Compilers: Principles and Practice by Pearson

A Simple Translator 37

and confirmed. We shall see in Chapter 4 why we need to

do this look-ahead. We cannot get the nextsymbol by

requesting Scanner(), as the meaning of a look-ahead

is that we peep at the next symbol, but without processing

it. Thus, we have a two-position “shifter” coded, which is

initially empty (see Fig. 2.2).

For each call to symbol(), a request goes to the

Scanner to supply the next token and the shifter is shifted

to the left. The very first call to symbol() results in two

calls to the Scanner. A call to nextsymbol() does not

result in a call to the Scanner, it simply allows a peep to

the nxtsym in the shifter.

The algorithms ...