bug in for loop?

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itamar luzon
itamar luzon on 30 Apr 2018
Commented: Jan on 30 Apr 2018
matlab run this code like there is a space between "5" and "a" in "for ii=4:5a" line.
a='aaaa';
for ii=4:5a
disp(ii);
end
answer:
a =
'aaaa'
4
a =
'aaaa'
5
while matlab cannot read a simpler line like:
5a
answer:
5a
Error: Unexpected MATLAB expression.
  2 Comments
Jan
Jan on 30 Apr 2018
@Dennis: Obviously the Matlab interpreter considers the code as valid. "5a" is not supposed to be a variable, but Matlab interprets this as "for ii=4:5 a".
@itamar luzon: I can imagine, that it took you a while to identify this behavior. A nice and confusing observation. Thanks for posting it, +1.

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Answers (1)

Walter Roberson
Walter Roberson on 30 Apr 2018
Cute. ;-)
While it is not what I would expect, I am not sure it could be called a bug.
The "for" loop can be followed by a statement on the same line:
for i = 4 : 5 disp(a), end
The parsing of the range of values for the "for" loop proceeds as far as possible to form an expression.
>> for j = 4:5i
disp(j)
end
Warning: Colon operands must be real scalars.
"i" is a valid numeric suffix so it was included in the parsing.
Now, what it it appears is that once it has gone as far as it can in parsing the expression for the loop bounds, that it is prepared to pick up from the very next character as the expression to be executed, without requiring a whitespace or ';' or ',' . Not what I would have guessed, but in the absence of any documentation saying that whitespace or separator is mandatory there, it is difficult to call it a bug.
... One thing that you need to understand is that the MATLAB command parser is hacked together. The MATLAB language "just grew" and was not defined in terms of BNF, so there are parsing oddities in corner cases like these.
  1 Comment
itamar luzon
itamar luzon on 30 Apr 2018
Thanks, Walter.
I agree it's not a bug, but it still very confusing.
"Not what I would have guessed", I also agree about the guess, I also thought the same, but in programming, things need to be clear and we don't need to guess.

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