Running multiple for loops

I am extremely new to MATLAB and I am trying to develop a code that will give me an output for a series of equations that are affected by one another. This is extremely simple in Excel; however, I am required to do this in MATLAB. The first for loop runs and works perfectly but I can not get the second for loop to give me an output. I am positive that this extremely simple, I am just not familiar with this program. Thanks.

%%First 4-Bar%%
O2O4=2; O2A=5.38517; O4B=4; AB=1:1.74457; 

%%Second 4-Bar%%
AC=1.41422; AG=6.96923; CG=5.87276; GE=1.82186; CD=4; DE=3:.967108; DE=3:.001:.967108;

for i=1:.001:1.74457

    thetaO2A=180-acosd((O2A^2+O2O4^2-(O4B+(i))^2)/(2*(O2A*O2O4)));
    thetaO4B=acosd((O2O4^2+(O4B+(i))^2-O2A^2)/(2*(O2O4*(O4B+(i)))));
    thetaAC=(180-156.8014)+thetaO2A;
    thetaAG=(180-121.46776)+thetaO2A;
    thetaCG=(180-136.66081)+thetaAC;
    pointA=[O2A*cosd(thetaO2A);+O2A*sind(thetaO2A)];
end

for ii=3:0.001:.967108
  thetaGE=(acosd((CG^2+GE^2-(CD+(ii))^2)/(2*CG*GE)))-(180-CG)
end

for iii=3:0.001:1.19133
  %another theta equation I have not derived yet that will reference the
  %previous two for loops and the third variable iii
end

Answer 1

[Andrew Newell]

In the second and third loops, the starting value is higher than the end. Maybe you want

for ii=3:-0.001:.967108

(this counts down from 3 to 0.968).

[John]

I do actually need to count down instead of up on the second two for loops. Would this matter for the output?

[Andrew Newell]

If you mean, will it change the output for a given value of ii? No. Note that, if you want to keep all the values of thetaGE, you could replace the loop by a vector expression:

x = .968:.001:3;
thetaGE=(acosd((CG^2+GE^2-(CD+x).^2)/(2*CG*GE)))-(180-CG);

The .^2 represents element-by-element taking of the power.

Answer 2

[Stephen23]

It seems like the basic problem is a lack of indexing to keep the results of each loop. Currently each loop simply redefines the variable pointA to be the result of the last line in the loop, but this is just a 2x1 array, and so you loose all of the results from the previous iterations.

However an even better solution is to vectorize your code, which is faster, neater and reads more like the maths it represents. Note the use of array operations , e.g. .^ instead of ^.

%%First 4-Bar%%
O2O4=2; O2A=5.38517; O4B=4; AB=1:1.74457; 
%%Second 4-Bar%%
AC=1.41422; AG=6.96923; CG=5.87276; GE=1.82186; CD=4; DE=3:.967108; DE=3:.001:.967108;
%
vec = 1:.001:1.74457;
%
thetaO2A = 180-acosd((O2A^2+O2O4^2-(O4B+vec).^2)/(2*(O2A*O2O4)));
thetaO4B = acosd((O2O4^2+(O4B+vec).^2-O2A^2)/(2*(O2O4*(O4B+vec))));
thetaAC  = (180-156.8014)+thetaO2A;
thetaAG  = (180-121.46776)+thetaO2A;
thetaCG  = (180-136.66081)+thetaAC;
pointA = [O2A*cosd(thetaO2A);+O2A*sind(thetaO2A)];
%
%for n=3:0.001:.967108
%  thetaGE = acosd((CG^2+GE^2-(CD+n)^2)/(2*CG*GE)) - (180-CG)
%end

I leave the second loop as an exercise for the reader...

Also note that you should not use i for your loop variable, as this is the name of the inbuilt imaginary unit .