Compare pair of rows in a matrix

Suppose I have a Matrix like A = [12.05; 12.10; 12.25; 12.45] and I want to find difference between A(1) and A(2) then A(2) and A(3) and so on. Basically I am trying to compare time column and I have a huge data set with maybe 900,000 rows. I used a simple for loop which seems to take a very long time (naturally). Can you help me optimize this issue. Here's my code.

for j = 1:(length(usr)-1)
  t11 = usr(j,5);
  t22 = usr(j+1,5);
  secs = fix((datenum(t22) - datenum(t11)) * 86400);
  if (secs > 600 && secs < 1800)
      placesLat = [placesLat;usr((j+1),2)];
      placesLong = [placesLong;usr((j+1),3)];
  end
end

Where placesLat and placesLong are just an array. I cannot assign memory before hand because I am not sure about the final result length.

Thanks in advance.

Answer 1

[Star Strider]

There are two possibilities, diff and filter:

A = [12.05; 12.10; 12.25; 12.45];
dA = filter([1 -1], 1, A);                                  % Use ‘filter’
dA = diff(A);                                               % Use ‘diff’

The advantage to using filter is that the resulting vectors are the same lengths. The diff output is one element shorter than the input.

[Neelabh Pant]

This makes sense. Thank you.

[Star Strider]

My pleasure.

If it solves your problem, please Accept it.