The parameter ctx (aka context) refers to a node in the par=
se tree. So they are instantiated within the Parser class generated by ANTL=
R. As it parses the input stream, it generates these nodes.

Because it=E2=80=99s using a heterogeneous tree representation,=
it actually can refer to a variety of different classes, in this case e>ProgContext to represent programs (the prog non-termi=
nal), StmtContext to represent statements, and ExprConte=
xt for expressions. The exact set of context classes will depend on =
your grammar and the use of the =E2=80=9Chash tags=E2=80=9D described in =
=E2=80=9CDistinguishing rule alternatives.=E2=80=9D s.net/cs664s16/visitor.html#distinguishing-rule-alternatives" class=3D"uri"=
>https://liucs.net/cs664s16/visitor.html#distinguishing-rule-alternativesa>

=E2=80=9CRuben.Safir=E2=80=9D =
ruben.safir-at-my.liu.edu writes:

can anyone tell me where this ctx object is defined and when it is insta=
ntiated?

 visitProg(ctx) top-level program
     where ctx.stmt(0).getText() is "n=3D5"
     and ctx.stmt(1).getText() is "print -3*(n+1)"
 visitStmt(ctx) assignment statement
     where ctx.ID().getText() is "n"
     and ctx.expr().getText() is "5"
 visitExpr(ctx) numeric literal
     where ctx.NUM().getText() is "5"
 visitStmt(ctx) print statementan
     where ctx.ID() is null
     and ctx.expr().getText() is "-3*(n+1)"
 visitExpr(ctx) multiplication
     where ctx.expr(0).getText() is "-3"
     and ctx.expr(1).getText() is "(n+1)"
 visitExpr(ctx) negation
     where ctx.expr(0).getText() is "3"
 visitExpr(ctx) numeric literal
     where ctx.NUM().getText() is "3"
 visitExpr(ctx) parentheses
     where ctx.expr(0).getText() is "n+1"
 visitExpr(ctx) addition
     where ctx.expr(0).getText() is "n"
     and ctx.expr(1).getText() is "1"
 visitExpr(ctx) variable
     where ctx.ID().getText() is "n"
 visitExpr(ctx) numeric literal
     where ctx.NUM().getText() is "1"