How to start loop when it finds the first maximum

Hello,

In my application, a sinusoidal excitation field is being applied. The value of the maximum excitation field is known. What I am trying to do is to start a loop when the first maximum is reached. I know it seems quite simple, but I can't really find a statement that works. In advantage, thank you so much for your response.

%This code doesn't work

max_value = 104;

%Line to obtain the actual value of the sinusoid (actual_value)

if actual_value == max_value %Start loop when it finds the FIRST maximum of the sinusoid
    
    for i=1:10
        %do something
    end
    
end

[Dyuman Joshi]

What is actual_value supposed to be? Is it the input data? If so, how does it vary?

[JORGE REVUELTA LOSADA]

Thank you for your answer. It is the actual value of a sinusoidal excitation field.

Answer 1

[Mathieu NOE]

hello

this code does work

maybe simply you have to pay attention that such condition statement may not work well simply because of rounding errors or lack of precision in your data between actual_value and max_value

if actual_value == max_value

it's more robust to make this way whith a given tolerance level

     if abs(actual_value - max_value) < tol

fullcode :

%This code does work
max_value = 104;

tol = 1e-1; % tolerance for max value finding (can be reduced if you use a higher sampling rate)

%% dummy data
% one sine period simulation
samples = 53; % samples per period (choose some dummy value to not have "perfect" data)
x = 1:samples-1;
y = max_value*sin(2*pi*x/samples);
plot(y,'-*')

%% main loop

for k = 1:numel(y) % this for loop simulates data acquisition (sample by sample)
    
        actual_value = y(k);    %Line to obtain the actual value of the sinusoid (actual_value)
        
    if abs(actual_value - max_value) < tol %Start loop when it finds the FIRST maximum of the sinusoid
        disp(['max value (' num2str(actual_value) ') reached ! at sample = ' num2str(k)]);

        for i=1:10
            %do something
        end

    end

end

[JORGE REVUELTA LOSADA]

Thank you for your answer. The tolerance was necessary in a first step and as I also had a noisy signal I had to make a fitting. Also I had to change the loop because it was only evaluating for once the actual value of the sinusoidal excitation field. Thank you once more.

[Mathieu NOE]

My pleasure !

Glad it helped

Answer 2

[Tushar Behera]

Hi Jorge,

It is difficult to give a proper solution with the information you have provided. However can you verify what are the values the variable "actual_value" takes during the codes run time. You can do this by debugging your code in debug mode.

I can only assume that probably "actual_value" never attains a value truly equal to "max_value". For example take a look at the below code:

a=2;
b=2.000000000000095
if a==b
    z=3% this statement will never get executed
end

However, for the code

a=2;
b=2.00000000000000000095
if a==b
    z=3%this will get executed
end

z will be assigned the value 3. Try to debug your code and take necessary steps so as "actual_value" will be numerically equal to "max_value". This is happenig because the error tolerance between "actual_value" and "maximu_value" might be too low.Some of the ways you can try is by using "floor" function in your code like below example:

a=2;
b=2.000000000000095
b=floor(b)
if a==b
    z=3% this statement will get executed
end

Also you can try out the suggestion given by Mathieu in order set a limit for tolerance band.

Hope this helps you.

Regards,

tushar

[JORGE REVUELTA LOSADA]

Thank you for your answer, I will take it into account in future developments.