Error using simple matrix multiplication

Question

I stumbled upon an error during a simple multiplication that rather surprised me. What is happening here, I always assumed * was only for matrix multiplication.

x = 2;
y = zeros(1,4);
y(1) = 1 *x;
y(2) = x* 1;
y(3) = (x *1);
y(4) = x *1;
y
x *1

Will give the following output:

y =

     2     2     2     1

Error: "x" was previously used as a variable,
conflicting with its use here as the name of a function or command.
See MATLAB Programming, "How MATLAB Recognizes Function Calls That Use Command Syntax" for details.

Does anyone understand what is going on here? Of course I verified that x is not a function.

@Dan Indeed it can easily be reproduced, I think the answers explain it. — Dennis Jaheruddin, Aug 13 '13 at 11:05
I think one clue is the syntax coloring of the `*1` in the line that gives the problem... — Floris, Aug 13 '13 at 12:44

score 11 · Accepted Answer · answered Aug 13 '13 at 10:55

11

It depends on the spacing. See also here for a longer explanation and some examples of when you could have genuine ambiguity, but basically the first three of these will work as you expected, and the last will assume you are trying to call a function x with input *1:

x*1  
x * 1 
x* 1
x *1

This doesn't happen if you assign the output to some variable, regardless of spacing:

y(2) = x *1
z = x *1
x = x *1

answered Aug 13 '13 at 10:55

nkjt

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Thanks, I did not realize this would be interpreted as a function call if the function doesn't exist. – Dennis Jaheruddin Aug 13 '13 at 11:11
3

@DennisJaheruddin Try this as well: `{[1+2], [1 + 2], [1+ 2], [1 +2]}`. This quirk is one of the things I dislike most about MATLAB. – Sam Roberts Aug 13 '13 at 12:01

score 9 · Answer 2 · answered Aug 13 '13 at 10:41

9

This happens because when you have x *1 in a separate line, MATLAB interprets x as a function an tries to pass '*1' as an argument to it, but then it realzes that x is a variable, hence the error.

answered Aug 13 '13 at 10:41

Mohsen Nosratinia

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Thanks, I did not realize this would be interpreted as a function call if the function doesn't exist. Wish I could accept this answer as well. – Dennis Jaheruddin Aug 13 '13 at 11:12

Error using simple matrix multiplication

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