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assignment - How to ignore list elements when extracting with pattern matching


If I have a simple list, say {1, 2, 3}, and I want to extract a subset of the elements, what is the correct syntax? e.g.


{ i, _, j } = {1, 2, 3}

I want i = 1 & j = 3 (which works), but I get the following warning:


Set::nosym: _ does not contain a symbol to attach a rule to.


so clearly I am using the wrong placeholder for elements I wish to ignore.



Answer



Turn off the warning


It should be noted that we can treat Set::nosym as a warning message rather than an error, and simply turn it Off:


Off[Set::nosym]

{i, _, j} = {1, 2, 3};

{i, j}



{1, 3}

throw-away Symbol


You could asko designate a Symbol for this purpose as a kind of dev/null, using e.g. $Post to clear it every time. I'll pick \[DoubleDagger], entered EscddgEsc and displayed as ‡:


$Post = ((‡ =.; #) &);

Now you could make your assignment:


{i, ‡, j} = {1, 2, 3};


The value of ‡ is cleared after each evaluation so as not to take up memory.



For reference, the definition of $Post above is not entirely neutral. For example, by default entering Sequence @@ {1, 2, 3} will return Sequence[1, 2, 3] whereas with the definition it will return 1. The ugly but proper definition would be something like:


$Post = Function[x, ‡ =.; Unevaluated@x, HoldAllComplete];

Since $Post is only one way to clear the Symbol I didn't want to clutter the top of the answer with this code. Other methods would be RunScheduledTask, CellEpilog, etc.


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