Add element to the front of a vector, without knowing if it is a row or column vector?

Question

If i'm adding a newvlaue to a row vector x, it would be

x = [newvlue, x] % use of ,

but if to a column vector x, it would be

x = [newvlue; x] % use of ;

so i have to know in advance if it's a row or column vector in order to perform this front insertion. But i might not always know as x is meant to be a user inputs. So every time i need to perform this row vector or column vector check beforehand. However, let's say I don't really want to care if it's a row or column vector, I just need to add one element at the front of the array. Is there any elegant way to write the code?

I'm not sure but adding to the end of vector should be more efficient than adding to the front — rahnema1, Apr 14 '20 at 17:27
@rahnema1 haha... that, of course, is the answer :) (see below...) — beaker, Apr 14 '20 at 19:13

Robert Seifert · Accepted Answer · 2020-04-14T14:44:18.400

You will have to check for the dimension of the input:

x = [1, 2, 3]
% or
x = [1; 2; 3]
new = 0;

% flexible concatenation
y = cat(~(size(x,1) > 1) + 1, new ,x)

Explanation

d = size(x,1) > 1   % check if column (=1) or row vector (>1)
z = ~(d) + 1        % results in either 1 or 2 for column or row vector
                    % as input for cat
y = cat(z, new ,x)  % concatenate in correct dimension

or by using isrow as suggested in ThomasIsCoding's answer, but I guess it almost does the same:

z = isrow(x) + 1;

In any way you should use isvector to check, whether the input is actually a vector and not a matrix. But actually I would recommend to convert any input, row or column vector, into a column vector with

x = x(:)

to allow for consisting coding within your underlying function.

Most important, the `x = x(:)`. Convert all vectors to a known orientation makes it much easier. — Daniel, Apr 14 '20 at 15:09

score 2 · Answer 2 · answered Apr 14 '20 at 17:13

2

I like one-liners, try this one

a(1:end+1)=[100;a(:)]

works for both row and column matrices.

answered Apr 14 '20 at 17:13

FangQ

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score 2 · Answer 3 · answered Apr 14 '20 at 19:14

2

As @rahnema1 said, just add the new value to the end:

x = flip(x); 
x(end+1) = newvalue; 
x = flip(x);

answered Apr 14 '20 at 19:14

beaker

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You can notice that adding to the end of vector is done with [amortized constant](https://stackoverflow.com/questions/48351041/matlab-adding-array-elements-iteratively-time-behavior) time while concatenation requires reallocation. But your method would be the most efficient answer if flip is done before and after a loop where a high mumber of appendings is performed. – rahnema1 Apr 14 '20 at 22:17
@rahnema1 I still think Daniel's and FangQ's answers would be faster, especially if the number of elements to be inserted is known beforehand. – beaker Apr 14 '20 at 22:32

score 1 · Answer 4 · answered Apr 14 '20 at 14:36

1

Maybe you can define your custom function like below

function y = addhead(x,val)
  if isrow(x)
    y = horzcat(val,x);
  else
    y = vertcat(val,x);
  end
end

answered Apr 14 '20 at 14:36

ThomasIsCoding

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score 1 · Answer 5 · answered Apr 14 '20 at 15:07

1

You can use indexing to shift all elements by 1, then insert the new element at the beginning.

x(2:end+1)=x;
x(1)=7;

answered Apr 14 '20 at 15:07

Daniel

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1

That's too simple. – Robert Seifert Apr 14 '20 at 15:32

Add element to the front of a vector, without knowing if it is a row or column vector?

5 Answers5

Explanation