CODEWITHSUNDEEP
matlabfunctionfor-loopoctave
Rick T
Rick T

Reputation: 3389

For loop equation into Octave / Matlab code

I'm using octave 3.8.1 which works like matlab.

I have an array of thousands of values I've only included three groupings as an example below:

(amp1=0.2; freq1=3; phase1=1; is an example of one grouping)

t=0;
amp1=0.2; freq1=3; phase1=1;   %1st grouping
amp2=1.4; freq2=2; phase2=1.7; %2nd grouping
amp3=0.8; freq3=5; phase3=1.5; %3rd grouping

The Octave / Matlab code below solves for Y so I can plug it back into the equation to check values along with calculating values not located in the array.

clear all
t=0;
Y=0;
a1=[.2,3,1;1.4,2,1.7;.8,5,1.5]
for kk=1:1:length(a1)
    Y=Y+a1(kk,1)*cos ((a1(kk,2))*t+a1(kk,3))
    kk
end
Y

PS: I'm not trying to solve for Y since it's already solved for I'm trying to solve for Phase

The formulas located below are used to calculate Phase but I'm not sure how to put it into a for loop that will work in an array of n groupings:

How would I write the equation / for loop for finding the phase if I want to find freq=2.5 and amp=.23 and the phase is unknown I've looked online and it may require writing non linear equations which I'm not sure how to convert what I'm trying to do into such an equation.

phase1_test=acos(Y/amp1-amp3*cos(2*freq3*pi*t+phase3)/amp1-amp2*cos(2*freq2*pi*t+phase2)/amp1)-2*freq1*pi*t
phase2_test=acos(Y/amp2-amp3*cos(2*freq3*pi*t+phase3)/amp2-amp1*cos(2*freq1*pi*t+phase1)/amp2)-2*freq2*pi*t
phase3_test=acos(Y/amp3-amp2*cos(2*freq2*pi*t+phase2)/amp3-amp1*cos(2*freq1*pi*t+phase1)/amp3)-2*freq2*pi*t

Image of formula below: image of formula

I would like to do a check / calculate phases if given a freq and amp values.

I know I have to do a for loop but how do I convert the phase equation into a for loop so it will work on n groupings in an array and calculate different values not found in the array?

Basically I would be given an array of n groupings and freq=2.5 and amp=.23 and use the formula to calculate phase. Note: freq will not always be in the array hence why I'm trying to calculate the phase using a formula.

Upvotes: 2

Views: 612

Answers (1)

barceloco
barceloco

Reputation: 458

Ok, I think I finally understand your question:

  • you are trying to find a set of phase1, phase2,..., phaseN, such that equations like the ones you describe are satisfied
  • You know how to find y, and you supply values for freq and amp.
  • In Matlab, such a problem would be solved using, for example fsolve, but let's look at your problem step by step.

For simplicity, let me re-write your equations for phase1, phase2, and phase3. For example, your first equation, the one for phase1, would read

amp1*cos(phase1 + 2 freq1 pi t) + amp2*cos(2 freq2 pi t + phase2) + amp3*cos(2 freq3 pi t + phase3) - y = 0

Note that ampX (X is a placeholder for 1, 2, 3) are given, pi is a constant, t is given via Y (I think), freqX are given.

Hence, you are, in fact, dealing with a non-linear vector equation of the form

F(phase) = 0

where F is a multi-dimensional (vector) function taking a multi-dimensional (vector) input variable phase (comprised of phase1, phase2,..., phaseN). And you are looking for the set of phaseX, where all of the components of your vector function F are zero. N.B. F is a shorthand for your functions. Therefore, the first component of F, called f1, for example, is

f1 = amp1*cos(phase1+...) + amp2*cos(phase2+...) + amp3*cos(phase3+...) - y = 0.

Hence, f1 is a one-dimensional function of phase1, phase2, and phase3.

The technical term for what you are trying to do is find a zero of a non-linear vector function, or find a solution of a non-linear vector function. In Matlab, there are different approaches.

For a one-dimensional function, you can use fzero, which is explained at http://www.mathworks.com/help/matlab/ref/fzero.html?refresh=true

For a multi-dimensional (vector) function as yours, I would look into using fsolve, which is part of Matlab's optimization toolbox (which means I don't know how to do this in Octave). The function fsolve is explained at http://www.mathworks.com/help/optim/ug/fsolve.html

If you know an approximate solution for your phases, you may also look into iterative, local methods.

In particular, I would recommend you look into the Newton's Method, which allows you to find a solution to your system of equations F. Wikipedia has a good explanation of Newton's Method at https://en.wikipedia.org/wiki/Newton%27s_method . Newton iterations are very simple to implement and you should find a lot of resources online. You will have to compute the derivative of your function F with respect to each of your variables phaseX, which is very simple to compute since you're only dealing with cos() functions. For starters, have a look at the one-dimensional Newton iteration method in Matlab at http://www.math.colostate.edu/~gerhard/classes/331/lab/newton.html .

Finally, if you want to dig deeper, I found a textbook on this topic from the society for industrial and applied math: https://www.siam.org/books/textbooks/fr16_book.pdf .

As you can see, this is a very large field; Newton's method should be able to help you out, though.

Good luck!

Upvotes: 1

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