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

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Nima on 9 Jul 2020
Edited: Subhadeep Koley on 9 Jul 2020
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
##### 1 CommentShowHide None
madhan ravi on 9 Jul 2020
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

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### Accepted Answer

Subhadeep Koley on 9 Jul 2020
Edited: Subhadeep Koley on 9 Jul 2020
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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