If statement not returning anticipated output

Purpose is to allow s1 to be printed as positive bearing in the 360 degree coordinate system. However, when s1 is printing as it's negative counterpart and it seems the if statement is being completely skipped. Beyond this, even if the statement is rewritten to evaluate if the variable sad is greater than 0 the result is still the same. It is probably trivial but I am stumped.

if sad < 0
    s1 = sad + 360;
else
    s1 = sad;
end

and disp(s1) returns -140 instead of the anticipated 220

Answer 1

[Star Strider]

There must be something elsewhere in your code that’s causing the problem.

This works fine for me:

sad = -140;
if sad < 0
    s1 = sad + 360;
else
    s1 = sad;
end

produces:

s1 =

     220

For sad = 140, s1 = 140.

[Thomas Robertson]

I noticed that but for some reason it would only operate externally to the main script. This is the bulk of my script up until now:

% Calculating wind's true bearing

if wid > 180
    w1 = wid - 175;
else
    w1 = wid + 185;
end

if w1 >= 360
    w2 = w1 - 360;
else
    w2 = w1;
end

% Langmuir/Stokes SaG

wv1 = wiv*0.03;

% Conversion for degrees to radians for pola2cart/cart2pola
w3 = degtorad(w2);
c1 = degtorad(cud);

% Polar to Cartesian
[wx,wy] = pol2cart(w3,wv1);
[cx,cy] = pol2cart(c1,cuv);

% Calculation of Sargassum velocity and direction
sx = wx + cx;
sy = wy + cy;

[sad,sav] = cart2pol(sx,sy);

% Conversion from radians to degrees for output display

sad = radtodeg(sad);

if sad < 0
    s1 = sad + 360;
else
    s1 = sad;
end

disp(s1);

I also found that the output varies by the size of input

% Single point parameters
>> waca(20,090,0.5,135)
  219.0239
% Single point parameters with just the paramater above
>> waca(wiv,wid,cuv,cud)
  219.0239
% Importing the data from .csv and applying script
>> waca(wiv,wid,cuv,cud)
   33.1318
    7.5604
   20.4237
   39.3418
   33.1318
   68.8177
   19.1261
   32.8604
   59.9695
   68.8177
  162.4274
 -140.9761
   58.9683
  113.1049
  162.4274
  122.2396
   82.5936
   60.4028
   91.8746
  122.2396

[Star Strider]

I’m lost. I have no idea what ‘waca’ is or what it does or what you want your code to do, or what is not working as it should.

I understand you’re correcting magnetic to true, but that’s it. What I see seems to work as it should.

The only suggestion I have is that you can replace your ‘w2’ calculation with:

w2 = rem(w1,360);

[Joseph Cheng]

Have you tried putting a break-point at or before the if statement for sad? Is there a difference between the sad values when it is positive and negative (size, type, etc?)

[Thomas Robertson]

Star:

The gist of it is that it takes the vector data for wind and current, calculates the true bearing and applies Stoke's drift ultimately to calculate the net set and drift of a point in the ocean. I am sure you can guess but the variables '--d' are the bearing data and '--v' are the velocity data. The deviations (s1,w1,wv1,etc.) are just my way of tracking where the changes were (or were supposed to) be made when I was attempting to debug. Appreciate the suggestion though. I like scripts to be as minimalistic as possible for my understanding of matlab.

Joseph:

I have not but am fairly new with matlab and am not sure I know how to. The issue with the variable sad being negative is that it shows the angle over the [180 -180] domain when the interested parties are going to want it to be [0 360] (I'm sure you knew that, just wanted to be clear). I know this is happening being the 'carttopol' function is calculating the arctan in QIII but am not sure how to remedy that in matlab.

[Star Strider]

We’re missing wiv, and I don’t completely understand the complexities of Stoke’s drift on a brief reading of the Wikipedia article, except that it seems to involve wave dynamics.

If you are simply interested in the velocity vector that results from the interaction of current and wind vectors on something floating on the surface (and neglecting the wave dynamics, drag, and other complications), so the resulting direction is the vector sum of these two velocity vectors, this example may do what you want. I did not do any magnetic-to-true conversions, assuming that you do them before you get this far. I included a compass plot to visualise the result:

wd = randi(360, 1, 9);      % CREATE WIND DIRECTIONS, VELOCITIES
wv = randi(30, 1, 9);

cd = randi(360, 1, 9);      % CREATE CURRENT DIRECTIONS, VELOCITIES
cv = randi(5, 1, 9);

wx = wv .* cosd(wd);        % CONVERT TO CARTESIAN
wy = wv .* sind(wd);

cx = cv .* cosd(cd);
cy = cv .* sind(cd);

rx = wx + cx;               % CALCULATE RESULTING VELOCITY (CARTEISIAN)
ry = wy + cy;

rd = atan2d(ry,rx);         % RESULTING DIRECTIONS (POLAR)
rd360 = rd;
rd360(rd360<0) = rd360(rd360<0) + 360; % 360° CORRECTION
rv = hypot(rx,ry);          % RESULTING VELOCITIES (POLAR)

figure(1)                   % PLOT TO CHEC ACCURACY
for k1 = 1:length(wd)
    subplot(3,3,k1)
    compass([wx(k1) cx(k1)], [wy(k1) cy(k1)], '-b') % ORIGINAL X,Y COORDINATES
    hold on
    rxp(k1) = rv(k1)*cosd(rd360(k1));   % CALCULATE RESULTING X,Y FROM ‘rd360’ & ‘rv’
    ryp(k1) = rv(k1)*sind(rd360(k1));
    compass(rxp(k1), ryp(k1), '-r')
    hold off
    title(sprintf('W(%.1f, %.1f), C(%.1f, %.1f), R(%.1f, %.1f)', wd(k1),wv(k1),cd(k1),cv(k1),rd360(k1),rv(k1)), 'FontName','Candara', 'FontSize',7)
end

I used nine-element vectors because they fit neatly on the subplot.

[Geoff Hayes]

@Thomas - it would be interesting to see what the negative sad value is before the code encounters the if statement and adds the 360 to it. Try changing the disp(s1) to disp([sad s1]). It may be, for this example, that sad is -500.9761.

If that is the case, then a while loop may be needed instead:

 while sad<0
     sad = sad + 360;
 end
 s1 = sad;