How to name every row of 100*2500 matrix?

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Md Monirul Islam 2017-11-1

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https://ww2.mathworks.cn/matlabcentral/answers/364374-how-to-name-every-row-of-100-2500-matrix

评论： Steven Lord 2020-11-27

currently i am working with a matrix. The dimension of the matrix is 100*2500. I need to assign each row the matrix to a new variable. How i can do this?

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Md Monirul Islam 2017-11-1

编辑：per isakson 2017-11-1

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Let me clear it. Let data = ones(3,5); So all I want is as followings:

row1= data(1,:), row2=data(2,:) and row3= data(3,:).

Les Beckham 2017-11-1

But Matlab is designed to allow you to operate on matrices and (more specifically, in this case) subsets of matrices using indexing. That is what per has suggested and, I think, you should try. You don't need to give a separate name like row1 to data(1,:), etc. -- just apply your operation to data(1,:), probably using a loop where the 1 is replaced by the loop index. Then you don't have to code a hundred lines to process row1 through row100.

Please note especially the last bullet in per's last comment.

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Answer 1

per isakson 2017-11-1

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编辑：per isakson 2017-11-1

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"So all I want is as followings" See TUTORIAL: Why Variables Should Not Be Named Dynamically (eval). However, I guess you need to make your own mistakes to understand that it's a really bad idea.

M = ones( 3, 5 );
for rr = 1 : size(M,1)
    variable_name = sprintf( 'row%03d', rr );
    assign( variable_name, M(rr,:) );
end

where

function    assign( varargin )
    switch nargin
        case { 2 },
            if  isvarname( varargin{ 1 } )
                Name = varargin{ 1 };
            else
                error( ['poi: First input argument, ', ...
                         inputname(1), ' must be a legal name'] ),
            end,
            Value = varargin{ 2 };
        otherwise
            error( 'poi: Wrong number of input arguments' ),
    end
    assignin( 'caller', Name, Value );
end

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Stephen23 2017-11-1

编辑：Stephen23 2017-11-1

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@Md Monirul Islam: if you had told us what you are actually trying to do, then we could have shown you better ways of doing it. For example, instead of wasting both your and MATLAB's time magically creating lots of separate variables just to save them in a .mat file, you can simply split the matrix rows into a structure, and then provide that structure to save:

>> M = rand(100,3);
>> C = sprintfc('row%d',1:size(M,1));
>> S = cell2struct(num2cell(M,2),C,1);
>> save('test.mat','-struct','S')

Did you see what I did there? Instead of using your complex and slow method using magical variable names, I used only simple, standard MATLAB concepts. My code uses just four lines, about one third the number of lines than your code does! It does not require any function to be defined. And is also easier to debug at the same time.

It is much better if you should tell us what your real task is, before deciding what slow, inefficient, and buggy code you will use:

http://xyproblem.info/

Muhammad Ammar bin Faizul Azli 2020-11-27

@Stephen Cobeldick

May i know what would your code be if i were to do the same for column?

Steven Lord 2020-11-27

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With functionality available now, I'd probably use a table array instead of a numeric array.

cities = ["Albuquerque"; "Boston"; "Chicago"; "Dallas"];
dist = array2table(magic(4), 'VariableNames', cities, 'RowNames', cities)
dist = 4x4 table
                   Albuquerque    Boston    Chicago    Dallas
                   ___________    ______    _______    ______

    Albuquerque        16            2         3         13  
    Boston              5           11        10          8  
    Chicago             9            7         6         12  
    Dallas              4           14        15          1  
distBC = dist{'Boston', 'Chicago'}
distBC = 10