How do I loop and save columns from a cell array as a double?

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lil brain 2022-3-24

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评论： Voss 2022-3-25

cell_of_doubles.mat

Hi,

I have a cell of doubles array which contains 19 cells. I am looking to build a for loop which saves the first three columns of each cell as a seperate three-column vector. In the end I would like to end up with 19 three-column vectors (19 variables).

How would that look?

Thank you!

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Answer 1

Voss 2022-3-25

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https://ww2.mathworks.cn/matlabcentral/answers/1680084-how-do-i-loop-and-save-columns-from-a-cell-array-as-a-double#answer_926389

编辑：Voss 2022-3-25

在 MATLAB Online 中打开

cell_of_doubles.mat

You don't want 19 separate variables. Just make another cell array.

S = load('cell_of_doubles.mat');
S.cell_of_double_balls
ans = 1×19 cell array
    {530×9 double}    {548×9 double}    {1045×9 double}    {1938×9 double}    {444×9 double}    {4548×9 double}    {715×9 double}    {1084×9 double}    {1079×9 double}    {592×9 double}    {3261×9 double}    {690×9 double}    {1063×9 double}    {543×9 double}    {2310×9 double}    {894×9 double}    {608×9 double}    {1050×9 double}    {10134×9 double}
% you don't need a loop:
C = cellfun(@(x)x(:,1:3),S.cell_of_double_balls,'UniformOutput',false)
C = 1×19 cell array
    {530×3 double}    {548×3 double}    {1045×3 double}    {1938×3 double}    {444×3 double}    {4548×3 double}    {715×3 double}    {1084×3 double}    {1079×3 double}    {592×3 double}    {3261×3 double}    {690×3 double}    {1063×3 double}    {543×3 double}    {2310×3 double}    {894×3 double}    {608×3 double}    {1050×3 double}    {10134×3 double}
% but here's one anyway:
C = cell(size(S.cell_of_double_balls));
for ii = 1:numel(C)
    C{ii} = S.cell_of_double_balls{ii}(:,1:3);
end
C
C = 1×19 cell array
    {530×3 double}    {548×3 double}    {1045×3 double}    {1938×3 double}    {444×3 double}    {4548×3 double}    {715×3 double}    {1084×3 double}    {1079×3 double}    {592×3 double}    {3261×3 double}    {690×3 double}    {1063×3 double}    {543×3 double}    {2310×3 double}    {894×3 double}    {608×3 double}    {1050×3 double}    {10134×3 double}

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lil brain 2022-3-25

Hi,

thanks!

The reason I wanted seperate variables is because when I use this formula,

dist = sqrt(sum(C{1,1}(:,1:3),1,1).^2);

it gives me the error,

Error using sum

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

Even though they are all doubles.

Any idea why?

Voss 2022-3-25

在 MATLAB Online 中打开

dist = sqrt(sum(C{1,1}(:,1:3),1,1).^2);
%                              ^^ third input to sum()

The third input argument to the sum function must be a character vector saying 'double', 'native', 'default', (to specify the output class) 'omitnan', or 'includenan' (to specify how the summation should treat NaNs). This is what the error message means.

If C{1,1}(:,1:3) is a matrix of (x,y,z) coordinates, then maybe you intend to calculate the distance between adjacent points in the matrix i.e., points in adjacent rows of C{1,1}(:,1:3), which involves taking the difference of adjacent (x,y,z) coordinates, to get (dx,dy,dz), then squaring those values, adding dx^2+dy^2+dz^2, and finally taking the square root (the distance formula):

dist = sqrt(sum(diff(C{1,1}(:,1:3),1,1).^2,2));

(Read about the second and third arguments to diff and the second argument to sum to know what they are for.)

If that's what you want it to do, then that will work whether the first argument to the sum function is its own variable or three columns from a matrix contained in a cell array, which it is here.

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