handles numeric expressions, and the interpreter, which actually executes the program. The expression parser is adapted from the one shown in Chapter 2. As it is used here, it parses a numeric expression that is contained within a larger program, rather than just parsing a self- contained expression as it did in Chapter 2.
Both the interpreter and the parser subsystems are contained within a single interpreter class, calledSBasic. Although it would in theory have been possible to use two separate classes, one for the interpreter and one for the expression parser, it was more efficient to combine both into a single class. The reason for this is that the expression parser and the interpreter code are highly intertwined. For example, both operate on the same character array that holds the program. Separating them into two classes would have added considerable overhead, a loss of performance, and duplication of functionality. Furthermore, because parsing an expression is simply a part of the larger task of interpreting a program, it makes sense to have the entire mechanism contained within a single class.
The interpreter works by reading the source code of a program one token at a time. When it encounters a keyword, it does whatever that keyword requests. For example, when the interpreter reads PRINT, it prints the value of the expression that follows. When it reads GOSUB, it executes the specified subroutine. This process continues until the end of the program is reached. Thus, the interpreter simply does what the program tells it to do!
The Small BASIC Interpreter
The code for the Small BASIC interpreter is fairly long—longer than one would normally put in a chapter of a book. However, don't be intimidated by its size. Despite its length, the interpreter is conceptually simple, and once you grasp its general mode of operation, each part is easy to understand.
The entire code for the Small BASIC interpreter is shown next. The remainder of this chapter will explain how it works, and how to use it.
// A Small BASIC Interpreter. import java.io.*;
import java.util.*;
// Exception class for interpreter errors. class InterpreterException extends Exception {
String errStr; // describes the error public InterpreterException(String str) {
errStr = str; }
public String toString() { return errStr;
} }
// The Small BASIC interpreter. class SBasic {
final int PROG_SIZE = 10000; // maximum program size // These are the token types.
final int NONE = 0; final int DELIMITER = 1; final int VARIABLE = 2; final int NUMBER = 3; final int COMMAND = 4; final int QUOTEDSTR = 5;
// These are the types of errors. final int SYNTAX = 0;
final int UNBALPARENS = 1; final int NOEXP = 2; final int DIVBYZERO = 3; final int EQUALEXPECTED = 4; final int NOTVAR = 5;
final int LABELTABLEFULL = 6; final int DUPLABEL = 7; final int UNDEFLABEL = 8; final int THENEXPECTED = 9; final int TOEXPECTED = 10; final int NEXTWITHOUTFOR = 11; final int RETURNWITHOUTGOSUB = 12; final int MISSINGQUOTE = 13;
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final int FILENOTFOUND = 14; final int FILEIOERROR = 15; final int INPUTIOERROR = 16;
// Internal representation of the Small BASIC keywords. final int UNKNCOM = 0;
final int PRINT = 1; final int INPUT = 2; final int IF = 3; final int THEN = 4; final int FOR = 5; final int NEXT = 6; final int TO = 7; final int GOTO = 8; final int GOSUB = 9; final int RETURN = 10; final int END = 11; final int EOL = 12;
// This token indicates end-of-program. final String EOP = "\0";
// Codes for double-operators, such as <=. final char LE = 1;
final char GE = 2; final char NE = 3; // Array for variables. private double vars[];
// This class links keywords with their keyword tokens. class Keyword {
String keyword; // string form
int keywordTok; // internal representation Keyword(String str, int t) {
keyword = str; keywordTok = t; }
}
/* Table of keywords with their internal representation. All keywords must be entered lowercase. */
Keyword kwTable[] = {
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new Keyword("input", INPUT), new Keyword("if", IF), new Keyword("then", THEN), new Keyword("goto", GOTO), new Keyword("for", FOR), new Keyword("next", NEXT), new Keyword("to", TO), new Keyword("gosub", GOSUB), new Keyword("return", RETURN), new Keyword("end", END)
};
private char[] prog; // refers to program array private int progIdx; // current index into program private String token; // holds current token private int tokType; // holds token's type
private int kwToken; // internal representation of a keyword // Support for FOR loops.
class ForInfo {
int var; // counter variable double target; // target value
int loc; // index in source code to loop to }
// Stack for FOR loops. private Stack fStack;
// Defines label table entries. class Label {
String name; // label
int loc; // index of label's location in source file public Label(String n, int i) {
name = n; loc = i; }
}
// A map for labels.
private TreeMap labelTable; // Stack for gosubs.
private Stack gStack;
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// Relational operators. char rops[] = {
GE, NE, LE, '<', '>', '=', 0 };
/* Create a string containing the relational operators in order to make checking for them more convenient. */
String relops = new String(rops); // Constructor for SBasic.
public SBasic(String progName) throws InterpreterException { char tempbuf[] = new char[PROG_SIZE]; int size;
// Load the program to execute.
size = loadProgram(tempbuf, progName); if(size != -1) {
// Create a properly sized array to hold the program. prog = new char[size];
// Copy the program into program array. System.arraycopy(tempbuf, 0, prog, 0, size); }
}
// Load a program.
