I am trying to write a function but it will not work.

function [ absorption ] = solar_rad( nc,lat,t,I)
%Calculating absorbed solar radiation
%   Calculating the absorbed solar radiaiton as a function of time of
%   day and year, cloud cover, and latitude, using a time step of one hour.

%BEGIN
for I=1:365;

      for T=[0:24];
          t=T*3600;
      end

     delta=-23.45*(pi/180)*cosd(2*pi)/365*(I+9); %Declination 
     S=1357+45*cosd((2*pi)/365)*(I); %Solar constant

    for lat=0;
      sinh=max(sind(lat)*sind(delta)+cosd(lat)*cosd(delta)*cosd((2*pi/86400)*(t+43200)));

      m=1/sinh; %optical length
      Cext=0.128-0.0253*(log(m)); %Extinction
    end

    for h=asin(max(sind(lat)*sind(delta)+cosd(lat)*cosd(delta)*cosd((2*pi)/86400)*(t+43200)));
      i=(pi/2)-h; %angle of incidence
      j=asin(0.75*sin(i)); %angle of reflection
      r=0.5*abs((sin(i-j)^2/sin(i+j)^2)+(tan(i-j)^2)/(tan(i+j)^2)); %reflectance

    end

end

for nc=0:100;

      if h>0
      Insd=S*(exp(-Cext*m))*(sin(h))*(1-(0.71*nc)); %direct incoming solar radiation at sea level
      end

      if h<=0
      Insd=0;
      end

      for Insg=0.52*nc*Insd; %global radiation
      Q_sun=Insd*(1-r)+0.97*Insg; 
      end

end

[absorption]=solar_rad(0,0,t,I);

Whenever I run it I get this error message:

Error in solar_rad (line 7)
for I=1:365;

Output argument "absorption" (and maybe others) not assigned during call to "\\fs-home-k\home-005\osp42f\My
Documents\MATLAB\Programmes\solar_rad.m>solar_rad".

Answer 1

[Star Strider]

You don’t have any variable named ‘absorption’ on the left-hand side of any statement anywhere in your code for your ‘solar_rad’ function. It has to have a variable by that specific name assigned somewhere in your code or it will not return any value for that variable.

[Meghan]

Yea I realised that and ended up fixing my code again! Now I'm just trying to figure out how to make a matrix joining I and T!

[Star Strider]

I don’t understand what you mean by ‘a matrix joining I and T’. ‘T’ and ‘I’ are indices.

Also, I don’t understand what you’re doing here:

cosd(2*pi)

since the trig functions ending with ‘d’ take their arguments in degrees. Those without take their arguments in radians. In any event, cos(2*pi) will always evaluate to 1, and cosd(2*pi) will always evaluate to 0.994.

Answer 2

[Adam]

r=0.5*abs((sin(i-j)^2/sin(i+j)^2)+(tan(i-j)^2)/(tan(i+j)^2));

That is your maths for 'r'. Without putting it into my Matlab it seems unlikely that will return an integer every time.

Q_sun=Insd(1-r)+0.97*Insg;

That is your maths indexing into Insd, which uses 'r'. Even if 'r' were an integer though the fact you use '1-r' means you will never end up with an integer index into your Insd array unless r == 0;

[Meghan]

I changed my code a little and now I get a new error message:

function [ absorption ] = solar_rad( nc,lat,t,I)
%Calculating absorbed solar radiation
%   Calculating the absorbed solar radiaiton as a function of time of
%   day and year, cloud cover, and latitude, using a time step of one hour.

%BEGIN
for I=1:365;

      for T=[0:24];
          t=T*3600;
      end

     delta=-23.45*(pi/180)*cosd(2*pi)/365*(I+9); %Declination 
     S=1357+45*cosd((2*pi)/365)*(I); %Solar constant

    for lat=0;
      sinh=max(sind(lat)*sind(delta)+cosd(lat)*cosd(delta)*cosd((2*pi/86400)*(t+43200)));

      m=1/sinh; %optical length
      Cext=0.128-0.0253*(log(m)); %Extinction
    end

    for h=asin(max(sind(lat)*sind(delta)+cosd(lat)*cosd(delta)*cosd((2*pi)/86400)*(t+43200)));
      i=(pi/2)-h; %angle of incidence
      j=asin(0.75*sin(i)); %angle of reflection
      r=0.5*abs((sin(i-j)^2/sin(i+j)^2)+(tan(i-j)^2)/(tan(i+j)^2)); %reflectance

    end

end

for nc=0:100;

      if h>0
      Insd=S*(exp(-Cext*m))*(sin(h))*(1-(0.71*nc)); %direct incoming solar radiation at sea level
      end

      if h<=0
      Insd=0;
      end

      for Insg=0.52*nc*Insd; %global radiation
      Q_sun=Insd*(1-r)+0.97*Insg; 
      end

end

[absorption]=solar_rad(0,0,t,I);

Error in solar_rad (line 7)
for I=1:365;

Output argument "absorption" (and maybe others) not assigned during call to "\\fs-home-k\home-005\osp42f\My
Documents\MATLAB\Programmes\solar_rad.m>solar_rad".

Any idea what I'm doing wrong? I've been trying to write this for 3 days!

[Adam]

You don't want a ';' at the end of that for loop line, although that maybe does no harm.

The error message tells you exactly what the problem is though despite perhaps erroneously pointing at that line.

You have stated there should be an output argument called 'absorption' yet you never create it.