Help writing a loop...please

I know this may be dumb but I was not properly taught how to use MATLAB. I am trying to write a loop that solves for a value using one variable (A), but when my input value exceeds a certain value I need it to solve using a different variable (B) EXAMPLE

I have a range of values: W = 0.188:0.001:1.256

when W <= 5.24/Tp (Tp = 11) Sigma = 0.07

but

when W > 5.24/Tp Sigma = 0.09

Here are my two equations I am using that uses my input variables:

Y = exp(-((0.191.*w*Tp-1)./(sqrt(2)*sigma)).^2)

S_w = 155.*((Hs^2)./((Tp^4).*(w.^5))).*exp((-944./((Tp^4).*(w.^4)))).*(3.3.^Y)

I have tried and can only get sigma to equal 0.09. It never uses 0.07. I really need help on this quick and I appreciate all help. Thanks

[Gurudatha Pai]

I might be able to help if you share the code you have written so far.

[Stephen]

I have no code written yet other than my basic input variables (w, Tp)...its the first part of the code that needs to be written, but yet have tried and failed

[Stephen]

here is what i have...but for some reason my Y and S_w values are not changing. Their staying the same

% Inputing Given Variables:

Hs = 12; % Significant wave height (meters)

Tp = 11; % Peak period (seconds)

% Setting a range of frequency with a lower bound and upper bound

w = 0.188:0.000001:1.256;

% Writting a loop to determine which sigma value to use

ii=0;

% For sigma:

for w=0.188:.021363:1.256

ii=1+ii;

if w <= 5.24/Tp,

sigma(ii)=.07;

else

sigma(ii)=.09;

end

end

% JONSWAP EQUATION:

Y = exp(-((0.191.*w*Tp-1)./(sqrt(2)*sigma)).^2)

S_w = 155.*((Hs^2)./((Tp^4).*(w.^5))).*exp((-944./((Tp^4).*(w.^4)))).*(3.3.^Y)';

% Plotting JONSWAP:

plot(w,S_w,'red')

xlabel('w (rad/sec)')

ylabel('...')

title('JONSWAP Prob. 1A')

grid on

Answer 1

[alex]

Yes 'Sigma' is evaluated according to the first value in array of'W' that's why you get only one value if 'Sigma' TO FIX IT: use array of sigmas,that has the same size as 'W' and the values in that sigma array have values either 0.07 or 0.09 according the condition on values of W i.e.: if W(1)<=5.24/Tp,then Sigma(1)=0.07 but if W(1)>5.24/Tp,then Sigma(1)=0.09 the same should be done with rest values of array -you can do it in LOOP IMPLEMENTATION -or you can do it in VECTOR MATH IMPLEMENTATION %however,enough with words here is the CODE that does it-in vector %way,:

function ask_from_portal W = 0.188:0.001:1.256; Tp=11; auxilary_array=W<=5.24/Tp;%where the condition is right has value '1' Sigma=ones(size(W));%allocating the ARRAY of sigmas

Sigma(auxilary_array)=0.09;% if W<=5.24/Tp,then Sigma=0.09; Sigma(~auxilary_array)=0.07;%else %W>5.24/Tp % ~ is logical 'not' in matlab bar(Sigma);%can clearly be seen thats Sigma has 0.07 and 0.09 values %Y=exp(-((0.191.*W*Tp-1)./sqrt(2)*sigma)).^2); %-------END CODE--------------