Hello everyone! I need your valuable help here.

I have a 3D matrix (12 x 14 x 360) which is 30 years monthly data. I need only months from Apri to September from all 30 years. The output matrix I expect is 12 x 14 x 180. I am getting this error: “Subscripted assignment dimension mismatch.” Correct me please.

 % code
h=1:180
for year=1:30;
    April=(year-1)*12+4;
    Sept=(year-1)*12+9;
    seasons(:,:,h)=mat(:,:,April:Sept);
end

Kindly, Emgdaw

Answer 1

[John D'Errico]

April = 4;
Sept = 9;
S = size(mat);
seasons = reshape(mat,[12 14 12 30]);
seasons = seasons(:,:,April:Sept,:);
seasons = reshape(seasons,[12,14,30*(Sept-April+1)]);

Simple, really. The reshape allows you to extract those months. Then reshape things again, to return the array into the desired shape. Or, I could have left it as an array of size [12,14,6,30]. That may help you to use it later. It depends on what you are doing.

[Engdaw Chane]

Hi John, Thank you for your swift replay. I will be doing matrix operations on the seasonal data once those months are separated.

I am getting this error: "Error using reshape. To RESHAPE the number of elements must not change.”

Kindly,

[Jan]

@Engdaw Chane: If your input is really "a 3D matrix (12 x 14 x 360)", the code runs fine:

mat = rand(12, 14, 360)

If you get an error, you must have done something else. Please post your code.

[Engdaw Chane]

Jan Simon Sure! it is working. thank you!

[Jan]

@Engdaw Chane: Think about selecting this as accepted answer, because it works and solves the problem.

[Engdaw Chane]

Thank you all. @John D'Errico and @Jan Simon thank you for the commitment you are showing.

Answer 2

[John BG]

Hi again Engdaw Chane

this is John BG <mailto:[e-mail] [e-mail]>

Mr D'Errico thanks for the correction, but I insist on the point that there is no need for any reshape, at all, neither on the data nor on the indices, look:

    A=randi([-10 10],12,14,360);     % data

    L=repmat([4:9],30,1).*repmat([1:30]',1,numel([4:9]));  % selection indices

    % or with Mr D'Errico correction that I must say I find it elegant
    % L=[4:9]'+[1:12:348]; 

    B=A(:,:,L(:));  % result

Now B is the expected shape.

if you find this answer useful would you please be so kind to consider marking my answer as Accepted Answer?

To any other reader, if you find this answer useful please consider clicking on the thumbs-up vote link

thanks in advance for time and attention

John BG

<mailto:[e-mail] [e-mail]>

[Engdaw Chane]

Thank you very much John! I accepted the answer now. :)

[John BG]

thanks Engdaw :) the more we ask the more we learn

[Engdaw Chane]

John, Sure. I am just starting to use matlab, and I may have questions to forward to you. :)

Thank you

[Jan]

The calculated indices are: [4 5 6 7 8 9 8 10 12 14 16 18 12 15, ...] This is not the searched "April to September" for each year. It is not even unique.

[John D'Errico]

This does not work. The result is the correct size and shape, but the answer is flat out wrong.

[4:9]'*[1:30]
ans =
  Columns 1 through 28
     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
     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
     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
     7    14    21    28    35    42    49    56    63    70    77    84    91    98   105   112   119   126   133   140   147   154   161   168   175   182   189   196
     8    16    24    32    40    48    56    64    72    80    88    96   104   112   120   128   136   144   152   160   168   176   184   192   200   208   216   224
     9    18    27    36    45    54    63    72    81    90    99   108   117   126   135   144   153   162   171   180   189   198   207   216   225   234   243   252
  Columns 29 through 30
   116   120
   145   150
   174   180
   203   210
   232   240
   261   270

If you look in the first column, you will see it generates months 4-9.

In the second year, you want months

12 + (4:9)
ans =
    16    17    18    19    20    21

NOT months [8 10 12 14 16 18].

So you will fail to extract the correct months. Right shape, but pure garbage for a result. You should see that in fact, some months are selected several times. Two of those months were actually selected and used 4 times. (Months 72 and 120, in case you care.)

Note that this WOULD have been correct with a different outer product for L. Had John BG written it as

L=[4:9]' + [0:12:348];

Thus, in a current MATLAB release, MATLAB will interpret this as a generalized outer product, creating what is essentially an addition table.

L =
  Columns 1 through 28
     4    16    28    40    52    64    76    88   100   112   124   136   148   160   172   184   196   208   220   232   244   256   268   280   292   304   316   328
     5    17    29    41    53    65    77    89   101   113   125   137   149   161   173   185   197   209   221   233   245   257   269   281   293   305   317   329
     6    18    30    42    54    66    78    90   102   114   126   138   150   162   174   186   198   210   222   234   246   258   270   282   294   306   318   330
     7    19    31    43    55    67    79    91   103   115   127   139   151   163   175   187   199   211   223   235   247   259   271   283   295   307   319   331
     8    20    32    44    56    68    80    92   104   116   128   140   152   164   176   188   200   212   224   236   248   260   272   284   296   308   320   332
     9    21    33    45    57    69    81    93   105   117   129   141   153   165   177   189   201   213   225   237   249   261   273   285   297   309   321   333
  Columns 29 through 30
   340   352
   341   353
   342   354
   343   355
   344   356
   345   357

You can see that modified as I showed it here, John's approach will extract the correct months, thus [4:9, 16:21, ... ]. What John BG did in posthaste to get a fast answer in, was carelessly assume because the shape of L was correct, that it did what he wanted it to do.