ANTLR4: grammar with runtime matching of number of integers not working correctly

Question

I am trying out a grammar in the ANTLR 4 book (p. 48-9), but the grammar isn't behaving as advertised.

The input is a series of integers. One integer indicates the number of following integers. For example, the first 2 in the following input indicates that there are two following integers and then the 3 indicates that there are three following integers:

2    9    10    3    1    2    3

At the bottom of this post is the grammar given in the book. When I run the ANTLR test rig (grun) with the -tree flag, using the grammar and the above input:

grun Data file -tree

I get this incorrect output tree:

(file (group 2 (sequence 9 10 3)) (group 1 (sequence 2 3)))

Through some experimentation I discovered that if I change the grammar from using this semantic predicate:

{$i<=$n}?

to using this incorrect semantic predicate:

{$i<$n}?

then I get the correct output tree:

(file (group 2 (sequence 9 10)) (group 3 (sequence 1 2 3)))

I am baffled. Any ideas on why this is happening? Is it a bug in the test rig (grun)? Or (more likely) am I not understanding something about the workings of ANTLR's semantic predicates and actions?

grammar Data;

file: group+ ;

group: INT sequence[$INT.int] ;

sequence[int n]
locals [int i = 1;]
      : ( {$i<=$n}? INT {$i++;} )*  // match n integers
      ;

INT  : [0-9]+ ;
WS  : [ \t\r\n]+ -> skip ;

Answer 1

Your local variable i is starting off uninitialized (at 0) instead of 1 because of a bug introduced in ANTLR 4.3.

The bug is described in more detail at https://github.com/antlr/antlr4/issues/672 .