This might be a very trivial question but as a beginner, I don't know the answer. I was exploring ? and after running a cell containing ?@, I got an interesting result:

enter image description here

However, I'm not sure about its exact interpretation. The answer contains two parts, a Global` part that shows the list of all the global variables that I have defined (or were created as a result of my code). But what is the System` part, and what does ?@do to get them?


1 Answer 1

  1. The full form of ?@ is Information["@", LongForm -> False].
  2. LongForm -> False means use the short form of Information, which calls Names["@"] (or a utility that both Information and Names use) to find the symbols which it should retrieve.
  3. Names interprets "@" as

    match one or more characters, but not uppercase letters

So you are getting a short form display from Information for all the symbols that match the wildcard character "@" as described under the 3rd bullet point of Details and Options under Names

  • $\begingroup$ Wow I was way over my head! Thanks. $\endgroup$
    – MathX
    Commented Mar 30, 2016 at 2:39
  • $\begingroup$ If I may ask, what are those that look like Greek letters with dots above and below that show in the system part? $\endgroup$
    – MathX
    Commented Mar 30, 2016 at 2:43
  • 1
    $\begingroup$ @MathX. See Formal Symbols under Letters and Letter-like Forms $\endgroup$
    – m_goldberg
    Commented Mar 30, 2016 at 2:47
  • $\begingroup$ Thanks! This is a super useful link. $\endgroup$
    – MathX
    Commented Mar 30, 2016 at 2:50
  • 1
    $\begingroup$ Yes. However, at that time, I was stuck by the special interpretation of ? by the front end, FullForm[Hold[?@]] won't work, one has to use prefix form FullForm@Hold@?@. $\endgroup$
    – luyuwuli
    Commented Mar 30, 2016 at 6:16

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