select selected numeber of rows from an huge array

Question

Damith 2014-5-12

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/129314-select-selected-numeber-of-rows-from-an-huge-array

评论： Damith 2014-5-16

Hi,

I need to select 365 (366,:) rows from array of [3652500 x 2] and write a matrix [366 x 10000]. Can somebody help me with that. I am new to MATLAB.

Thanks in advance.

5 个评论
显示 3更早的评论隐藏 3更早的评论

dpb 2014-5-12

编辑：dpb 2014-5-12

在 MATLAB Online 中打开

>>  (3652500*2)==(366*10000)
ans =
   0
>> num2str((3652500*2)/366,'%.5f')
ans =
19959.01639
>>

Don't fit.

doc reshape % but will have to be commensurate sizes

Walter Roberson 2014-5-12

Your first dimension is being reduced by a factor of about 10000, but your second dimension is being increased by a factor of 5000 (10000 / 2). Where is the remaining data to go?

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

per isakson 2014-5-12

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/129314-select-selected-numeber-of-rows-from-an-huge-array#answer_136539

编辑：per isakson 2014-5-14

在 MATLAB Online 中打开

There are a couple of magic numbers in your question. I guess it is the number of days in a year. "366" is leap year.

    >> M = cssm;
    >> whos M

returns

    Name        Size                  Bytes  Class     Attributes
    M         366x10000            29280000  double

where

    function M02 = cssm
        N01 = 3652500;
        N02 = 1e4;
        M01 = randi( [1,17], [N01,2] );
        M02 = nan( 366, N02 );
        x01 = 1;
        for jj = 1 : N02
            if rem( jj, 4 ) >= 1
                x02 = x01 + 365 - 1;
                M02(1:365,jj) = M01( x01:x02, 1 ); 
                x01 = x02+1;
            else % leap year
                x02 = x01 + 366 - 1;
                M02(1:366,jj) = M01( x01:x02, 1 ); 
                x01 = x02+1;
            end
        end
    end

.

Comment: I chose to build the new array from the first column of your array. You can easily modify the code to use the second column or both.

.

Answer to comment:

I downloaded 0001.txt, but I still lack information

What is the first column of whole numbers? It is not time information.
Which of the first and second column shall be included in the result?
There is no way to decide which data belongs to leap years.

Try

    M02 = cssm;
    whos M02

which returns

    Name        Size                  Bytes  Class     Attributes
    M02       366x10000            29280000  double

where

    function    M02 = cssm( varargin )
        narginchk( 0, 1 )
        if nargin == 1
            filespec = varargin{1};
        else
            filespec = 'h:\m\cssm\0001.txt';
        end
        fid = fopen( filespec, 'r' );
        cac = textscan( fid, '%f%*f%*f%f' );
        fclose( fid );
        M01 = cat( 2, cac{:} );
        M02 = cssm_( M01 );
    end
    function    M02 = cssm_( M01 )
        N02 = 1e4;
        M02 = nan( 366, N02 );
        x01 = 1;
        for jj = 1 : N02
            if rem( jj, 4 ) >= 1
                x02 = x01 + 365 - 1;
                M02(1:365,jj) = M01( x01:x02, 2 ); 
                x01 = x02+1;
            else % "leap" year
                x02 = x01 + 366 - 1;
                M02(1:366,jj) = M01( x01:x02, 2 ); 
                x01 = x02+1;
            end
        end
    end