Searching for minimum x root for known y

Hello. I am beginner with matlab and I'm getting confused so I would like some help.

I have the equation y = -0,1042x^5 + 1,9167x^4 - 13,062x^3 + 39,333x^2 - 45,083x + 21 and I have also inserted a matrix of 8760 y values.

I want to build a table that displays the minimum root of x every time for given y values.

Any help?

[Alex Mcaulley]

You can use the function roots combined with min for each case

[Panos Katsa]

i dont know exaclty how to make equations between this function for instance and each and every column of the matrix

[Alex Mcaulley]

For each value in y you can obtain the roots using roots function and then take the minimum value using min

Hints:

• for loop might be useful
• read the documentation of roots

Answer 1

[the cyclist]

This code finds the minimum root, for y == 43.

    y = 43;
    p = [-0.1042 1.9167 13.062 39.333 45.083 21-y];
    min(roots(p))

You'll need to run a loop over your values of y.

[Panos Katsa]

what if y is a column extracted from excel?

thanks a lot

[the cyclist]

Is y a numeric vector in the MATLAB workspace? I would probably do it like this:

y_count = numel(y);

minValues = zeros(y_count,1);

for iy = 1:y_count;
    p = [-0.1042 1.9167 13.062 39.333 45.083 21-y(iy)];
    minValues(iy) = min(roots(p))
end

If y is some other class (e.g. a table) after importing from Excel, then you might need a type conversion in there.

[Panos Katsa]

it is a table extracted from excel

[the cyclist]

Here is one way to convert from table to numeric. I also documented what the prior code was doing.

% Create a variable y that is a table (where the variable *inside* the table is also named
% "y"). You should use your real data here instead
y = table([22; 44],'VariableNames',{'y'});

% Convert it to numeric
y_numeric = table2array(y);

% Get the number of values
y_count = numel(y_numeric);

% Preallocate the array for the minimum values
minValues = zeros(y_count,1);

% Loop over each value of y, and store the minimum root
for iy = 1:y_count
    p = [-0.1042 1.9167 13.062 39.333 45.083 21-y_numeric(iy)];
    minValues(iy) = min(roots(p));
end

[Panos Katsa]

thanks a lot