how to remove specific rows of columns

Hey everyone. I have one matrix [a] with 5 columns. i am removing some rows of b=a(:,4) based on some threshold, and then I need to construct a new matrix with the new column 4 . but I also need to include the first three columns. how can I remove the same rows from the first three columns? Thanks !

Answer 1

[James Tursa]

Save the indexing that you use to do the removing, and then apply that to the original matrix during the reconstruction. E.g.,

>> a = reshape(1:30,6,5)
a =
     1     7    13    19    25
     2     8    14    20    26
     3     9    15    21    27
     4    10    16    22    28
     5    11    17    23    29
     6    12    18    24    30
>> b = a(:,4)
b =
    19
    20
    21
    22
    23
    24
>> rows_to_remove = mod(b,2)==1
rows_to_remove =
     1
     0
     1
     0
     1
     0
>> result = [a(~rows_to_remove,1:3) b(~rows_to_remove)]
result =
     2     8    14    20
     4    10    16    22
     6    12    18    24

Or suppose instead of logical indexing as in the above example, you had row numbers to remove. E.g.,

rows_to_remove = [1 3 5]

Then just transform this into a logical indexing vector first. E.g.,

rows_to_remove = ismember(1:size(a,1),rows_to_remove)

[farfar]

Thank you !