Why do I get: "Error using sum Invalid option. Option must be 'double' " even though my variable is a double?

Hi,

I have this for loop:

n = numel(first_three);
test_first_three = {}; 
for dd = 1:n
    test_first_three{dd} = sqrt(sum(first_three{dd}(:,1:3),1,1).^2); 
end  

Whenever I try to run it I get the error,

Error using sum

Invalid option. Option must be 'double', 'native', 'default', 'omitnan', or 'includenan'.

It isn't clear to me why that is happening since my cell of doubles array is a double.

What I am trying to achieve with this loop is to apply this function: dist = sqrt( dx.^2 + dy.^2 + dz.^2 ); to every row and its adjacent row.

Here dx, dy and dz are the differences (subtracted from one another) between one row (ex: row 2) and its adjacent row (ex: row 1). That way row 2 - row 1, row 3 - row 2, row 4 - row 3 ... and so on.

Does anyone know how I can solve this?

[yilak alemu]

Error using sum

Invalid option. Option must be 'double', 'native', 'default', 'omitnan' or 'includenan'.

how to correct

[Walter Roberson]

@yilak alemu

Please show your current code, and include your MATLAB version number.

Answer 1

[Walter Roberson]

At the moment you have code which is effectively

sum(x,1,1)

where x happens to have 3 columns.

The first 1 there would give the dimension to sum over (so, sum each column), but the second 1 there is in the slot where MATLAB expects only output class 'double', 'native', 'default', or else the flag about how to treat nans. No numeric value is expected in that third parameter.

[lil brain]

Thank you @Walter Roberson for pointing that out. So now I get the sums of the 3 entire columns stored as three values in a vector.

But would you know how I can get the differences of adjacent rows as described above?

[Walter Roberson]

sqrt(sum(diff(first_three{dd}(:,1:3), [], 1).^2, 2))

[lil brain]

Is there perhaps also a solution without using diff? Using diff seems to mess with later steps in my data analysis as I am not able to 2nd order derivative twice.

[Walter Roberson]

temp = first_three{dd}(:,1:3);
sqrt(sum( (temp(2:end,:) - temp(1:end-1,:)).^2, 2))

But you should expect exactly the same problem with secod order derivative later, as subtracting adjacent rows like this is exactly what diff() is going to do (but with a more efficient internal implementation.)