MATLAB: Does MATLAB use the same memory address for variables that have different names but the same value

copyMATLABonsamevaluevariablewrite

I would like to know if MATLAB uses the same memory address for variables that have different names but the same value. I would also like to know if MATLAB uses "copy-on-write".

For example, if I issue the following commands in the MATLAB Command Window (assuming a is already declared):

b=a;
c=a;

I can do a WHOS and it looks like MATLAB is allocating memory for each variable.

Best Answer

When MATLAB creates a variable, it actually creates two elements of data. One part is the data itself, the other part is a header that contains information about the data (such as where in memory the data lives, how big it is, etc). For more information see the External Interfaces/API Manual which may be found at:

https://www.mathworks.com/help/releases/R13sp2/techdoc/matlab_external/matlab_external.html

When executing an assignment such as 'b = a', MATLAB creates a new header for 'b', but just tells the data element for 'b' to point to the data element for 'a'. This is done for efficiency. Until there is a definite need to create a separate copy of data for 'b', 'a' and 'b' will share the same data. When the value of the data for 'a' or 'b' changes, MATLAB will make a separate copy of the data for each variable. This is called "copy on write".

The output of WHOS may make it seem like MATLAB is allocating memory for each variable, but this is not always the case. WHOS will show you the memory usage assuming each variable has its own data set.

For example, after entering in 'b=a', any of the commands below will cause MATLAB to allocate new storage space for 'b'.

b(1)=7;
b=2*a;
a(1)=2;
b = b+0;

This feature allows MATLAB to be more efficient, allocating memory only when needed.

Related Solutions

MATLAB: Is there a function in Stateflow to determine how a particular “Data Store Memory” data object is being used by a Stateflow chart

The ability to determine how a particular "Data Store Memory" data object is used in a Stateflow chart is not available in Stateflow.

To work around this issue, you can write your own MATLAB file function that uses the Stateflow API to obtain the expressions in a transition, state, or other chart object that indicate how the "Data Store Memory" is being used. As an example, you might use the following commands in the Stateflow API to access expressions in a transition and state:

rt = sfroot;
m = rt.find('-isa', 'Simulink.BlockDiagram', '-and', 'Name','myModel');
chart = m.find('-isa','Stateflow.Chart','-and','Name','mychart');
trans = find(chart, '-isa','Stateflow.Transition'); %Get all transitions
states = find(chart, '-isa','Stateflow.State'); %Get all states
transString = trans(1).LabelString; %Get the expressions in the first transition
statesString = states(1).LabelString; %Get the expressions in the first state

One way to determine if the "Data Store Memory" data object is being read from or written to is to see if it is on the right or left side of an "=" sign. You can determine this by parsing the transString and statesString strings. You can also use the STRFIND command to get the location of the "Data Store Memory" variable and the "=" sign in the expressions contained in transString and statesString strings.

MATLAB: Do I get the error “Invalid default value for property” when trying to define one default instance variable value in terms of another

It is easier to see the reason for this issue once you fully-qualify the name of an instance variable with which you are defining another.

e.g.

classdef Example
    properties
        A = {'a','b','c'}
        B = [Example.A,'d']
    end
end

yields the error

>> Example

Invalid default value for property 'B' in class 'Example':

The property 'A' in class 'Example' must be accessed from a class instance because it is not a Constant property.

Because 'B' is defined in terms of A, we must either define 'A' to be a "Constant" property or we must initialize B in the constructor.

i.e.

classdef Example
    properties (Constant)
        A = {'a','b','c'}
    end
    properties
        B = [Example.A,'d']
    end
end

classdef Example
    properties
        A = {'a','b','c'}
        B
    end
    methods
        function obj = Example()
            obj.B = [obj.A,'d'];
        end
    end
end

Best Answer

Related Solutions

MATLAB: Is there a function in Stateflow to determine how a particular “Data Store Memory” data object is being used by a Stateflow chart

MATLAB: Do I get the error “Invalid default value for property” when trying to define one default instance variable value in terms of another

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