MATLAB: Matrix full of zeros

ideal gass lawMATLABstructuresvolume

load('air.txt')
Air.temp = air(:,1);
Air.press = air(:,2);
Air_R = 0.287;
for k = 1:length(air)
    Air(k).vol = (Air_R*Air(k).temp)/Air(k).press;
end

I am using the ideal gas law to solve for volume and I made a structure with all the information needed (pressure, temperature). I am trying to make a subfield in that structure called vol (for volume) but the matrix that comes out as a result when I look at Air.vol is a maxtrix full of zeros and then colum 35 has all the values I need. Is there a way to minimize this so there's only one colum with the values I need?

Best Answer

Use element-wise division

Air(k).vol = (Air_R*Air(k).temp)./Air(k).press;
                               %^ add dot here

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MATLAB: “Index exceeds the number of array elements (1).”

The way you define your struct in the first few lines results in Air being a 1x1 struct with two fields (temp and press), each of which is 38x1. So calling Air(2).temp throws an error because Air is only 1x1. To use the indices correctly, your first for loop should read

for k = 1:length(air)
    Air.vol(k) = (Air_R*Air.temp(k))./Air.press(k);
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MATLAB: Using ode45 for high altitude balloon trajectory: need some variables to update every iteration and need to plot altitude vs time.

or variables that change with time or state you should put their calculations inside the function xp. In your case, your states x seem to be [z;z']. Set z = x(1) in the beginning of the function and calculate the variable parameters before you calculate xp(2). It seems you will run into a problem trying to access the z value of the previous timestep (old_z) unless you use a persistent variable. You can try the following.

function xp = F(t,x)
    persistent old_z;
    z = x(1);
    dz = z - old_z;
    old_z = z; % set the value of old_z for next timestep
    % calculate variable parameters
    ...
    xp = zeros(2,1);
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    xp(2) = (g*(RhoA*vol-mass)-.5*RhoA*realCD*x(2)*abs(x(2))*Ca)/(mass+cb*RhoA*vol);
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The persisent variable will remain in memory between calls to the function.

Best Answer

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MATLAB: “Index exceeds the number of array elements (1).”

MATLAB: Using ode45 for high altitude balloon trajectory: need some variables to update every iteration and need to plot altitude vs time.

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