Insert elements before a specific value in vector

I have a vector of time, twtt. At every point where twtt(i)=0 (except for i=1) I need to insert a value, 1.5, before it. For example:

twtt=[0 0.25 0.5 0 0.25 0.26 0.5 0 0.1 0.25 0.4 0.5 0.6 0 0.25 0.5]';

should become

twtt_new=[0 0.25 0.5 1.5 0 0.25 0.26 0.5 1.5 0 0.1 0.25 0.4 0.5 0.6 1.5 0 0.25 0.5 1.5]';

The distance between 0's is variable so I cannot use a predictor.

The code I have currently is almost there but I cannot figure out where I am going wrong. Any help would be appreciated.

Code:

idx=find(twtt(2:end)==0); % To exclude 0 at i=1

twtt_new=zeros(length(twtt)+length(idx),1); % New vector is larger
first_val=idx(1);
counter=1;
j=1;
for i=1:length(twtt)-1
    if j>length(idx);
        break;
    else

          if i~=idx(j) && i<first_val;
              twtt_new(i)=twtt(i);
          elseif i~=idx(j) && i>first_val;
              twtt_new(i+counter)=twtt(i);
          elseif i==idx(j) && i==first_val;
              twtt_new(i)=1.5;
              j=j+1;
          elseif i==idx(j) && i>first_val;
              twtt_new(i+counter)=1.5;
              j=j+1;
              counter=counter+1;
          end
      end
  end

Resultant and wrong twtt_new produced by code above:

twtt_new=[0 0.2500 1.5000 0 0 0.2500 0.2600 1.5000 0 0 0.1000 0.2500 0.4000 0.5000 1.5000 0 0 0 0]'

Answer 1

[Image Analyst]

Wow, so complicated. Obviously you've never heard of the strrep() trick. Simply call strrep() like this:

% Create row vector, not column vector, sample data.
twtt = [0 0.25 0.5 0 0.25 0.26 0.5 0 0.1 0.25 0.4 0.5 0.6 0 0.25 0.5]
% Replace 0 with [1.5, 0]
out = strrep(twtt, 0, [1.5, 0])

If you want a column vector, you can simply transpose out.

[K G]

Fantastic! Thank you

[K G]

I would now like to put a velocity value (1472) into another vector, rms_vel, at every corresponding element to each 1.5 value in twtt_new/out. Do you know of a simple way to do this using the index position of each 1.5 in twtt_new?

[dpb]

Well, if you had the find indices, it should be pretty obvious what to do with them I'd think...but you don't need find at all--

rms_vel(twtt_new==1.5)=1472;

Look up "logical addressing" in the doc; extremely powerful topic.

Answer 2

[dpb]

Alternate ways, but a "deadahead" route is

iz=[find([nan twtt(2:end)]==0) length(twtt)];   % locations except first of zero keeping length
t=[];
for i=length(iz)-1:-1:1   % work from end backwards...
  t=[[1.5 twtt(iz(i):iz(i+1))] t];
end
t=[twtt(1:iz(1)-1) t];  % add first section before first zero