Angles from XY coordinates from a matrix

Hello,

I am generally new to matlab, and any help would be appreciated.

I am currently trying to collect data on joint angles over time. I have figured out how to get joint data from XY coordinates at a single point in time. I would like matlab to be able to calculate joint angle at each time interval.

Here is some example data...

[x1  y1  x2  y2  x3  x4
23  32  45  56  66  54
24  34  46  55  66  55
22  36  44  54  65  56
33  23  43  57  64  53]

Here is the matlab code I have working with...

x1=a(1,1);
y1=a(1,2);
x2=a(1,3);
y2=a(1,4);
x3=a(1,5);
y3=a(1,6);

x1=23;
y1=32;
x2=45;
y2=56;
x3=66;
y3=54;

% Create vectors describing sides of triangle
P1=[x1 y1];
P2=[x2 y2];
P3=[x3 y3];

P12=P2-P1;
P13=P3-P1;

% Calculate the angle between two of the sides

ang = acosd(dot(P12,P13)/sqrt(sum(P12.^2)*sum(P13.^2)))

Matlab will now pump out the angle for line 1 of the data above.

I would like to be able to have a code that will read the data from a matrix, and then pump out the angles from each line.

Example:

ang=

20.049
20.056
..........
..........
etc.

Thank you so much for your assistance.

Answer 1

[Matt Fig]

Seems like a simple FOR loop would do the trick.

for ii = size(a,1):-1:1
    P12 = [a(ii,3)-a(ii,1), a(ii,4)-a(ii,2)];
    P13 = [a(ii,5)-a(ii,1), a(ii,6)-a(ii,2)];
    V(ii) = acosd(dot(P12,P13)/sqrt(sum(P12.^2)*sum(P13.^2)));
end

Or even a vectorized version:

P12 = [a(:,3)-a(:,1), a(:,4)-a(:,2)];
P13 = [a(:,5)-a(:,1), a(:,6)-a(:,2)];
V = acosd(dot(P12,P13,2)./sqrt(sum(P12.^2,2).*sum(P13.^2,2)))

[Michael]

Thank you so much. This worked perfectly!

Answer 2

[Jürgen]

Hi,

two possibilities if you want to process a lot of data

: you can use loops or vectorize,

in this case, with few iterations I would use a for loop

so for Counter= 1: NumberOfRowsInMatrixa ... assign values ... do your calculations ... assign results to ResultsMatrix

end

regardsJ