private int loadProgram(char[] p, String fname) throws InterpreterException
{
int size = 0; try {
FileReader fr = new FileReader(fname); BufferedReader br = new BufferedReader(fr); size = br.read(p, 0, PROG_SIZE);
fr.close();
} catch(FileNotFoundException exc) { handleErr(FILENOTFOUND);
} catch(IOException exc) { handleErr(FILEIOERROR); }
// If file ends with an EOF mark, back up. if(p[size-1] == (char) 26) size--;
return size; // return size of program }
// Execute the program.
public void run() throws InterpreterException { // Initialize for new program run.
vars = new double[26]; fStack = new Stack(); labelTable = new TreeMap(); gStack = new Stack(); progIdx = 0;
scanLabels(); // find the labels in the program sbInterp(); // execute
}
// Entry point for the Small BASIC interpreter. private void sbInterp() throws InterpreterException {
// This is the interpreter's main loop. do {
getToken();
// Check for assignment statement. if(tokType==VARIABLE) {
putBack(); // return the var to the input stream assignment(); // handle assignment statement } else // is keyword switch(kwToken) { case PRINT: print(); break; case GOTO: execGoto(); C h a p t e r 3 : I m p l e m e n t i n g L a n g u a g e I n t e r p r e t e r s i n J a v a
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break; case IF: execIf(); break; case FOR: execFor(); break; case NEXT: next(); break; case INPUT: input(); break; case GOSUB: gosub(); break; case RETURN: greturn(); break; case END: return; } } while (!token.equals(EOP)); }
// Find all labels.
private void scanLabels() throws InterpreterException {
int i;
Object result;
// See if the first token in the file is a label. getToken();
if(tokType==NUMBER)
labelTable.put(token, new Integer(progIdx)); findEOL();
do {
getToken();
if(tokType==NUMBER) {// must be a line number result = labelTable.put(token,
new Integer(progIdx)); if(result != null)
}
// If not on a blank line, find next line. if(kwToken != EOL) findEOL();
} while(!token.equals(EOP));
progIdx = 0; // reset index to start of program }
// Find the start of the next line. private void findEOL()
{
while(progIdx < prog.length &&
prog[progIdx] != '\n') ++progIdx; if(progIdx < prog.length) progIdx++; }
// Assign a variable a value.
private void assignment() throws InterpreterException {
int var; double value; char vname;
// Get the variable name. getToken(); vname = token.charAt(0); if(!Character.isLetter(vname)) { handleErr(NOTVAR); return; }
// Convert to index into variable table.
var = (int) Character.toUpperCase(vname) - 'A'; // Get the equal sign.
getToken();
if(!token.equals("=")) { handleErr(EQUALEXPECTED); return;
}
// Get the value to assign. value = evaluate();
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// Assign the value. vars[var] = value; }
// Execute a simple version of the PRINT statement. private void print() throws InterpreterException {
double result; int len=0, spaces; String lastDelim = ""; do {
getToken(); // get next list item
if(kwToken==EOL || token.equals(EOP)) break; if(tokType==QUOTEDSTR) { // is string System.out.print(token); len += token.length(); getToken(); } else { // is expression putBack(); result = evaluate(); getToken(); System.out.print(result);
// Add length of output to running total. Double t = new Double(result);
len += t.toString().length(); // save length }
lastDelim = token;
// If comma, move to next tab stop. if(lastDelim.equals(",")) {
// compute number of spaces to move to next tab spaces = 8 - (len % 8);
len += spaces; // add in the tabbing position while(spaces != 0) { System.out.print(" "); spaces--; } } else if(token.equals(";")) { System.out.print(" "); len++;
}
else if(kwToken != EOL && !token.equals(EOP)) handleErr(SYNTAX);
} while (lastDelim.equals(";") || lastDelim.equals(",")); if(kwToken==EOL || token.equals(EOP)) {
if(!lastDelim.equals(";") && !lastDelim.equals(",")) System.out.println();
}
else handleErr(SYNTAX); }
// Execute a GOTO statement.
private void execGoto() throws InterpreterException {
Integer loc;
getToken(); // get label to go to // Find the location of the label. loc = (Integer) labelTable.get(token); if(loc == null)
handleErr(UNDEFLABEL); // label not defined else // start program running at that loc
progIdx = loc.intValue(); }
// Execute an IF statement.
private void execIf() throws InterpreterException {
double result;
result = evaluate(); // get value of expression /* If the result is true (non-zero),
process target of IF. Otherwise move on to next line in the program. */
if(result != 0.0) { getToken();
if(kwToken != THEN) { handleErr(THENEXPECTED); return;
} // else, target statement will be executed }
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else findEOL(); // find start of next line }
// Execute a FOR loop.
private void execFor() throws InterpreterException {
ForInfo stckvar = new ForInfo(); double value;
char vname;
getToken(); // read the control variable vname = token.charAt(0);
if(!Character.isLetter(vname)) { handleErr(NOTVAR);
return; }
// Save index of control var.
