Error: Invalid use of operator with nc files

Hi

I have a code where i run a couple of nc-files. I have make a for loop so I can run it over all the time steps and for all the longitudes and latitudes.

But I got an error called "Invalid use of opretor in the line called "zx(:,1:ny) = (:,ny:-1:1)z(n);"

I dont know why and what can I do?

I want to find blocking in the northern hemisphere where I will call if GHGS > 0 or GHGN < -10 there is a blocking. Did I have make a correct if loop?

for i = 1:Nfiles
  lon{i} = ncread(ncfiles(i).name, 'longitude'); nx = length(lon{i});
  lat{i} = ncread(ncfiles(i).name, 'latitude'); ny = length(lat{i});
  time{i} = ncread(ncfiles(i).name, 'time'); nt = length(time{i});
  z{i} = ncread(ncfiles(i).name, 'z'); nz = length(z{i});                                                                                                                                      
end


%Midler geopotentialet til et månedsmiddel
zmean = zeros([nx ny]);
blocks = zeros([nx]);
for n = 1:nt
    z = ncread(ncfiles(i).name,'z',[1 1 n],[nx ny 1]);
    zx(:,1:ny) = (:,ny:-1:1)z(n);
    zmean = zmean + zx;
    %pcolor(lon,lat,z');
    %shading interp 
    %drawnow
    GHGS = (zx(:,[151+[-1 0 1]])-zx(:,[131+[-1 0 1]]))/20;
    GHGN = (zx(:,[171+[-1 0 1]])-zx(:,[151+[-1 0 1]]))/20;
    for i=1:ny
        blocks(i)=blocks(i)+1;
        if GHGS > 0;
           disp('The point is blocked')
        elseif GHGN < -10; 
           disp('The point is blocked')  
        end  
    end
end

[Jan]

Please do not post a question multiple times, because this confuses the readers. See https://www.mathworks.com/matlabcentral/answers/436656-index-exceeds-array-bounds-and-some-other-mistakes

Answer 1

[Jan]

I have no idea, what (:,ny:-1:1)z(n) should do. Something essential is missing here. Maybe you mean - a bold guess:

zx(:,1:ny) = z(:,ny:-1:1) * z(n);
%            ^            ^

In you other question this line is:

zx(:,1:ny) = z(:,ny:-1:1);

So something went wrong during editing.

By the way, follow the hints given in the editor:

% [151+[-1 0 1]]   easier and more efficient:
151+[-1 0 1]  % or
[150, 151, 152]

and

blocks = zeros(nx);   % Without brackets

Note that this is equivalent to:

blocks = zeros(nx, nx);  % not blocks = zeros(nx, 1)!

Because you use a single index running from 1 to ny:

for i=1:ny
        blocks(i) = ...

I guess, that you want this instead:

    blocks = zeros(1, ny);   % or zeros(ny,1)

The line

    if GHGS > 0;

does not need a trailing semicolon and might not do, what you expect: if requires a scalar condition, therefore Matlab evaluates implicitly:

    if all(GHGS > 0) && ~isempty(GHGS)

I assume you mean GHGS(i).

[Jonas Damsbo]

Hi Jan

I got a question to my code line:

 z = ncread(ncfiles(i).name,'z',[1 1 1],[nx ny 1]);

Here I use the syntax for ncread:

 vardata = ncread(source,varname,start,count)

I got the error "Index exceeds array bounds. Where nx and ny is longitudes and latitudes:

nx = 360

ny = 181

 Index exceeds array bounds.

Error in Blocking (line 39)
    z = ncread(ncfiles(i).name,'z',[1 1 1],[nx ny 1]);

[Jan]

Use the debugger to find out, which index is exceeded:

dbstop if error

Then start the code again. When Matlab stops at the error, check the variables and indices:

which ncread  % is it a variable or the function?
size(ncfiles)
i

[Walter Roberson]

Try

    z = ncread(ncfiles(i).name,'z',[1 1 1],[ny nx 1]);

NCF uses rows and columns reversed from MATLAB notation.

[Jonas Damsbo]

Okay, I have found something thats maybe could say something?

If I writing return just here:

for i = 1:Nfiles
  lon{i} = ncread(ncfiles(i).name, 'longitude'); 
  lat{i} = ncread(ncfiles(i).name, 'latitude'); 
  time{i} = ncread(ncfiles(i).name, 'time'); 
  z{i} = ncread(ncfiles(i).name, 'z');  
end

nx = length(lon{i});
ny = length(lat{i});
nt = length(vertcat(time{:}));


zmean = zeros([nx ny]);
blocks = zeros(nx);

return
%Writing the data over all time steps
    for n = 1:nt
    z = ncread(ncfiles(i).name,'z',[1 1 n],[nx ny 1]); 
    zx(:,1:ny) = z(:,ny:-1:1);
    zmean = zmean + zx;
    end

I can see my z variable is a cell array like this:

So it's longitudes x latitudes x time.

Are there a way to combine these cells?

[Walter Roberson]

cell2mat() can combine them. But you need to ask which dimension you want to combine them over. You would probably need to

cell2mat(reshape(z, 1, 1, []))

to combine them over the third dimension (you cannot combine them over the 4th dimension because the third dimensions do not match.)