MATLAB: Removing matrix rows after the first element of a row becomes too big

findMATLABmatricesremoving rows

I want to remove all of the rows in A after the element of the first column of the nth row passes, say 10. Let A be

A = [ 1  1  1  1  1  1  1  1  1 
      3  3  3  3  3  3  3  3  3 
      4  4  4  4  4  4  4  4  4 
      7  7  7  7  7  7  7  7  7 
      8  8  8  8  8  8  8  8  8 
      10 10 10 10 10 10 10 10 10 
      11 11 11 11 11 11 11 11 11
      25 25 25 25 25 25 25 25 25 ]
 How do I do this without knowing what row the first element will be larger than 10 at?

Best Answer

[ii,jj]=find(A>10,1);
A(ii:end,:)=[]

Related Solutions

MATLAB: Removing rows in big data

In your example, your result saves every 5-th row of the input matrix to the output matrix.

Try this:

M = [(380:400)'  randi(9, size((380:400)'))]               % Create Matrix
Out = M(1:5:end,:)

If you want the result to have every 3-rd row, do this:

Out = M(1:3:end,:)

EDIT — Clarifying the discrepancy.

MATLAB: Rows mean collapse in big matrix

If the number of rows of M was an integer multiple of N then this would be easy to achieve by simply reshaping the matrix M. Because your data does not fit this we have to take into account that there may be from 1 to N last rows to be "collapsed" together. This can be achieved using mat2cell to split the matrix into groups, and then performing the mean inside a cellfun call:

% Number of rows to "collapse" together:
N = 3;
% Your data:
M = [...
    92    99     1     8    15    67    74    51    58    40
    98    80     7    14    16    73    55    57    64    41
     4    81    88    20    22    54    56    63    70    47
    85    87    19    21     3    60    62    69    71    28
    86    93    25     2     9    61    68    75    52    34
    17    24    76    83    90    42    49    26    33    65
    23     5    82    89    91    48    30    32    39    66
    79     6    13    95    97    29    31    38    45    72
    10    12    94    96    78    35    37    44    46    53
    11    18   100    77    84    36    43    50    27    5];
% vector of row counts:
row = N*ones(1,ceil(size(M,1)/N));
row(end) = mod(size(M,1)-1,N)+1;
% scalar of column count:
col = size(M,2);
% split data into cell, with the specified rows and columns in groups:
C = mat2cell(M,row,col);
% calculate mean of each group and concatenate back into one matrix:
out = cell2mat(cellfun(@(m)mean(m,1),C,'UniformOutput',false));

and the output in the command window:

>> out
out =
    64.667       86.667           32           14       17.667       64.667       61.667           57           64       42.667
    62.667           68           40       35.333           34       54.333       59.667       56.667           52       42.333
    37.333       7.6667           63       93.333       88.667       37.333       32.667           38       43.333       63.667
        11           18          100           77           84           36           43           50           27            5

Best Answer

Related Solutions

MATLAB: Removing rows in big data

MATLAB: Rows mean collapse in big matrix

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