Insert elements before a specific value in vector

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K G
K G 2017-1-7
评论： dpb 2017-1-9
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]'

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采纳的回答

Image Analyst
Image Analyst 2017-1-7
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.
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K G
K G 2017-1-9
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
dpb 2017-1-9
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.

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更多回答（1 个）

dpb
dpb 2017-1-7
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

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