Unusal error "Current use of the variable is inconsistent with its previous use or definition".

Hi,

I recently updated to Matlab version R2021b and noticed an unusal error. Suppose, I delacre two variables 'a' and 'b' and when I add them together, I get an error "Unrecognized variable 'a' ". But If I write the same equation differently by adding or removing some spaces, it works fine (see the code snippet below). When using in a script, I get a error "the current use of 'a' is inconsistent with its previous use or definition".

Am I missing something obvious? I don't remember seeing this issues in the previous versions of Matlabs. Any clarification is appreciated.

a = 10;
b = 10;
a +b    %shows error Unrecognized variable

Unrecognized function or variable 'a'.

a+b    % this is fine
a+ b    % this is fine
a + b    % this is fine
a + b    % this is fine
c = a +b    % this is fine

Answer 1

[James Tursa]

See how the +b is purple? That means MATLAB thinks you are invoking a as a function with +b as a character string input. Hence the error since a is not a function. This form of calling functions with character string inputs has been around for a long, long time. E.g., here is an example of using this form of function calling with a different function that takes character string inputs (see how the ThisIsAString is highlighted purple in both cases, meaning MATLAB is treating it as a character string):

disp ThisIsAString

ThisIsAString

Does the same thing as

disp('ThisIsAString')

ThisIsAString

Spacing matters with your case, so use one of the other forms that works.

[Topojit Debnath]

Thank you for quick reply.

1. Yes, I noticed the purple texts, but didn't know we can declare function that way. I normally declare using the tag 'function'. Can you show some example or documentation of this type of function declaration?
2. Do you know when this was implemented? I used Matlab R2019 and R2020, but didn't notice this behaviour. I have a lot of older Matlab codes and this error is driving me mad. Assuming I can't turn of this behaviour, I have to go back to an older version for now which doesn't have this implemented.

[Walter Roberson]

a +b

has three different potential meanings in MATLAB:

1. add a and b -- that is, equivlaent to plus(a,b)
2. the + immediately followed by a value might be unary plus uplus, + so another interpretation would be "10 space unary-plus-10" and since unary plus of a simple numeric value is the same as the value, that would potentially be "10 space 10" which would be a syntax error unless it were inside a [] or {}
3. or since this is the start of a line, it could mean "call function a passing in the character vector '+b'" which is an example of command syntax in MATLAB; see https://www.mathworks.com/help/matlab/matlab_prog/command-vs-function-syntax.html

MATLAB has to choose one of those behaviours -- or else just generally give an error saying "That's ambiguous". MATLAB happens to choose command equivalance in this case.

If it were something that was not in a place that permits a command, then MATLAB might have interpreted it differently. For example 3 + a +b in a slot of a possible command would be interpreted as additions, and [3 + a +b] would be interpreted as [(3 + a), (+b)] which would be [13, 10]

You do not always want any + between numbers or variables to be interpreted as addition . For example,

[-1 -1 +1 +1 -1 -1 +1 +1]

is obviously intended to be a list of positive and negative values rather than an expression intended to be calculated.

[John D'Errico]

You are not actually declaring a as a function. Instead, you have managed to confuse the parser.

This function/command duality capability has been in MATLAB for many years. Whenever you use a command, you are actually using the command form of the function by that name. For example,

load XYZ.mat

is equivalent to the function call

load('XYZ.mat')

So when you wrote

a +b

the parser decides from the context that a must be a function, with the argument the string +b. But then it realizes that a was actually a variable, so it gets upset and espresses its confusion by the error.

We can see this happening in the call to a function I named a.

a +b

  Name      Size            Bytes  Class    Attributes

  arg       1x2                 4  char               

+b

function a(arg)
   whos arg
   disp(arg)
end

Again, it has been this way for many years.

[James Tursa]

@Topojit Debnath This behavior has probably been around for decades. It is certainly present in your R2019 and R2020 versions. I just checked my R2018b version and it behaves this way. If you have code that behaves differently in the versions you have, please post it.

[Walter Roberson]

@Topojit Debnath

As you can see from this, the behaviour was present R2016a or before.

The exact message has changed over the years, but this is the way MATLAB has behaved for a long long time.

[Topojit Debnath]

I see. Then I might have confused this with python. This syntax doesn't show error in python and gives the addition as output result (just checked in Jupyterlab).

Thank you everyone for the help. Have a nice Weekend.

[Walter Roberson]

Does python have a "command" syntax (that is not down to restricted keywords) ?

There is a situation in MATLAB in which a + in that context would not be a clue that mathematics was intended.

MATLAB has a "which" function / command which is most often used in command form, such as

which classify

but it is expected to also be able to handle lines such as

which +java/+lang/serialport
which sym/beta
which double>empty

and of course there are command syntaxes that are expected to work such as

ls C:\Users\该帽子\Desktop

[Steven Lord]

There are some functions in Cleve Moler's original MATLAB that can be called in command form. The Appendix section of this blog post says that you can use the commands "help \ , help eig" to get some help. The help for the CLEAR function also says "CLEAR X" works. So this has existed for at least 40 years.

In fact you've almost certainly used command form without realizing it, for one or more of the following six functions.

• help
• doc
• close
• clear
• hold
• edit

If you've ever written help plot, doc ode45, close all, clear all, hold on, or edit myfun you've used command form. You could have called those functions using function form as help('plot'), doc('ode45'), close('all'), clear('all'), hold('on'), or edit('myfun') with only a few more keystrokes.

The rules for what counts as command form and what's interpreted as an operator call can get complicated.