How do I declare a blank column vector of unknown size?

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I need to declare a column vector whose elements will be determined as the code runs. How do I declare such a vector of size n (entered by user) in MATLAB.
Thanks and Regards

Accepted Answer

KSSV on 16 Nov 2021
Edited: KSSV on 16 Nov 2021
O = zeros(n,1) ; % Initiate a zeros column vector
N = NaN(n,1) ; % initiate a NaN column vector
iwant = zeros([],1) ; % empty vector which can be filled if length is not known

More Answers (2)

Walter Roberson
Walter Roberson on 16 Nov 2021
MATLAB does not offer any way of creating blank column vectors of unknown size.
Any numeric column must have some numeric value assigned to the entries. If you write out numeric data to an Excel file, then NaN values will show up empty.
MATLAB allow you to create empty columns. However, if you write data further down in the column, the emptiness is filled with numeric 0 for numeric or logical columns, and by the character with binary value 0 for character columns.
A = repmat('', 5, 1)
A = 0×0 empty char array
A(5,1) = 'Z'
A = 5×1 char array
' ' ' ' ' ' ' ' 'Z'
ans = 0
A(1,1) == ' '
ans = logical
The display of the expanded array makes it look like blanks were put in, but you can see that they are binary 0 instead.
Generally speaking, if you have not created a numeric column yet, and at some point in the program you find out how large it will be, then you can create it as the needed size using zeros() or ones() or inf() or nan() . For example,
NumRows = 4
NumRows = 4
B = ones(NumRows, 2, 'uint8')
B = 4×2
1 1 1 1 1 1 1 1
If you have not created a character column yet, and at some point in the program you find out how large it will be, then repmat() can sometimes be the easiest way to fill it with blanks:
NumRows = 4
NumRows = 4
NumCols = 7
NumCols = 7
C = repmat(' ', NumRows, NumCols)
C = 4×7 char array
' ' ' ' ' ' ' '
C(1,1) == ' '
ans = logical
and here you can see this gives true blanks.

Mohit Kumar
Mohit Kumar on 16 Nov 2021
Thanks to both of you. Your responses were helpful.

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