Multiple names for a single variable.

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Joshua Arenson 2021-1-27

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https://ww2.mathworks.cn/matlabcentral/answers/728913-multiple-names-for-a-single-variable

评论： Walter Roberson 2021-2-15

Hi folks, I'm wondering if this is possible and how one would go about doing it.

I have a structure data. It has a field say, ActiveInd, so data.ActiveInd. I want to be able to have some kind of shorthanded name to keep the code easier to read and shorter, say AInd. Now I could (and have) code that says soemthing like:

AInd = data.ActiveInd

[do stuff to AInd]

data.ActiveInd = AInd.

Of course the doing stuff to AInd means I now have two variables, AInd and data.ActiveInd increasing memory usage. With tons of spare memory this isn't an issue but I'm starting to push up against memory caps. I'd love to able use both AInd and data.ActiveInd to point to and alter the same stuff in memory without using excess memory. Is this possible?

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Answer 1

James Tursa 2021-1-27

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https://ww2.mathworks.cn/matlabcentral/answers/728913-multiple-names-for-a-single-variable#answer_607858

There are no "variable pointers" in MATLAB, so no generic alias capability as you would probably want. Other options to consider:

Use data.Activelnd spelled out in all your code, instead of an assigned copy
Use functions to modify data.Activelnd with special syntax that allows "in-place" modification
Use classdef objects derived from handle that would essentially allow the alias behavior you want

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Walter Roberson 2021-1-27

在 MATLAB Online 中打开

Could someone else run this in a real .m file, not in LiveScript, and see if you see the same thing I see?

format debug
data.ActiveInd = rand(1, 20);
fprintf('data\n');
data
fprintf('data.ActiveInd\n');
data.ActiveInd
fprintf('doStuff about to be applied to data.ActiveInd\n');
data.ActiveInd = doStuff(data.ActiveInd);
fprintf('doStuff has been applied\n')
data
fprintf('New data.ActiveInd\n');
data.ActiveInd
function AInd = doStuff(AInd)
fprintf('doStuff: AInd before\n');
AInd
AInd(13) = 27;
fprintf('doStuff: AInd after\n');
AInd
end

In particular, look at the second time that data is displayed. First time,

data =

struct with fields:

ActiveInd: [0.8147 0.9058 0.1270 0.9134 0.6324 0.0975 0.2785 0.5469 0.9575 0.9649 0.1576 0.9706 0.9572 0.4854 0.8003 0.1419 0.4218 0.9157 0.7922 0.9595]

Then after the value in the field is updated to 27:

data =

struct with fields:

ActiveInd: [0.8147 0.9058 0.1270 0.9134 0.6324 0.0975 0.2785 0.5469 0.9575 0.9649 0.1576 0.9706 0 0.4854 0.8003 0.1419 0.4218 0.9157 0.7922 0.9595]

Notice that the 13th field has become 0 -- not remained the same and not become 27!

And yet when the field content is specifically requested immediately after that, it displays as 27.

James Tursa 2021-1-28

在 MATLAB Online 中打开

Run from function file:

data
data = 
  struct with fields:
    ActiveInd: [1×20 double]
data.ActiveInd
ans =
Structure address = 208ffd1c1c0
m = 1
n = 20
pr = 208964b25c0
  Columns 1 through 8
    0.9593    0.5472    0.1386    0.1493    0.2575    0.8407    0.2543    0.8143
  Columns 9 through 16
    0.2435    0.9293    0.3500    0.1966    0.2511    0.6160    0.4733    0.3517
  Columns 17 through 20
    0.8308    0.5853    0.5497    0.9172
doStuff about to be applied to data.ActiveInd
doStuff: AInd before
AInd =
Structure address = 208ffd1c1c0
m = 1
n = 20
pr = 208964b25c0
  Columns 1 through 8
    0.9593    0.5472    0.1386    0.1493    0.2575    0.8407    0.2543    0.8143
  Columns 9 through 16
    0.2435    0.9293    0.3500    0.1966    0.2511    0.6160    0.4733    0.3517
  Columns 17 through 20
    0.8308    0.5853    0.5497    0.9172
doStuff: AInd after
AInd =
Structure address = 208ffb68400
m = 1
n = 20
pr = 208970333a0
  Columns 1 through 8
    0.9593    0.5472    0.1386    0.1493    0.2575    0.8407    0.2543    0.8143
  Columns 9 through 16
    0.2435    0.9293    0.3500    0.1966   27.0000    0.6160    0.4733    0.3517
  Columns 17 through 20
    0.8308    0.5853    0.5497    0.9172
doStuff has been applied
data = 
  struct with fields:
    ActiveInd: [1×20 double]
New data.ActiveInd
ans =
Structure address = 208ffb68400
m = 1
n = 20
pr = 208970333a0
  Columns 1 through 8
    0.9593    0.5472    0.1386    0.1493    0.2575    0.8407    0.2543    0.8143
  Columns 9 through 16
    0.2435    0.9293    0.3500    0.1966   27.0000    0.6160    0.4733    0.3517
  Columns 17 through 20
    0.8308    0.5853    0.5497    0.9172

Jan 2021-2-15

在 MATLAB Online 中打开

@Walter Roberson: In my Win10/R2018b system, your code shows expected behavior: The element is set to 27 immediately. But I see this

format debug
data.ActiveInd = 1:3;
% data.ActiveInd   % Uncommenting does not let the zeros disappear
data
data.ActiveInd
% Output:
data =   struct with fields:
    ActiveInd: [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
ans =
Structure address = 100266a0
m = 1
n = 20
pr = ff92c9c0
   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20   

With data.ActiveInd=rand(1,3) this effect does not appear.

Walter Roberson 2021-2-15

The test I just did with R2020b on Mac has data display with field ActiveInd that is [8 8 8] -- right length but wrong content.

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Multiple names for a single variable.

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