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Ensure the syntax is what we guess

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Raphael Cautain
Raphael Cautain on 17 Aug 2012
It would be nice to publish (as in Old School Languages) a syntax summary of Matlab, reflecting the exact behaviour of the interpreter.
It could help to prevent surprise from unexpected results, such as when one evaluates the expression : 5 + 1*0:10
A single transcription of the formal grammar contained in the interpreter's code would be sufficient.
Thanks.

  2 Comments

Walter Roberson
Walter Roberson on 17 Aug 2012
Yes, a BNF would be good.
Walter Roberson
Walter Roberson on 17 Aug 2012
I would not call it a bug. It just reflects that ":" is a low priority operator, which is consistent. If you want to raise the priority of the ":" then use brackets.

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Accepted Answer

Jan
Jan on 17 Aug 2012
You do not have to _guess_ the details of the syntax. It is explained exhaustively in the documentation already. Your example |5 + 1*0:10| is clear after reading:
doc precedence

  2 Comments

Raphael Cautain
Raphael Cautain on 18 Aug 2012
That's right, thanks. This precedence rule is very useful when we write subscripts, like in "subimg = img(x-r:x+r, y-r:y+r);" We just lost the equality of sizes between 0:10 and 1+0:10, which is really less useful and requires then to write 1+(0:10). No problem, it's just an habit.
Jan
Jan on 18 Aug 2012
Note, that X((1+0):(1+10)) will be processed faster than X(1 + (0:10)): In the 1st case the limits are checked for the marginal elements only, while in the 2nd case Matlab wastes time with boundary checks. Of course this is more important for larger index vectors.

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