Help writing a loop...please

Question

Stephen 2012-2-26

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/30352-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

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Stephen 2012-2-26

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 2012-2-26

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

% 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

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Answer 1

alex 2012-2-26

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/30352-help-writing-a-loop-please#answer_38812

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--------------