How to store the results from a for loop into a matrix?

Asked by John Stahura on 20 Sep 2019

Latest activity Edited by madhan ravi on 20 Sep 2019

4 views (last 30 days)

clear all
clc
F = 2; 
x = [.1:.05:1.9];
for i = x;
    theta1 = atan((2-i)/1);
    theta2 = atan(1/(2-i));
    theta3 = atan(1/i);
    A = [-1 -cos(theta2) 0 0 0 0 0 0 0 0; %Fab
        0 sin(theta2) 0 0 0 0 0 0 0 0; %Fac
        1 0 -1 0 0 0 0 0 0 0; %Fbd
        0 0 0 1 0 0 0 0 0 0; %Fbc
        0 sin(theta1) 0 0 -1 -cos(theta3) 0 0 0 0; %Fce
        0 -cos(theta1) 0 -1 0 -sin(theta3) 0 0 0 0; %Fcd
        0 0 1 0 0 cos(theta3) -1 0 0 0; %Dx
        0 0 0 0 0 sin(theta3) 0 1 0 0; %Fde
        0 0 0 0 1 0 0 0 -1 0; %Ex
        0 0 0 0 0 0 0 -1 0 1]; %Ey
        
    b = [0 F 0 F/2 0 0 0 0 0 0]';
    Reactions = A\b
end

How do i store the "Reactions" results from each iteration of the loop into one matrix?

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Products

MATLAB

Release

R2016a

2 Answers

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Answer by thoughtGarden on 20 Sep 2019

Edited by thoughtGarden on 20 Sep 2019

Accepted Answer

Note that there area some poor practices in place here, but with the most minimal change to what you already have is to create a blank matrix Reactions and then add each column to that matrix as they are developed.

clear all
clc
F = 2; 
x = [.1:.05:1.9];
Reactions = [];
for i = x;
    theta1 = atan((2-i)/1);
    theta2 = atan(1/(2-i));
    theta3 = atan(1/i);
    A = [-1 -cos(theta2) 0 0 0 0 0 0 0 0; %Fab
        0 sin(theta2) 0 0 0 0 0 0 0 0; %Fac
        1 0 -1 0 0 0 0 0 0 0; %Fbd
        0 0 0 1 0 0 0 0 0 0; %Fbc
        0 sin(theta1) 0 0 -1 -cos(theta3) 0 0 0 0; %Fce
        0 -cos(theta1) 0 -1 0 -sin(theta3) 0 0 0 0; %Fcd
        0 0 1 0 0 cos(theta3) -1 0 0 0; %Dx
        0 0 0 0 0 sin(theta3) 0 1 0 0; %Fde
        0 0 0 0 1 0 0 0 -1 0; %Ex
        0 0 0 0 0 0 0 -1 0 1]; %Ey
        
    b = [0 F 0 F/2 0 0 0 0 0 0]';
    Reactions = [Reactions, A\b];
end

a better solution would be to loop over index value (instead of the values of x) and preallocate the matrix.

clear all
clc
F = 2; 
x = [.1:.05:1.9];
Reactions = zeros(10,size(x,1));
for ii = 1:length(x)
    theta1 = atan((2-x(ii))/1);
    theta2 = atan(1/(2-x(ii)));
    theta3 = atan(1/x(ii));
    A = [-1 -cos(theta2) 0 0 0 0 0 0 0 0; %Fab
        0 sin(theta2) 0 0 0 0 0 0 0 0; %Fac
        1 0 -1 0 0 0 0 0 0 0; %Fbd
        0 0 0 1 0 0 0 0 0 0; %Fbc
        0 sin(theta1) 0 0 -1 -cos(theta3) 0 0 0 0; %Fce
        0 -cos(theta1) 0 -1 0 -sin(theta3) 0 0 0 0; %Fcd
        0 0 1 0 0 cos(theta3) -1 0 0 0; %Dx
        0 0 0 0 0 sin(theta3) 0 1 0 0; %Fde
        0 0 0 0 1 0 0 0 -1 0; %Ex
        0 0 0 0 0 0 0 -1 0 1]; %Ey
        
    b = [0 F 0 F/2 0 0 0 0 0 0]';
    Reactions(:,ii) = A\b;
end
disp(Reactions)

Both are valid, but the later is more efficient and clear to future readers.

1 Comment

madhan ravi on 20 Sep 2019

Preallocation is preferred over appending datas.

https://in.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html - for the importance of preallocation

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Answer by madhan ravi on 20 Sep 2019

Edited by madhan ravi on 20 Sep 2019

F = 2; 
x = .1:.05:1.9;
Reactions = cell(numel(x),1);
b = [0 F 0 F/2 0 0 0 0 0 0]'; % there's no point in defining it over and over again inside the loop.
for ii = 1:numel(x);
    theta1 = atan((2-x(ii))/1);
    theta2 = atan(1/(2-x(ii)));
    theta3 = atan(1/x(ii));
    A = [-1 -cos(theta2) 0 0 0 0 0 0 0 0; %Fab
        0 sin(theta2) 0 0 0 0 0 0 0 0; %Fac
        1 0 -1 0 0 0 0 0 0 0; %Fbd
        0 0 0 1 0 0 0 0 0 0; %Fbc
        0 sin(theta1) 0 0 -1 -cos(theta3) 0 0 0 0; %Fce
        0 -cos(theta1) 0 -1 0 -sin(theta3) 0 0 0 0; %Fcd
        0 0 1 0 0 cos(theta3) -1 0 0 0; %Dx
        0 0 0 0 0 sin(theta3) 0 1 0 0; %Fde
        0 0 0 0 1 0 0 0 -1 0; %Ex
        0 0 0 0 0 0 0 -1 0 1]; %Ey
    Reactions{ii} = A\b;
end
celldisp(Reactions)
cat(2,Reactions{:}) % if you want a matrix

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John Stahura (view profile)