Stacks and compilers - Data Structures and Problem Solving Using Java - page 453

1 /**
2 * Print an error message for unbalanced symbols.
3 * @return number of errors detected.
4 */
5 public int checkBalance( )
6 {
7 char ch;
8 Symbol match = null;
9 Stack<Symbol> pendingTokens = new Stack<Symbol>( );
10
11 while( ( ch = tok.getNextOpenClose( ) ) != '\0' )
12 {
13 Symbol lastSymbol = new Symbol( ch, tok.getLineNumber( ) );
14
15 switch( ch )
16 {
17 case '(': case '[': case '{':
18 pendingTokens.push( lastSymbol );
19 break;
20
21 case ')': case ']': case '}':
22 if( pendingTokens.isEmpty( ) )
23 {
24 errors++;
25 System.out.println( "Extraneous " + ch +
26 " at line " + tok.getLineNumber( ) );
27 }
28 else
29 {
30 match = pendingTokens.pop( );
31 checkMatch( match, lastSymbol );
32 }
33 break;
34
35 default: // Cannot happen
36 break;
37 }
38 }
39
40 while( !pendingTokens.isEmpty( ) )
41 {
42 match = pendingTokens.pop( );
43 System.out.println( "Unmatched " + match.token +
44 " at line " + match.theLine );
45 errors++;
46 }
47 return errors + tok.getErrorCount( );
48

}

figure 11.8
The checkBalance algorithm