Vector definition, what is expected behavior when separating elements by comma and spaces are used in the expression?

I recently found an unexpected behavior in some of my code. When I define a vector:

ind = [1,1+2,3]

Matlab returns (as expected):

ind =

       1     3     3

If I am a little loose with my spaces, and say

ind = [ 1 , 1 + 3 , 3 ]

Matlab still returns the expected answer as stated above. However, if I am less consistent with using spaces and define ind as follows:

ind = [1, 1 +2, 3]

Matlab gives me:

ind =

       1     1     2     3

I did also test other arithmetic and logical operators like

[1, 1 -2 -2, 3] 
ans =
     1     1    -2    -2     3  % acts as [1,1,-2,2,3]
[1, 1 -2 + 2, 3]
ans =
     1     1     0     3        % acts as [1,1,-2+2,3]
[1, 1 *2 + 2, 3]
ans =
     1     4     3              % acts as [1,1*2+2,3]
[1, 1 *2 +2, 3]
ans =
     1     2     2     3        % acts as [1,1*2,2,3]
[1, 1 *2 >2, 3]
ans =
     1     0     3              % acts as [1,1*2>2,3]

It seems like +- indicates a new element in the array while other arithmetic operations and logical operations do not. So my question is when does Matlab decide how to separate elements in a vector?

I'm using Matlab 9.0.0.341360 (R2016a)

Answer 1

[Stephen23]

This is explained in the MATLAB documentation:

https://www.mathworks.com/help/matlab/matlab_prog/case-and-space-sensitivity.html

https://www.mathworks.com/help/matlab/matlab_prog/operator-precedence.html

And has also been covered several times on this forum, e.g.:

https://www.mathworks.com/matlabcentral/answers/320231-either-i-or-matlab-is-going-crazy-with-vertcat-using-brackets

Really the differences between your examples come down to whether an operator can be both unary and binary (e.g. + and -) or is binary only (e.g. * and <):

1. If there is no space a unary operator has a higher priority than the binary.
2. If the operator is only binary then the space is always optional.

NOTE: All of the examples are code that should never be used in real life. Always use commas for separating elements of an array, if only because it makes the intention clear.

[TordFO]

Thank you. That was the documentation page I was looking for. However, it does not account for the fact that

[1 +1]

and

[1 + 1]

are treated differently, other than "Because of this flexibility, check to ensure that MATLAB stores the correct values.". Can you post the link to where the question is answered? (I think the link is missing from your answer)

[Stephen23]

Read the page about operator precedence.

[TordFO]

Thank you for the answer. This was a detour, but helpful reading.