Extracting data based on categories

Question

Allie 2019-7-11

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/471303-extracting-data-based-on-categories

编辑： Guillaume 2019-7-12

I have a table (z) with two columns, year and annual precipitation. I have a separate vector (E) of three categories (0,1,2) that correspond to the years in the first table. The categories are unevenly spaced. I would like to create three new tables of year and annual precipitation, one for each category. How do I extract the correct year and precipitation data for each category and populate new tables?

z=['01-Jan-1973'	0.0114583333333333
'01-Jan-1974'	0.0918518518518519
'01-Jan-1975'	0.0529032258064516
'01-Jan-1976'	0.213571428571429
'01-Jan-1977'	0.0857692307692308]
E=[0 1 0 2 1]

I would like to create three new tables in the same format as z, like below.

A=['01-Jan-1973'	0.0114583333333333
   '01-Jan-1975'	0.0529032258064516]
B=['01-Jan-1974'	0.0918518518518519
   '01-Jan-1977'	0.0857692307692308]
C=['01-Jan-1976'	0.213571428571429]

My actual tables and vectors are 1x79, making extracting the three new tables challenging to do by hand.

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Guillaume 2019-7-11

Can you give an example of inputs and desired output. Preferably using valid matlab syntax.

Allie 2019-7-11

I updated the question. Do you need more information still?

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Answer 1

Star Strider 2019-7-11

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The splitapply function (R2015b and later releases) is perfect for this!

z = {'01-Jan-1973'	0.0114583333333333
    '01-Jan-1974'	0.0918518518518519
    '01-Jan-1975'	0.0529032258064516
    '01-Jan-1976'	0.213571428571429
    '01-Jan-1977'	0.0857692307692308};
E=[0 1 0 2 1];
Out = splitapply(@(x){x}, z, E(:)+1);
A = Out{1}
B = Out{2}
C = Out{3}

producing:

A =
  2×2 cell array
    {'01-Jan-1973'}    {[0.0114583333333333]}
    {'01-Jan-1975'}    {[0.0529032258064516]}
B =
  2×2 cell array
    {'01-Jan-1974'}    {[0.0918518518518519]}
    {'01-Jan-1977'}    {[0.0857692307692308]}
C =
  1×2 cell array
    {'01-Jan-1976'}    {[0.213571428571429]}

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Star Strider 2019-7-12

I did not know your dates were in datetime format, so that is likely the reason the splitapply code failed.

I added the accumarray code later when I realised that you may not have splitapply. I’m glad it worked!

Guillaume 2019-7-12

编辑：Guillaume 2019-7-12

在 MATLAB Online 中打开

Note: If you are actually using a table (or a timetable, which would be better probably), the best solution might be not to split the table at all and just add the E as an extra table variable.

Matlab has plenty of aggregation functions (rowfun, varfun, splitapply as shown here, groupsummary) which allows you to apply the same function to each group of a table all at once. More often, the code is actually simpler and faster if the data is not split beforehand.

e.g:

%demo table
T = table(datetime({'01-Jan-1973'; '01-Jan-1974'; '01-Jan-1975'; '01-Jan-1976'; '01-Jan-1977'}), ...
          [0.0114583333333333; 0.0918518518518519; 0.0529032258064516; 0.213571428571429; 0.0857692307692308], ...
          'VariableNames', {'Date', 'Value'});
T.group = [0; 1; 0; 2; 1]
groupsummary(T, 'group', {'mean', 'sum', 'std'}, 'Value')

outputs:

ans =
  3×5 table
    group    GroupCount        mean_Value            sum_Value              std_Value     
    _____    __________    __________________    __________________    ___________________
      0          2         0.0321807795698924    0.0643615591397849     0.0293059645132893
      1          2         0.0888105413105414     0.177621082621083    0.00430106261490964
      2          1          0.213571428571429     0.213571428571429                      0