Gdtyjr

Is it possible to add non-number characters on the left of a code block?

Specifically, I want to add ... to correspond with some source code (see image).

I am currently using commands to stop / start the line numbers.
Here is the code:

documentclass[12pt]article

usepackage[a4paper, margin = 20mm, top = 30mm, bottom = 25mm]geometry
usepackagexcolor, soul
usepackagelistings

definecolorcodebackgroundrgb0.95, 0.95, 0.92

lstset

 frame = tb, % hrule above and below
 keepspaces = true,
 columns = flexible,
 basicstyle = normalsizettfamily,
 escapeinside = (*@@*), % for escaping
 backgroundcolor = colorcodebackground,
 showstringspaces = false,
 language = C,
 keywordstyle = colorblue,
 stringstyle = colorred,
 commentstyle = colorteal,
 numbers = left, % none, left, right
 firstnumber = 1,
 numberstyle = scriptsizecolorblack,
 numbersep = 5pt


letorigthelstnumberthelstnumber
makeatletter
% for stopping line numbers
newcommand*stopnumber

 lst@AddToHookOnNewLine
 
 letthelstnumberrelax
 advancec@lstnumber-@nerelax
 

% for starting line numbers
newcommand*startnumber[1]

 setcounterlstnumbernumexpr#1-1relax
 lst@AddToHookOnNewLine
 
 letthelstnumberorigthelstnumber
 refstepcounterlstnumber
 

makeatother

begindocument
beginlstlisting[firstnumber = 456]
int third_function(const int c)

 /* error checking */
 if(c < 0)
 
 return;
 (*@ stopnumber @*)

 ...
 (*@ startnumber512 @*)
 /* other stuff */
 int d = 4;
 char* word = "third function"; (*@ stopnumber @*)

 ...
 (*@ startnumber678 @*)
 return (c * d);

endlstlisting
enddocument

Here's a simple solution that makes use of the mathescape option to modify the counter and printing of line numbers. You could also use escapeinside if you want to use different escape characters than the standard $...$.

documentclassarticle
usepackagelistings

lstsetbasicstyle=ttfamily, language=c, numbers=left

makeatletter
letorig@lstnumber=thelstnumber

newcommandlstsetnumber[1]gdefthelstnumber#1
newcommandlstresetnumbergloballetthelstnumber=orig@lstnumber
makeatother

begindocument
beginlstlisting[firstnumber=100, mathescape=true]
int foo(int x) 
 // some code$lstsetnumberldots$
 ...$lstresetnumbersetcounterlstnumber199$

 // more code with extra spacing$lstsetnumber$
 $lstsetnumberldots$
 ...$lstsetnumber$
 $lstresetnumbersetcounterlstnumber299$

 return x;

endlstlisting
enddocument

The lstsetnumber macro redefines the output macro for line numbers, in the above example to the fixed value ldots. lstresetnumber resets the output to the standard definition.

Note that you have to subtract 1 from the next line number if you modify the line counter via setcounterlstnumber as listings automatically increases the counter on the next line.

The code for adding extra empty lines is a bit clumsy, not sure how to add vertical space in a more flexible way here.

So here's another, easier to use approach that is syntactically the same as in your example:

documentclass[12pt]article

usepackage[a4paper, margin = 20mm, top = 30mm, bottom = 25mm]geometry
usepackagexcolor, soul
usepackagelistings

definecolorcodebackgroundrgb0.95, 0.95, 0.92
lstset ... 

letorigthelstnumberthelstnumber
makeatletter

renewcommandthelstnumber%
 ifnumvaluelstnumber>0
 origthelstnumber
 else
 ifnumvaluelstnumber=-1
 ldots
 fi
 fi


% for starting line numbers
newcommand*startnumber[1]%
 setcounterlstnumbernumexpr#1-1relax%


% for stopping line numbers
newcommand*stopnumber%
 startnumber-2%


makeatother

begindocument
beginlstlisting[firstnumber = 456]
int third_function(const int c)

 /* error checking */
 if(c < 0)
 
 return;
 (*@ stopnumber @*)

 ...
 (*@ startnumber512 @*)
 /* other stuff */
 int d = 4;
 char* word = "third function"; (*@ stopnumber @*)

 ...
 (*@ startnumber678 @*)
 return (c * d);

endlstlisting
enddocument

In your code you are using lst@AddToHook multiple times to change thelstnumber. This is a bit problematic because adding hooks is accumulative, i.e. it adds the same commands everytime you use it. Also, the listings documentation explicitly doesn't guarantee that the order of hook execution is the same as the order of definition.

My approach is based on redefining thelstnumber only once and (mis)using negative line numbers for the ellipses in your code. Positive numbers are printed as usual, -1 denotes ldots, other negative numbers don't yield any output at all. So in stopnumbers we just init lstnumber with -2 to map the following sequence of -2, -1, 0 to empty, ldots, empty.

In case you'd want a short ellipsis without empty lines around, you could define a new command that inits lstnumber with -1.