Merge a selected rows to creat a single row

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salah roud 2020-2-28

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https://ww2.mathworks.cn/matlabcentral/answers/508080-merge-a-selected-rows-to-creat-a-single-row

编辑： dpb 2020-2-28

采纳的回答： dpb

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I [have] a long (2000x2) (array/matrix) " the data is double "

and i want to Merge each 8 rows to get a (667x6 array)

if the last row is less than 6 , then it's gonna fill the begging of it with zeros .

a small example :

x =    1    2
         3    4 
         5    6
         7    8
result = 1    2    3    4    5    6
             0    0    0    0    7    8

thank you in advance

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Answer 1

dpb 2020-2-28

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https://ww2.mathworks.cn/matlabcentral/answers/508080-merge-a-selected-rows-to-creat-a-single-row#answer_417771

编辑：dpb 2020-2-28

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EDITED for ROW ORDER CORRECTION -- dpb

function X=reshapeNperRow(x,Nper)
% X = reshapeNperRow(x,Nper)
% reshapes input array x to have Nper elements per row
% if not evenly divisible number elements total, augment
% last row by prepending zeros
  x=x.';                            % transpose for column major order internal storage
  N=numel(x);                       % total number elements
  Nrep=fix(N/Nper);                 % how many output full rows in input array
  Nused=Nrep*Nper;                  % how many elements used for even rows
  X=reshape(x(1:Nused),Nper,[]).';  % output those in desired shape
  Nleft=N-Nused;                    % are there any left over?
  if Nleft, X=[X;[zeros(1,Nper-Nleft) x(Nused+1:end)]];,end % if so add them to end
end

OK, I'll actually test it this time... :)

>> a=reshape(1:15,[],3).'          % test input...
a =
     1     2     3     4     5
     6     7     8     9    10
    11    12    13    14    15
>> reshapeNperRow(a,6)
ans =
     1     2     3     4     5     6
     7     8     9    10    11    12
     0     0     0    13    14    15
>> 

looks as advertised!

Added some comments so can see what's going on each line.

NB: Think about how to build such arrays from builtins rather than having to construct manually. Learning about memory storage patterns is big key for many optimizations in writing ML code!

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salah roud 2020-2-28

编辑：dpb 2020-2-28

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thank you so much you are a legend

Could you tell me how to edit the code so I can get the desired values ..

because the output is not exactly as I asked :/

For example

>> a = [ 1 2 3 4 5 ; 6 7 8 9 10 ; 11 12 13 14 15]
>> Y = reshapeNperRow(a,6)

I get this result

Y =
   6    11     2     7    12
   8    13     4     9    14
   0     0     5    10    15

But i want it this way

Y = 
  2    3     4    5    6
  8    9    10    11    12
  0    0    13    14    15

you got what i mean :/

and thank you so much

dpb 2020-2-28

Huh. I thought I had done that but see I missed a code line in the final...

You should be able to recognize what went wrong; the orientation array to select the first N elements has to be in column major order, but you want row major. But, if the array is transposed initially, then the row,column counts are wrong in computing things so have to do that first, then transpose...

In fact, the final is somewhat different logic than I started with; the ending version doesn't need [nr,nc] at all...it was in editing that I see I deleted one line off from where I intended in cleaning up the posted version.

Thanks for pointing out the error in my ways! :)

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