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3: Implementing Language Interpreters in Java
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return result; } // Get the value of a number or variable. private double atom() throws InterpreterException { double result = 0.0; switch(tokType) { case NUMBER: try { result = Double.parseDouble(token); } catch (NumberFormatException exc) { handleErr(SYNTAX); } getToken(); break; case VARIABLE: result = findVar(token); getToken(); break; default: handleErr(SYNTAX); break; } return result; } // Return the value of a variable. private double findVar(String vname) throws InterpreterException { if(!Character.isLetter(vname.charAt(0))){ handleErr(SYNTAX); return 0.0; } return vars[Character.toUpperCase(vname.charAt(0))-'A']; } // Return a token to the input stream. private void putBack() { if(token == EOP) return; for(int i=0; i < token.length(); i++) progIdx--; }
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// Handle an error. private void handleErr(int error) throws InterpreterException { String[] err = { "Syntax Error", "Unbalanced Parentheses", "No Expression Present", "Division by Zero", "Equal sign expected", "Not a variable", "Label table full", "Duplicate label", "Undefined label", "THEN expected", "TO expected", "NEXT without FOR", "RETURN without GOSUB", "Closing quotes needed", "File not found", "I/O error while loading file", "I/O error on INPUT statement" }; throw new InterpreterException(err[error]); } // Obtain the next token. private void getToken() throws InterpreterException { char ch; tokType = NONE; token = ""; kwToken = UNKNCOM; // Check for end of program. if(progIdx == prog.length) { token = EOP; return; } // Skip over white space. while(progIdx < prog.length && isSpaceOrTab(prog[progIdx])) progIdx++;
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3: Implementing Language Interpreters in Java
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// Trailing whitespace ends program. if(progIdx == prog.length) { token = EOP; tokType = DELIMITER; return; } if(prog[progIdx] == '\r') { // handle crlf progIdx += 2; kwToken = EOL; token = "\r\n"; return; } // Check for relational operator. ch = prog[progIdx]; if(ch == '<' || ch == '>') { if(progIdx+1 == prog.length) handleErr(SYNTAX); switch(ch) { case '<': if(prog[progIdx+1] == '>') { progIdx += 2;; token = String.valueOf(NE); } else if(prog[progIdx+1] == '=') { progIdx += 2; token = String.valueOf(LE); } else { progIdx++; token = "<"; } break; case '>': if(prog[progIdx+1] == '=') { progIdx += 2;; token = String.valueOf(GE); } else { progIdx++; token = ">"; } break; }
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tokType = DELIMITER; return; } if(isDelim(prog[progIdx])) { // Is an operator. token += prog[progIdx]; progIdx++; tokType = DELIMITER; } else if(Character.isLetter(prog[progIdx])) { // Is a variable or keyword. while(!isDelim(prog[progIdx])) { token += prog[progIdx]; progIdx++; if(progIdx >= prog.length) break; } kwToken = lookUp(token); if(kwToken==UNKNCOM) tokType = VARIABLE; else tokType = COMMAND; } else if(Character.isDigit(prog[progIdx])) { // Is a number. while(!isDelim(prog[progIdx])) { token += prog[progIdx]; progIdx++; if(progIdx >= prog.length) break; } tokType = NUMBER; } else if(prog[progIdx] == '"') { // Is a quoted string. progIdx++; ch = prog[progIdx]; while(ch !='"' && ch != '\r') { token += ch; progIdx++; ch = prog[progIdx]; } if(ch == '\r') handleErr(MISSINGQUOTE); progIdx++; tokType = QUOTEDSTR;
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3: Implementing Language Interpreters in Java
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} else { // unknown character terminates program token = EOP; return; } } // Return true if c is a delimiter. private boolean isDelim(char c) { if((" \r,;<>+-/*%^=()".indexOf(c) != -1)) return true; return false; } // Return true if c is a space or a tab. boolean isSpaceOrTab(char c) { if(c == ' ' || c =='\t') return true; return false; } // Return true if c is a relational operator. boolean isRelop(char c) { if(relops.indexOf(c) != -1) return true; return false; } /* Look up a token's internal representation in the token table. */ private int lookUp(String s) { int i; // Convert to lowercase. s = s.toLowerCase(); // See if token is in table. for(i=0; i < kwTable.length; i++) if(kwTable[i].keyword.equals(s)) return kwTable[i].keywordTok; return UNKNCOM; // unknown keyword } }
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The Small BASIC Expression Parser
At the core of the interpreter is the expression parser. As mentioned, the parser used by Small BASIC is adapted from the one shown in 2. If you have not yet read 2, do so now because it provides the detailed description of the parser. Although its fundamental operation is unchanged, a special version of the parser is required for Small BASIC. Many of the changes to the parser allow it to handle the syntax of the Small BASIC language. For example, the parser must be able to recognize the keywords of the language, it must not treat the equal sign (=) as an operator, and it must evaluate relational operators. The getToken( ) method is substantially enhanced to handle the expanded demands placed on it. Other differences between the parser in 2 and the one used here are caused by efficiency considerations. For example, in 2, a reference to the expression was passed to the parser. In the Small BASIC version, a reference to the program being interpreted is held in an instance variable shared by both the interpreter and the parser. Thus, the overhead associated with passing the reference is avoided. Because interpreters are slow by nature, these types of efficiency enhancements are important. Another change in the parser is caused by the fact that the Small BASIC version operates on an array of characters rather than on a string. Recall that the parser developed in 2 was passed a string containing the expression to evaluate. The reason for the change is efficiency. As you know, a program is stored in a normal text file, which is a sequence of characters, not a string. Thus, when the interpreter loads the file prior to execution, it reads the file into a character array. Although it would have been possible to convert this array into a string, to do so would introduce a needless inefficiency. Since the Small BASIC expression parser uses the same techniques as described in 2, you will have no trouble following its operation and we will not examine it in detail here. However, before moving on to the interpreter, a few general comments are in order. We will begin by explaining precisely what an expression is as it relates to Small BASIC.
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