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Matrix help in MatLab, Not the simple matrix I have searched, found nothing!

by frozenguy
Tags: matlab, matrix, searched, simple
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frozenguy
#1
Feb23-12, 01:54 AM
P: 190
Please help me with this.

I need to code into matlab a function that results in a matrix answer.

It is this:
phi(i,j)=atan((eta(j)-eta(i))/(zeta(j)-zeta(i)))*180/pi]

Here is my code so far:
for i=1:4;

N=4;

w(i)=(360/N)*(i-0.5);

x(i)=-cos(w(i)*pi/180);

y(i)=sin(w(i)*pi/180);

x(5)=x(1);

y(5)=y(1);

zeta(i)=0.5*(x(i)+x(i+1));

eta(i)=0.5*(y(i)+y(i+1));

theta(i)=atan2((y(i+1)-y(i)),(x(i+1)-x(i)))*180/pi;

s(i)=sqrt((x(i+1)-x(i))^2+(y(i+1)-y(i))^2);

if theta(i)<0;

theta(i)=theta(i)+360;

end
j=1:4;

phi(i)=[atan((eta(i:N)-eta(i:N))/(zeta(i:N)-zeta(i:N)))*180/pi];

end
See I need a 4X4 matrix for phi.

I only have four etas and four zetas. both i and j = 4. With some work I can get a 4x4 matrix but it isn't coded right. It gives me the same answers in rows. It should have a diagonal line of zero's from top left to bottom right because that is where i=j

And I need to be able to change N to 250 so it needs to be easy to write for large N's

Thank you for your help!
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frozenguy
#2
Feb23-12, 03:20 AM
P: 190
Something is wrong with MatLab. Both my copy and the one at school. I have been using it only for a day or so and I have found a big bug.

When I restart MatLab, my code that was working just before quitting wont work and will get error.

All I have to do is cut it from the file, run the file, then put it back in and presto. No error.

What a complete waste of time.


Anyways, I constructed my matrix using this:

phi(i,j)=atan2((eta(j)-eta(i)),(zeta(j)-zeta(i)))*180/pi;

And right above that I have j=1:4

My matrix looks like this:
0 -45.0000 -90.0000 -135.0000
135.0000 0 -135.0000 -180.0000
90.0000 45.0000 0 135.0000
45.0000 0.0000 -45.0000 0

Which is basically correct BUT! Because of atan2 function, those negatives need 360 deg added. So this is what I did (worked on theta earlier in code). But it wont work this time for phi(i,j):
if phi(i,j)<0;

phi(i,j)=phi(i,j)+360
end
Pythagorean
#3
Feb23-12, 04:09 AM
PF Gold
Pythagorean's Avatar
P: 4,287
the "error" your having might have simply been that you had variables in your work space from previous runs and playing in the command line. When you restart, you wipe your work space.

Try putting 'clear all' at the very beginning of your code, which will mimic having an empty workspace (like when you first start matlab). If you get the same error using 'clear all' then that's your problem. It means you're missing something in your code that you took for granted.

Also, you want to nest the for loops if you want to go along the matrix like you're imagining:

for i = i range
for j = j range

stuff
phi(i,j) = blahblah
stuff

end
end

frozenguy
#4
Feb23-12, 05:01 PM
P: 190
Matrix help in MatLab, Not the simple matrix I have searched, found nothing!

Quote Quote by Pythagorean View Post
the "error" your having might have simply been that you had variables in your work space from previous runs and playing in the command line. When you restart, you wipe your work space.

Try putting 'clear all' at the very beginning of your code, which will mimic having an empty workspace (like when you first start matlab). If you get the same error using 'clear all' then that's your problem. It means you're missing something in your code that you took for granted.

Also, you want to nest the for loops if you want to go along the matrix like you're imagining:

for i = i range
for j = j range

stuff
phi(i,j) = blahblah
stuff

end
end
What is nesting the for loops?

And also, why am I getting negative angles for phi when I said
If phi(i,j)<0
phi(i,j)=phi(i,j)+360
end

I can say
else phi(i,j)=phi(i,j)+360 but then it just adds 360 to everything.

I just need to add 360 to phi only if phi is negative.


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