Need little coding help

[catenaryworks]

The code below plots 'k' along the y-axis and 'theta' along the x-axis.

I would like help on the following, for which I do not know how to write out the code in MATLAB:
1. I want to extract only those 'theta' values corresponding to integer values of 'k'
2. Then, use the above 'theta' values and solve for corresponding 'alpha' values from the equation:
d*sin(alpha) - r*sin(theta-alpha) = 0
3. I would also like to print an array of x and y coordinates corresponding to the above theta values
x = r*cos(theta) and y = r*sin(theta)

n=12;
d = 200;
r = 3200;
dbyr=d/r;
theta=0:0.0001:2*pi;
k=(n/(2*pi))*(theta+dbyr*sin(theta));
plot(theta,k)

Thanks.

 

[IRstuff]

So, this is for school?

TTFN
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[catenaryworks]

No, not for school assignment.

 

[GregLocock]

A quick look suggests that there are no integer values of k

Cheers

Greg Locock


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[catenaryworks]

k does have integer values: 1,2,.......,12. The y-axis of the plot generated shows them.

n=12;
d = 200;
r = 3200;
dbyr=d/r;
theta=0:0.0001:2*pi;
k=(n/(2*pi))*(theta+dbyr*sin(theta));
plot(theta,k)

Thanks.

 

[IRstuff]

Did you not take trig in high school? If this is not homework, why must you do this in MATLAB, since the solutions for theta can be found by inspection?x`

TTFN
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[GregLocock]

OK, I've watsed a minute on this. theta is in integer multiples of 0.0001

k is an integer divided by pi times (x.sin(theta)+theta)

dividing things that aren't pi, by pi, rarely results in integers.

Cheers

Greg Locock


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[IRstuff]

Unless his equation is incorrect, Theta as multiples of pi result in integer values of k, since the sin term gets zeroed, leaving n*m*pi/2*pi = 6m, since n=12.

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[LiteYear]

To be precise, there is exactly one integer value in k. You can find it like so:

for j=1:62832
if(round(k(j)) == k(j))
j
end
end

Unsurprisingly, it's the value for when theta is 0. As already stated, everything else gives you an expression with pi, which is never going to be exactly integer.

To suggest "k does have integer values: 1,2,.......,12. The y-axis of the plot generated shows them." is wrong - the plot shows a continuous line. The values of k are discrete, and are never exactly integer (except at the origin). If you want values that are "close enough" to integers, you need to use numerical methods. Time to rethink the problem you're trying to solve. Which is?

 

[IRstuff]

and what about theta = pi, 2*pi, 3*pi, etc.

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[LiteYear]

IRStuff, the input is never pi, 2*pi or 3*pi. The input is "0:0.0001:2*pi;". So 0 is in but the closest to pi you get is 3.1416 and oddly enough, even the last input is not 2*pi (Matlab rounds it to something a bit surprising - I can't remember exactly what).

I agree, if the input were exactly pi, 2*pi or 3*pi, the equation would give a rational result since the sin term goes to zero and the remaining theta term cancels with the pi on the denominator.

 

[GregLocock]

This isn't helping the OP much, but then if you are talking about mathematics and you aren't using the words correctly then you either get with the program or you get out of the kitchen.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[IRstuff]

The OP is wandering aimlessly. Look at what he says he wants:
"I want to extract only those 'theta' values corresponding to integer values of 'k' " The fact that he can't even figure out his own Matlab code suggests that his vector for theta is arbitrarily constructed.

TTFN
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[FeX32]

The OP's question is illogical and makes no sense.


Fe (IronX32)