";" is described under Compound Expression as a delimiter, such that only the output of the last item in the compund expression is given.


1/ Print[] produces output whether followed by ";" or not. [Clearly an override is desirable so this makes sense for compound expressions]

2/ When combining graphics using Show[], appending ";" still suppresses output. Since "Show" is as conceptually active as "Print", is there a principle at work here (are there other instances)?

3/ If in procedural code a syntactically complete line is not terminated with a ";", the following line is indented... which means Show[] in the middle of several lines of code upsets indentation and code legibility

Question: if one has several graphics that one wishes to display without combining them what is the recommended approach, given that multiple Show[] should be separated by ";" but doing so suppresses output.

PS There is an incredibly minor-but-irritating-to-the-beginner "quirk" of commenting: semi-colon before comment at the end of code before a closing "]" is marked as a syntax error - is it really? If so, what is the nature of the error (it might just be redundant)?

 Print[a];(*the ; preceding is magenta, but is it really a syntax error?*)]

One is tempted to say the ";" is superflous because the expression is terminated by "]", in which case why is it not still an error when extra line breaks are inserted?

Block[{a}, Print[a];
 (*the ; preceding is not magenta, so why is this not a syntax error?*)
  • $\begingroup$ 1) Print[expr] is different than expr, evaluation will send expr via Print to $Output. 2) If you have a;, b, c; in consecutive lines in the cell then only b will be printed. It is not a CompoundExpression unless you wrap it with something, e.g: (a; b c;) (each in new line) Take a look at first part of this answer too. $\endgroup$ – Kuba Mar 24 '14 at 13:57
  • $\begingroup$ I think the recommended way is to use Print[Show[...]]; per line. Unlike Print, Show has no side effects (at least not since the major changes in how graphics are handled in version 6). The magenta highlighting doesn't really indicate a syntax error, I'd rather consider it an error in the highlighting. If you find that highlighting irritating you could put the ; after the comment, add another ; after the comment or explicitly put Null after it. As documented a;b; is actually CompoundExpression[a,b,Null] ... $\endgroup$ – Albert Retey Mar 24 '14 at 13:58
  • $\begingroup$ and if your Shows are CompoundExpressions I'd do (a; Return[b, CompoundExpression]; c;), if you really can't put it in a separate line. $\endgroup$ – Kuba Mar 24 '14 at 14:05
  • $\begingroup$ @Kuba, thanks for the expert perspective: the documentation doesn't seem to say anything about a Compound Expression needing to be wrapped and the basic examples suggest otherwise! As for Print to Stdout - I see: I might Print to a file. Per Print documentation though, does that mean that if $Output is not Stdout then it would not "generate a cell styled print"? If so, the documentation could be clearer... $\endgroup$ – Julian Moore Mar 24 '14 at 14:10
  • $\begingroup$ @AlbertRetey. Hmmm... I see that as Print refers to \$Output, Show refers to \$DisplayFunction... so if Print is pointing to a file, and Show is e.g. a PopupWindow, what happens? Anyway, quirk in the highlighting it is... this beginner did spend rather too much time trying to find a synatx error though! $\endgroup$ – Julian Moore Mar 24 '14 at 14:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.