Creating a loop within a loop

[EDIT: 20111027 17:42 CDT - merge another revised question - WDR]

[EDIT: 20111027 17:10 CDT - merge revised question - WDR]

Hello everyone,

I'm presently trying to construct a loop within a loop that based on changing date (1:365) and changing latitude (0:90), and the subsequent effects on the below equation:

    x=sin(lat).*sin(Dec)+cos(lat).*cos(Dec).*cos(wd.*((t+1)+43200));

I've tried this:

    for t=1:365
        x(t+1)=sin(lat).*sin(Dec)+cos(lat).*cos(Dec).*cos(wd.*((t+1)+43200));
        for lat=0:90
            x(lat+1)=sin(lat+1).*sin(Dec)+cos(lat+1).*cos(Dec).*cos(wd.*(t+43200));
        end
    end
    x

and this:

    for t=365;
    lat=1;
    x=sin(lat).*sin(Dec)+cos(lat).*cos(Dec).*cos(wd.*(t+43200));
    while length(lat)==90;
        lat=lat+1;
        while length(t)==365;
            t=t+1;
        end
    end
    end

    x;

Anyone have any ideas?

[Revised Question]

Hello everyone,

I'm presently trying to construct a loop with two variables, changing date (1:365) and changing latitude (0:90), with the aim of the loop to calculate all the possible outputs of the below equation:

    x=sin(lat).*sin(Dec)+cos(lat).*cos(Dec).*cos(wd.*(t+43200));

So far i've tried this but it doesn't work:

    lat=1;
    t=1;
    x=sin(lat).*sin(Dec)+cos(lat).*cos(Dec).*cos(wd.*(t+43200));
    while length(lat)==90;
        lat=lat+1;
        x=[x,sin(lat).*sin(Dec)+cos(lat).*cos(Dec).*cos(wd.*(t+43200))];
        while length(t)==365;
            t=t+1;
            x=[x,sin(lat).*sin(Dec)+cos(lat).*cos(Dec).*cos(wd.*(t+43200))];
        end
    end

    x

[Revision #2]

I'm just trying to construct a loop that calculates all the possible outputs based on two variables (lat=0:90, t=1:365), with the equation:

    x=sin(lat).*sin(Dec)+cos(lat).*cos(Dec).*cos(wd.*(t+43200));

So far, my latest attempt is:

    for lat=0:90;
        for t=1:365;
            x(lat,t)=sin(lat).*sin(Dec)+cos(lat).*cos(Dec).*cos(wd.*(t+43200));
        end
    end

    x

Can anyone recommend a workable method?

[the cyclist]

Please make your code more readable by using the "Code" markup button.

Answer 1

[Walter Roberson]

For your revision #2:

[LAT,T] = ndgrid(0:90, 1:365);
x=sin(LAT).*sin(Dec)+cos(LAT).*cos(Dec).*cos(wd.*(T+43200));

[Josh]

Apologies, I thought the question had been deleted.

[Walter Roberson]

Might I point out that LAT of 0 to 90 looks suspiciously like "degrees", but that sin() and cos() work on RADIANS ?

[Josh]

??? Error using ==> times

Matrix dimensions must agree.

The above code creates two grids but does not successfully calculate x, tried to manipulate it a bit but with no success yet.

[Josh]

That is an excellent point ^Radians^ - I may have had a rather embarrassing moment, I'll just check...

[Walter Roberson]

Is there a possibility that Dec or wd might not be scalars? The above code requires Dec and wd to be scalars. I did test it with a couple of sample scalar values, so I know that normally it would not give the message about matrix dimensions not agreeing.

When you are checking radians, be sure to check Dec and wd .

[Josh]

Dec and wd were already in radians, changed the grid to make it more radian appropriate but still have matrix dimensions error:

[LAT,T]=ndgrid(0:RC:RC*90,RC*1:RC:RC*365);

x=sin(LAT).*sin(Dec)+cos(LAT).*cos(Dec).*cos(wd.*(T+43200));

RC=0.0018 as an approximate radians calculation.

Dec is a 1x365 matrix and wd is a conversion coefficient.

[Walter Roberson]

Nothing like omitting some fundamental information :(

Your desired output is not well defined. You want one output per lat and t value, but sin(lat) .* sin(Dec) is going to result in a vector of values for a single lat and t value.

Perhaps what you want to calculate is

sin(lat) .* sin(Dec(t)) + cos(lat) .* cos(Dec(t)) .* cos(wd.*(t+43200))

If so...

[LAT,T] = ndgrid(0:90,1:365);

x = sind(LAT) .* sin(Dec(T)) + cosd(LAT) .* cos(Dec(T)) .* cosd(wd .* (T+43200));

Please be sure to cross-check degrees vs radians here. sind() and cosd() work on degrees, and are a lot easier to read in your situation.

[Josh]

Sorry, thank you so much :-)

Answer 2

[Walter Roberson]

[T,LAT] = ndgrid(1:365,0:90);
x=sin(LAT).*sin(Dec)+cos(LAT).*cos(Dec).*cos(wd.*((T+1)+43200));

[Josh]

This creates the size of the grid I would explain as an output, but it doesn't calculate the outputs in relation to the equation.