Taking averages of only a few rows out of several?

Question

Lee 2022-1-15

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https://ww2.mathworks.cn/matlabcentral/answers/1629610-taking-averages-of-only-a-few-rows-out-of-several

回答： Image Analyst 2022-1-16

I have a data set that is 6X100. I need to take the average of the first three rows for all 100 and then of the last three rows for all 100 columns. How do I take averages of only a few rows?

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Answer 1

Voss 2022-1-15

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https://ww2.mathworks.cn/matlabcentral/answers/1629610-taking-averages-of-only-a-few-rows-out-of-several#answer_875160

A = (1:6).'.*(1:100)
A = 6×100
     1     2     3     4     5     6     7     8     9    10    11    12    13    14    15    16    17    18    19    20    21    22    23    24    25    26    27    28    29    30
     2     4     6     8    10    12    14    16    18    20    22    24    26    28    30    32    34    36    38    40    42    44    46    48    50    52    54    56    58    60
     3     6     9    12    15    18    21    24    27    30    33    36    39    42    45    48    51    54    57    60    63    66    69    72    75    78    81    84    87    90
     4     8    12    16    20    24    28    32    36    40    44    48    52    56    60    64    68    72    76    80    84    88    92    96   100   104   108   112   116   120
     5    10    15    20    25    30    35    40    45    50    55    60    65    70    75    80    85    90    95   100   105   110   115   120   125   130   135   140   145   150
     6    12    18    24    30    36    42    48    54    60    66    72    78    84    90    96   102   108   114   120   126   132   138   144   150   156   162   168   174   180
mean(A(1:3,:))
ans = 1×100
     2     4     6     8    10    12    14    16    18    20    22    24    26    28    30    32    34    36    38    40    42    44    46    48    50    52    54    56    58    60
mean(A(1:3,:),'all')
ans = 101
mean(A(end-2:end,:))
ans = 1×100
     5    10    15    20    25    30    35    40    45    50    55    60    65    70    75    80    85    90    95   100   105   110   115   120   125   130   135   140   145   150
mean(A(end-2:end,:),'all')
ans = 252.5000

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Answer 2

Image Analyst 2022-1-16

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https://ww2.mathworks.cn/matlabcentral/answers/1629610-taking-averages-of-only-a-few-rows-out-of-several#answer_875250

That's kind of ambiguous. Do you want a mean from each row, so you'll get 3 values? Or do you want all 300 values to be averaged into a single value.

A = (1:6).'.*(1:100)
A = 6×100
     1     2     3     4     5     6     7     8     9    10    11    12    13    14    15    16    17    18    19    20    21    22    23    24    25    26    27    28    29    30
     2     4     6     8    10    12    14    16    18    20    22    24    26    28    30    32    34    36    38    40    42    44    46    48    50    52    54    56    58    60
     3     6     9    12    15    18    21    24    27    30    33    36    39    42    45    48    51    54    57    60    63    66    69    72    75    78    81    84    87    90
     4     8    12    16    20    24    28    32    36    40    44    48    52    56    60    64    68    72    76    80    84    88    92    96   100   104   108   112   116   120
     5    10    15    20    25    30    35    40    45    50    55    60    65    70    75    80    85    90    95   100   105   110   115   120   125   130   135   140   145   150
     6    12    18    24    30    36    42    48    54    60    66    72    78    84    90    96   102   108   114   120   126   132   138   144   150   156   162   168   174   180
% Get 3 means -- one for each row.
meanFirst3 = mean(A(1:3,:), 2)
meanFirst3 = 3×1
   50.5000
  101.0000
  151.5000
meanLast3 = mean(A(end-2:end,:), 2)
meanLast3 = 3×1
  202.0000
  252.5000
  303.0000
% Get 1 mean covering all 3 rows
meanFirst3 = mean2(A(1:3,:))
meanFirst3 = 101
meanLast3 = mean2(A(end-2:end,:))
meanLast3 = 252.5000