Colon operands must be real scalars

I tested the following code:

x = rand(1,3);
x([1,2]:[1,2]) 

Warning: Colon operands must be real scalars. This warning will become an error in a future release.

ans = 0.1995

In R2024a it gives a warning but it is ok in R2023b! Some existing code that I use relies on this feature. Any reason why it will become an error in the future?

For a better example of how using non-scalar colon operands may be useful:

clear,clc
Params.agejshifter.College=21;
Params.J.College=100-Params.agejshifter.College;
Params.agej.College=1:1:Params.J.College;
Params.J.College=100-Params.agejshifter.College; % Age ends at 100
dj_temp=interp1([0,30,60,65,70,100],[0.00587,0.00116,0.01086,0.01753,0.02785,0.39134],0:1:100,'linear');
Params.sj.College=1-dj_temp((Params.agej.College+Params.agejshifter.College):100);

Warning: Colon operands must be real scalars. This warning will become an error in a future release.

[Matt J]

For a better example of how using non-scalar colon operands may be useful:

It's not an effective example, because you haven't told us what you think it should do.

Is the following simpler example doing what you expect? It seems like an overlay complicated way of pulling out the first element of a matrix.

A=reshape(1:20,4,5)*10

A = 4x5

10 50 90 130 170 20 60 100 140 180 30 70 110 150 190 40 80 120 160 200

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A([1,2]:[1,2])

Warning: Colon operands must be real scalars. This warning will become an error in a future release.

ans = 10

[Image Analyst]

Your example had a 3 element vector

x = rand(1,3);

Which element(s) do you think you want to refer to when you do x([1,2]:[1,2]) ? The first one, second one, third one, or the first and second one (like x(1:2))???

[Stephen23]

"Any reason why it will become an error in the future?"

Because it causes plenty of bugs, misleads users, and is trivially replaced with some indexing.

Answer 1

[Walter Roberson]

The colon operator has always been defined as using the first element of each operand.

The change is that soon using non-scalar operands will be an error instead of silently permitted.

Params.sj.College=1-dj_temp((Params.agej.College+Params.agejshifter.College):100);

That code does not create some kind of ragged array indexing from each Params.agej.College+Params.agejshifter.College pair to 100. That code is equivalent to

Params.sj.College=1-dj_temp((Params.agej.College(1)+Params.agejshifter.College(1)):100);

It has always been equivalent to that.

[Alessandro]

Thanks for your clear answer!

Answer 2

[Steven Lord]

For the expression you gave there are a few possible "reasonable" solutions for what it returns. [But it may not do what you think.]

actual = [1 2]:[1 2]

Warning: Colon operands must be real scalars. This warning will become an error in a future release.

actual = 1

whatItDoes = 1:1

whatItDoes = 1

alternate1 = [1:1 2:2] % Each element of a is "combined" with the corresponding element of b

alternate1 = 1×2

1 2

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alternate2 = [1 2:1 2] % 2:1 is empty

alternate2 = 1×2

1 2

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But there are other cases where it's difficult to give a reasonable alternative (other than the "just use the first element" approach it's been using for a while.)

actual2 = [1 2]:[1 2 3]

Warning: Colon operands must be real scalars. This warning will become an error in a future release.

actual2 = 1

actual3 = [1 2]:[3 999]

Warning: Colon operands must be real scalars. This warning will become an error in a future release.

actual3 = 1×3

1 2 3

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In addition, upon reviewing usage of : with non-scalar inputs in MathWorks code we've found that often that usage is a bug!

A = ones(3, 4);
v = 1:size(A)

Warning: Colon operands must be real scalars. This warning will become an error in a future release.

v = 1×3

1 2 3

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Did you expect v to be 1:3 or 1:4 or did you expect it somehow to be a combination of those two vectors? If you expected it to be 1:height(A) then you got what you expected; if you expected 1:width(A) or 1:numel(A) you weren't getting what you wanted!

v = 1:size(A, 1)

v = 1×3

1 2 3

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v = 1:height(A)

v = 1×3

1 2 3

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v = 1:size(A, 2)

v = 1×4

1 2 3 4

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v = 1:width(A)

v = 1×4

1 2 3 4

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v = 1:numel(A)

v = 1×12

1 2 3 4 5 6 7 8 9 10 11 12

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Let's look at your example.

Params.agejshifter.College=21;
Params.J.College=100-Params.agejshifter.College;
Params.agej.College=1:1:Params.J.College;
Params.J.College=100-Params.agejshifter.College; % Age ends at 100
dj_temp=interp1([0,30,60,65,70,100],[0.00587,0.00116,0.01086,0.01753,0.02785,0.39134],0:1:100,'linear');
a = (Params.agej.College+Params.agejshifter.College)

a = 1×79

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

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indices = a:100

Warning: Colon operands must be real scalars. This warning will become an error in a future release.

indices = 1×79

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

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What exactly were you hoping indices would be when you used it as an index into dj_temp? Did you somehow hope that it would be a collection of vectors, first 22:100 then 23:100 then 24:100 etc.? It is not. So if you thought it was behaving that way, your code has a bug.