- Preallocate the required output arrays.
- Use indexing in the nested loops.
What is the best way to set up a nested for loop?
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Hi,
I need help creating an efficient, nested for-loop. I have to run through a couple of equations on a few of variables but need to know the best way to increment these variables and to store their new values.
Thanks,
Steve
5 Comments
Answers (2)
Sulaymon Eshkabilov
on 22 Sep 2019
Hi Steven,
You loop is not doing much. SInce you are saving the last value from your nested loops. If this is what you want then, just remove the loops as follows:
load('Triplets');
load('F_points');
i=953; j=953;
xC = Triplets{i}(2,1);
xB = Triplets{i}(2,2);
xA = Triplets{i}(2,3);
yC = Triplets{i}(1,1);
yB = Triplets{i}(1,2);
yA = Triplets{i}(1,3);
yb = F_points{j}(2,1);
xb = F_points{j}(1,1);
xyb = [yb,xb];
xye = [Triplets{j}(1,3), Triplets{j}(2,3); Triplets{j}(1,2) Triplets{j}(2,2); Triplets{j}(1,1) Triplets{j}(2,1)];
CosTheta1 = dot(xye(1,:)-xyb(1,:),xye(2,:)-xyb(1,:))/(norm(xye(1,:)-xyb(1,:))*norm(xye(2,:)-xyb(1,:)));
ThetaInDegrees1 = acosd(CosTheta1);
CosTheta2 = dot(xye(2,:)-xyb(1,:),xye(3,:)-xyb(1,:))/(norm(xye(2,:)-xyb(1,:))*norm(xye(3,:)-xyb(1,:)));
ThetaInDegrees2 = acosd(CosTheta2);
CosTheta3 = dot(xye(3,:)-xyb(1,:),xye(1,:)-xyb(1,:))/(norm(xye(3,:)-xyb(1,:))*norm(xye(1,:)-xyb(1,:)));
ThetaInDegrees3 = acosd(CosTheta3);
degsum=ThetaInDegrees1+ThetaInDegrees2+ThetaInDegrees3;
If you do wnat to save all values of variables (xC, xB, ... xye, ... degsum) from your computations, then you'd need to specify the indexes and then you'd need to do the memory allocation:
xA = zeros(1,953); % Memory allocation
xB = zeros(1,953);
xC = zeros(1,953);
yA = zeros(1,953);
yB = zeros(1,953);
yC = zeros(1,953);
...
for i = 1 : length (Triplets)
for j = 1 : i
xC(i) = Triplets{i}(2,1);
xB(i) = Triplets{i}(2,2);
xA(i) = Triplets{i}(2,3);
yC(i) = Triplets{i}(1,1);
yB(i) = Triplets{i}(1,2);
yA(i) = Triplets{i}(1,3);
yb(j) = F_points{j}(2,1);
xb(j) = F_points{j}(1,1);
...
end
end
Sulaymon Eshkabilov
on 23 Sep 2019
Hi Steven,
Here is the complete solution of your problem:
clearvars
%% Part 0. Loading data
load('Triplets');
load('F_points');
N = length(Triplets);
%% Part 1. Saving all values of arrays from the calculations
% Memory allocation
xA = zeros(1,N);
xB = zeros(1,N);
xC = zeros(1,N);
yA = zeros(1,N);
yB = zeros(1,N);
yC = zeros(1,N);
xb = zeros(1,N);
yb = zeros(1,N);
for ii = 1 : N
for jj = 1 : N
xC(ii) = Triplets{ii}(2,1);
xB(ii) = Triplets{ii}(2,2);
xA(ii) = Triplets{ii}(2,3);
yC(ii) = Triplets{ii}(1,1);
yB(ii) = Triplets{ii}(1,2);
yA(ii) = Triplets{ii}(1,3);
yb(jj) = F_points{jj}(2,1);
xb(jj) = F_points{jj}(1,1);
end
end
xyb = [yb; xb]'; % Augmented matrix array = xyb
% Memory allocation:
xye1 = zeros(2,N);
xye2 = zeros(2,N);
xye3 = zeros(2,N);
for k = 1:N
xye1(:,k) = [Triplets{k}(1,3), Triplets{k}(2,3)];
xye2(:,k) = [Triplets{k}(1,2) Triplets{k}(2,2)];
xye3(:,k) = [Triplets{k}(1,1) Triplets{k}(2,1)];
end
XYE0 = [xye1', xye2', xye3']; % Augmented matrix array = 'xye'
%% Part 2. Compute Angles
% Memory allocation:
CosTheta1 = zeros(1,N);
ThetaInDegrees1=zeros(1,N);
CosTheta2 = zeros(1,N);
ThetaInDegrees2 = zeros(1,N);
CosTheta3 = zeros(1,N);
ThetaInDegrees3 = zeros(1,N);
for jj = 1:N
CosTheta1(jj) = dot(XYE0(jj,1:2)-xyb(jj, :),XYE0(jj,3:4)-xyb(jj,:))/(norm(XYE0(jj,1:2)-xyb(jj,:))*norm(XYE0(jj,3:4)-xyb(jj,:)));
ThetaInDegrees1(jj) = acosd(CosTheta1(jj));
CosTheta2(jj) = dot(XYE0(jj,3:4)-xyb(jj, :),XYE0(jj,5:6)-xyb(jj,:))/(norm(XYE0(jj,3:4)-xyb(jj,:))*norm(XYE0(jj,5:6)-xyb(jj,:)));
ThetaInDegrees2(jj) = acosd(CosTheta2(jj));
CosTheta3(jj) = dot(XYE0(jj,5:6)-xyb(jj, :),XYE0(jj,1:2)-xyb(jj,:))/(norm(XYE0(jj,5:6)-xyb(jj,:))*norm(XYE0(jj,1:2)-xyb(jj,:)));
ThetaInDegrees3(jj) = acosd(CosTheta3(jj));
degsum(jj)=ThetaInDegrees1(jj)+ThetaInDegrees2(jj)+ThetaInDegrees3(jj);
end
good luck
4 Comments
Sulaymon Eshkabilov
on 24 Sep 2019
Ok. Glad that can of some help. Hit accept of this answer, pl.
Make sure that your formualtions are giving the values in degrees not radians. If other way around, then you should use conversion the values rad2deg(). With rad2deg() you will get the values in degrees. Always watch out the size match.
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