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

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

[madhan ravi]

Couple of suggestions:

1) Never name a variable which is the same as MATLAB’s in - built function (xlim.., etc)

2) i and j are imaginary units use ii and jj instead.

3) preallocation is really significant

4) Use cell arrays for preallocation which avoids ambiguities

Answer 1

[Subhadeep Koley]

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