Java Reference
In-Depth Information
1
Start
→
Num
ans
$
call
print
(
ans
)
2 Num
ans
→
x SetBase Digs
baseans
ans
←
baseans
3
|
SetBaseTen Digs
decans
ans
←
decans
4 SetBase
→
d
val
base
←
val
5 SetBaseTen
→ λ
base
←
10
6 Digs
up
→
Digs
below
d
next
if
next
≥
base
then
3
( "Digit outside allowable range")
up
←
below
×
base
+
next
error
7
|
d
f irst
if
f irst
≥
base
then
4
error
( "Digit outside allowable range")
up
←
f irst
Figure 7.7: Strings with an optionally specified base.
This grammar avoids copying the productions that generate a string of digits;
instead, the grammar assigns and references a global variable (
base
)asa
side-
e
ff
ect
of processing an o (Rule 4) or
λ
(Rule 5).
We expand our example yet again, using the terminal x to indicate that the
next digit is the base to which the subsequent digits should be interpreted. If
no x is present, then the digits should be interpreted base-10, as before. Some
examples of legal inputs and their interpretation are as follows:
Input
Meaning Value (base-10)
431$
431
10
431
x8431$
431
8
281
x5431$
431
5
116
A grammar for the new language is shown in Figure 7.7. Unit- and
λ
-
productions are introduced to set the global variable
base
properly.
