MATLAB Answers


Variable declaration in matlab

Asked by Shalini
on 5 Mar 2012
Latest activity Commented on by Walter Roberson
on 16 Nov 2015

Is it necessary to declare a variable in matlab?




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5 Answers

Answer by Walter Roberson
on 5 Mar 2012
 Accepted answer

There are some conditions under which it is necessary to initialize a variable ahead of time. These conditions have to do with "closures" and nested functions, and "poofing" variables.

Also, variables that will be used for with the code generator take their generated data type from the first assignment to the variable within the code.

Variables that are marked "global" must be declared as global before they are used; likewise with "persistent" variables.

There are no declarations such as in C or PASCAL, but there is enough leakage from the original "no declarations" model that the answer is more "not usually" than "no".


Yes, I have used empty structs with the appropriate fields.

@Oleg: yes, cell arrays. Behind the scenes, cell arrays are arrays of pointers to memory locations, so you suffer the same performance penalty if you don't preallocate them (even though you don't need to allocate what will go inside the cells). I've had situations where I've had large cell arrays, with each cell containing nothing particularly big, and I was filling the cells in with a loop.

(I'm interpreting "declare" in the MATLAB sense of x = cell(m,n);)

Sideways Comment: You can declare an unitialized numeric, char, or logical variable with a mex routine using unofficial API functions. E.g., see the UNINIT function from this FEX submission:

Also, to clarify Jan's comment, when an unitialized cell or struct element is referenced (i.e., one with a NULL pointer behind the scenes), MATLAB will create a temporary empty 0x0 double matrix on the fly.

Answer by Aldin
on 5 Mar 2012

Yes, when in you type in command window:

a = 2; it's a number (in JAVA: integer)
a = '2'; it's a string (in JAVA String)
a = [1 2 3 4 5]  it's a vector(array) (in JAVA: int array[] = new int[5])
a = [ 1 2; 3 4] it's a matrix (in JAVA: int array[][] = new int[2][2])
a = {'a',2;'b',3} it's a cell (in JAVA structure)


I do not agree, because a proper declaration exists without assignment of a value.

on 5 Mar 2012

MATLAB works with only with matrix variable!!!

What is a matrix variable?

Answer by Shalini
on 5 Mar 2012

Can you give an example of declaration (integer/real) in a simple code?


If the variable is already declared in other datatype,then it'll converts to int32()


a=2; % 'a' is double
b=int32(a) % 'b' is int32

else, it'll directly take datatype,which we mentioned or default

In older versions of MATLAB, int32(12) worked by having 12 evaluated in double precision first, creating a temporary (nameless) double precision variable. That temporary variable was then passed to the int32() routine, which did the conversion, creating a second temporary variable but of int32 type. That second temporary variable was returned from int32, after which the first temporary variable was deleted. The second temporary variable was then assigned to "second", which would happen by creating a permanent name for the nameless variable (not by copying its content, just naming it and holding on to it.)

In newer versions of MATLAB, int32() and similar numeric datatypes applied to constant numeric expressions of a very limited number of forms (e.g., uint64(134324342332432)) is handled at parsing time, special processing to ensure that the full precision is used; the parsing would directly create a temporary int32 variable. The assignment of that temporary variable to "second" would proceed like above, by assigning a permanent name to it rather than by copying it.

However, some testing I did a few weeks ago showed that if the numeric constant was not in one of a very limited number of formats, then even though it was numeric and constant, the special case would not be recognized, and instead the same sort of processing would be applied as for older MATLAB.

i want to initialize some double and some integer in a structure, can anyone help me how to do that

Answer by KJDS SRINIVASA RAO on 27 May 2013

yes sometimes required to remove confusion


Answer by Jon Camilleri
on 15 Nov 2015

So how do I initialize a variable and read the data types available?

  1 Comment

You initialize a variable by assigning a value to it.

You can informally find the data type by using whos() on the variable to see information about it. You can find its data type name in a program by asking for class() of the variable.

There is no complete list of data types available, because objects created through MATLAB Object Oriented Programming are considered distinct datatypes.

The "basic" data types are:

  • numeric: single(), double(), int8(), uint8(), int16(), uint16(), int32(), uint32(), int64(), uint64(). Each of these can also have a complex component
  • logical: logical(), false, true (can often be converted to numeric), cannot have complex component, stored as 1 byte each
  • char / string: char(), quoted strings, stored as 2 bytes each, using Unicode code points
  • cell array: cell()
  • structures: struct()
  • double() and logical() can also exist in sparse array form

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