15
$\begingroup$

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?

Improve this question
$\endgroup$

1 Answer 1

Reset to default
19
$\begingroup$
  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

Improve this answer
$\endgroup$
8
  • $\begingroup$ 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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.