How can I read the elements of an array backwards?

I have an array with x = 250000 elements. I want the code to read backwards a part of it:

k = pks3(1):-1:1; 
%read data points backwards starting from pks3 and ending at the beginning

So even though I have i = 10 pks3 peak points, I want to create an array k from the first point pks3(1). But, the problem is that in the end I get k = 255997855 elements, in which the pks3 point for example is repeated for several hundreds of times. The rest of the code is this:

      k = pks3(1):-1:1;
%read data points backwards starting from pks3 ending to pks3 again.

      first = k(k<xfb1); % find all k such that k is smaller than xfb1
      if ~isempty(first)
          first = first(1); %store the first number which is smaller than xfb1
      else
          disp('No value of k is smaller than xfb1.')
      end
      distQ = length(first:pks3(1));
      Qstart = locs_RwaveB(1)*distQ;  %distance from locs_RwaveB to Qstart point

What am I doing wrong?

Thanks.

Answer 1

[Ahmet Cecen]

It appears to me that you are trying to do something very simple in a very convoluted way, probably due to programming practices in another language.

If you need to read x in reverse order, assuming x is a column vector simply do:

 x = flipud(x);  % flips the vector.

And now you can access it in forward order.

[Konstantinos]

I used the fliplr function instead, since all elements are placed on one row, but there was no success.

[Steven Lord]

Show a smaller concrete example with a ten element vector not a 250000 element vector.

theVector = sin(1:10)

What do you expect the result to be when you operate on that vector and how specifically did you obtain that result? Explain in words, if possible, not code.

[Konstantinos]

It's difficult to explain with such a small vector since the data I have come from a digital signal, but let's say if I put a threshold on a vector sin(1:100), where 1 is on the bottom of the signal and 100 is on the peak, I want to put a threshold at i.e. 20 and tell the code to return the first point which is smaller than the threshold.

[Konstantinos]

After the fliplr function, I wrote this line:

k = B(pks3(1):end);

and I get the error message:

Warning: Integer operands are required for colon operator when used as index

I understant that it might be due to the "end" I've inserted, but how else could I define the end of the vector? I tried the fix, ceil, floor and round functions, but I get the same error message.

[Stephen23]

The problem is not the end, it is the pks3(1). What value does pks3(1) have?

[Konstantinos]

pks3(1) = 2.3937e+08

Answer 2

[Johannes Kalliauer]

a horizontal vector can be fliped using fliplr

x=1:4
x=fliplr(x)