I expected 3 from both of these
j = 2; new = {1, 1, 1, 1, 1}; new[[++j]] += 2; j
j = 2; new = {1, 1, 1, 1, 1}; new[[++j]] = 2; j
4
3
Why does the first one return 4?
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1 Answer
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That's really a nasty one. I don't think that it is supposed to work this way, but apparently,
new[[++j]] += 2
is equivalent to
new[[++j]] = new[[++j]] + 2
That's why ++j gets executed twice.
I am not sure, but maybe AddTo is implemented as follows:
SetAttributes[addto, HoldFirst];
addto[a_, b_] := a = a + b;
Since the first argument is held, new[[++j]] is evaluated twice under execution of addto[new[[++j]],2].
answered Jul 21, 2018 at 18:27
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AddToandPreIncrementhave the attributeHoldFirst$\endgroup$ Jul 21, 2018 at 21:00 -
$\begingroup$ My uncertainty about the implementation was more about the true internals than about the fact that
AddTohas that attribute. In principle, the held expression could be analyzed for occurences ofPartand this behavior could be prevented by enforcing that the second arguments ofPartget evaluated only once. $\endgroup$ Jul 21, 2018 at 21:11
list, not the 3rd. $\endgroup$