + y.^2 + z.^2);
THETA = linspace(0, pi, 31);                          % Trapz

PHI = linspace(0, 2*pi, 31);                          % Trapz
%% 
for i=1:numel(RHO)
    for j=1:numel(THETA)
        for k=1:numel(PHI)
        
        
F_x{i,j,k} = (RHO(i)>= rho1_max) .* 2/3*M0*mu0 .* sin(theta) .* (RHO(i) .* (sin(THETA(j)) .* cos(theta) .* cos(PHI(k)-phi) - cos(THETA(j)) .* sin(theta)) ./ ...
                                   (RHO(i).^2 + rho1_max.^2 - 2.*RHO(i) .* rho1_max .* (sin(THETA(j)) .* sin(theta) .* cos(PHI(k)-phi) + cos(THETA(j)).* cos(theta))).^3/2) .* rho1_max.^2 .* sin(theta);
                                 
B1x(i,j,k) = -trapz(PHI,trapz(THETA,F_x{i,j,k},2)) ; 
        end
     end 
end

Best Answer

Pre allocate B1x as cell array instead of numeric array to solve the problem.
close
clear
clc
%%


mu0 = 4*pi*1e-7; % Vs/Am
M0 = 1e3; % A/m
maxnum = 31;
rho1_min = 0;
rho1_max = 0.25;
xlimit = [-1, 1];
ylimit = xlimit;
zlimit = xlimit;
x = linspace(min(xlimit), max(xlimit), maxnum);
y = linspace(min(ylimit), max(ylimit), maxnum);
z = linspace(min(zlimit), max(zlimit), maxnum);
[Xg, Yg, Zg] = ndgrid(x, y, z);
rho = sqrt(Xg.^2 + Yg.^2 + Zg.^2);
phi = angle(Xg + 1i*Yg);
theta = angle(Zg + 1i*sqrt(Xg.^2 + Yg.^2));
%%
RHO = sqrt(x.^2 + y.^2 + z.^2);
THETA = linspace(0, pi, 31);                          % Trapz

PHI = linspace(0, 2*pi, 31);                          % Trapz
%%
% Pre-allocate F_x and B1x as cell array
F_x = cell(numel(RHO), numel(THETA), numel(PHI));
B1x = cell(numel(RHO), numel(THETA), numel(PHI));
for ii = 1:numel(RHO)
    for jj = 1:numel(THETA)
        for kk = 1:numel(PHI)
            F_x{ii, jj, kk} = (RHO(ii)>= rho1_max) .* 2/3*M0*mu0 .* sin(theta) .* (RHO(ii) .* (sin(THETA(jj)) .* cos(theta) .* cos(PHI(kk)-phi) - cos(THETA(jj)) .* sin(theta)) ./ ...
                (RHO(ii).^2 + rho1_max.^2 - 2.*RHO(ii) .* rho1_max .* (sin(THETA(jj)) .* sin(theta) .* cos(PHI(kk)-phi) + cos(THETA(jj)).* cos(theta))).^3/2) .* rho1_max.^2 .* sin(theta);
            B1x{ii, jj, kk} = squeeze(-trapz(PHI,trapz(THETA,F_x{ii,jj,kk},2)));
        end
    end
end
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