How to insert elements in a vector at regular intervals in Matlab

问题

I have a vector of 13 entities in Matlab.

a=[3 4 6 8 1 5 8  9  3 7 3 6 2]

I want to append values [1 2 3 4 5] at regular intervals at position 1 5 9 13 & 17. The final value of a looks like this.

a=[1 3 4 6 2 8 1 5 3 8 9 3 4 7 3 6 5 2].

The values with italics show the appended values. How can I do it?

回答1:

Since you are looking for regular intervals, you can take advantage of the reshape and cat function:

a = [3 4 6 8 1 5 8 9 3 7 3 6 2];
v = [1 2 3 4 5];
l = [1 5 9 13 17];

interval = l(2)-l(1)-1; %computes the interval between inserts
amax = ceil(size(a,2)/interval) * interval; %calculating maximum size for zero padding
a(amax) = 0; %zero padding to allow `reshape`
b = reshape (a,[interval,size(v,2)]); %reshape into matrix
result = reshape(vertcat (v,b), [1,(size(b,1)+1)*size(b,2)]); %insert the values into the right position and convert back into vector

%remove padded zeros
final = result(result ~= 0) %remove the zero padding.

>>final =

Columns 1 through 16

 1     3     4     6     2     8     1     5     3     8     9     3     4     7     3     6

Columns 17 through 18

 5     2

回答2:

Here's an approach using boolean-indexing -

% Inputs
a = [3 4 6 8 1 5 8 9 3 7 3 6 2]
append_vals = [1 2 3 4 5]
append_interval = 4 % Starting at 1st index

% Find out indices of regular intervals where new elements are to be inserted.
% This should create that array [1,5,9,13,17]
N_total = numel(a) + numel(append_vals)
append_idx = find(rem(0:N_total-1,append_interval)==0)

% Get boolean array with 1s at inserting indices, 0s elsewhere
append_mask = ismember(1:N_total,append_idx)

% Setup output array and insert new and old elements
out = zeros(1,N_total)
out(~append_mask) = a
out(append_mask) = append_vals

Alternatively, we can also use linear-indexing and avoid creating append_mask, like so -

% Setup output array and insert new and old elements
out = zeros(1,N_total)
out(append_idx) = append_vals
out(setdiff(1:numel(out),append_idx)) = a

回答3:

a=[3 4 6 8 1 5 8 9 3 7 3 6 2]; % // Your original values
pos = [1 5 9 13 17]; % // The position of the values you want to insert
b=[1 2 3 4 5]; % // The values you want to insert

% // Pre-allocate a vector with the total size to hold the resulting values
r = zeros(size(a,2)+size(pos,2),1);

r(pos) = b % // Insert the appended values into the resulting vector first
r3 = r.' <1 % // Find the indices of the original values. These will be zero in the variable r but 1 in r3   
ans =

   0   1   1   1   0   1   1   1   0   1   1   1   0   1   1   1   0   1

ind= find(r3==1) % // Find the indices of the original values

ind =

    2    3    4    6    7    8   10   11   12   14   15   16   18

r(ind) = a; % // Insert those into the resulting vector.

r.'
ans =

   1   3   4   6   2   8   1   5   3   8   9   3   4   7   3   6   5   2

回答4:

You can use this function to append a bunch of values to an existing vector, given their positions in the new vector:

function r=append_interval(a,v,p)

% a - vector with initial values
% v - vector containing values to be inserted 
% p - positions for values in v

lv=numel(v); % number of elements in v vector
la=numel(a); % number of elements in a vector

column_a=iscolumn(a); % check if a is a column- or row- wise vector

tot_elements=la+lv;

% size of r is tha max between the total number of elements in the two vectors and the higher positin in vector p (in this case missing positions in a are filled with zeros)
lr=max([max(p) tot_elements]);

% initialize r as nan vector
r=zeros(column_a*(lr-1)+1,~column_a*(lr-1)+1)/0;

% set elements in p position to the corresponding values in v
r(p)=v;

% copy values in a in the remaining positions and fill with zeros missing entries (if any)
tot_missing_values=lr-tot_elements;
if(tot_missing_values)
    remaining_values=cat(2-iscolumn(a),a,zeros(column_a*(tot_missing_values-1)+1,~column_a*(tot_missing_values-1)+1));
else
    remaining_values=a;
end

% insert values
r(isnan(r))=remaining_values;

You can use row-wise or column-wise vectors; the orientation of r will be the same of that of a.

Input:

a =

     3     4     6     8     1     5     8     9     3     7     3     6     2

v =

     1     2     3     4     5


p =

     1     5     9    13    17

Output:

>> append_interval(a,v,p)

ans =

     1     3     4     6     2     8     1     5     3     8     9     3     4     7     3     6     5     2

Every sequence of positive positions is allowed and the function will pad for you with zeros the final vector, in case you indicate a position exceding the sum of the original vector and added items.

For example, if:

v3 =

     1     2     3     4     5     6    90

p3 =

     1     5     9    13    17    30    33

you get:

append_interval(a,v3,p3)

ans =

  Columns 1 through 19

     1     3     4     6     2     8     1     5     3     8     9     3     4     7     3     6     5     2     0

  Columns 20 through 33

     0     0     0     0     0     0     0     0     0     0     6     0     0    90

Hope this will help.

来源：https://stackoverflow.com/questions/38845713/how-to-insert-elements-in-a-vector-at-regular-intervals-in-matlab

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matlab

append