stckvar.var = Character.toUpperCase(vname) - 'A'; getToken(); // read the equal sign
if(token.charAt(0) != '=') { handleErr(EQUALEXPECTED); return;
}
value = evaluate(); // get initial value vars[stckvar.var] = value;
getToken(); // read and discard the TO if(kwToken != TO) handleErr(TOEXPECTED);
stckvar.target = evaluate(); // get target value /* If loop can execute at least once,
push info on stack. */
if(value >= vars[stckvar.var]) { stckvar.loc = progIdx;
fStack.push(stckvar); }
else // otherwise, skip loop code altogether while(kwToken != NEXT) getToken();
// Execute a NEXT statement.
private void next() throws InterpreterException {
ForInfo stckvar; try {
// Retrieve info for this For loop. stckvar = (ForInfo) fStack.pop();
vars[stckvar.var]++; // increment control var // If done, return.
if(vars[stckvar.var] > stckvar.target) return; // Otherwise, restore the info.
fStack.push(stckvar);
progIdx = stckvar.loc; // loop } catch(EmptyStackException exc) {
handleErr(NEXTWITHOUTFOR); }
}
// Execute a simple form of INPUT.
private void input() throws InterpreterException { int var; double val = 0.0; String str; BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); getToken(); // see if prompt string is present if(tokType == QUOTEDSTR) {
// if so, print it and check for comma System.out.print(token);
getToken();
if(!token.equals(",")) handleErr(SYNTAX); getToken();
}
else System.out.print("? "); // otherwise, prompt with ? // get the input var
var = Character.toUpperCase(token.charAt(0)) - 'A'; try {
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str = br.readLine();
val = Double.parseDouble(str); // read the value } catch (IOException exc) {
handleErr(INPUTIOERROR);
} catch (NumberFormatException exc) { /* You might want to handle this error
differently than the other interpreter errors. */
System.out.println("Invalid input."); }
vars[var] = val; // store it }
// Execute a GOSUB.
private void gosub() throws InterpreterException {
Integer loc; getToken();
// Find the label to call.
loc = (Integer) labelTable.get(token); if(loc == null)
handleErr(UNDEFLABEL); // label not defined else {
// Save place to return to.
gStack.push(new Integer(progIdx)); // Start program running at that loc. progIdx = loc.intValue();
} }
// Return from GOSUB.
private void greturn() throws InterpreterException {
Integer t; try {
// Restore program index. t = (Integer) gStack.pop(); progIdx = t.intValue();
handleErr(RETURNWITHOUTGOSUB); }
}
// **************** Expression Parser **************** // Parser entry point.
private double evaluate() throws InterpreterException {
double result = 0.0; getToken();
if(token.equals(EOP))
handleErr(NOEXP); // no expression present // Parse and evaluate the expression.
result = evalExp1(); putBack();
return result; }
// Process relational operators.
private double evalExp1() throws InterpreterException {
double l_temp, r_temp, result; char op;
result = evalExp2();
// If at end of program, return. if(token.equals(EOP)) return result; op = token.charAt(0);
if(isRelop(op)) { l_temp = result; getToken();
r_temp = evalExp1();
switch(op) { // perform the relational operation case '<':
if(l_temp < r_temp) result = 1.0; else result = 0.0;
break;
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case LE:
if(l_temp <= r_temp) result = 1.0; else result = 0.0;
break; case '>':
if(l_temp > r_temp) result = 1.0; else result = 0.0;
break; case GE:
if(l_temp >= r_temp) result = 1.0; else result = 0.0;
break; case '=':
if(l_temp == r_temp) result = 1.0; else result = 0.0;
break; case NE:
if(l_temp != r_temp) result = 1.0; else result = 0.0; break; } } return result; }
// Add or subtract two terms.
private double evalExp2() throws InterpreterException { char op; double result; double partialResult; result = evalExp3(); while((op = token.charAt(0)) == '+' || op == '-') { getToken(); partialResult = evalExp3(); switch(op) { case '-':
result = result - partialResult; break;
case '+':
result = result + partialResult; break;
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}
return result; }
// Multiply or divide two factors.
private double evalExp3() throws InterpreterException { char op; double result; double partialResult; result = evalExp4(); while((op = token.charAt(0)) == '*' || op == '/' || op == '%') { getToken(); partialResult = evalExp4(); switch(op) { case '*':
result = result * partialResult; break;
case '/':
if(partialResult == 0.0) handleErr(DIVBYZERO);
result = result / partialResult; break;
case '%':
if(partialResult == 0.0) handleErr(DIVBYZERO);
result = result % partialResult; break; } } return result; } // Process an exponent.
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if(token.equals("^")) { getToken(); partialResult = evalExp4(); ex = result; if(partialResult == 0.0) { result = 1.0; } else for(t=(int)partialResult-1; t > 0; t--) result = result * ex;
}
return result; }
// Evaluate a unary + or -.
private double evalExp5() throws InterpreterException {
double result; String op; op = "";
if((tokType == DELIMITER) &&
token.equals("+") || token.equals("-")) { op = token;
getToken(); }
result = evalExp6();
if(op.equals("-")) result = -result; return result;
}
// Process a parenthesized expression.
private double evalExp6() throws InterpreterException { double result; if(token.equals("(")) { getToken(); result = evalExp2(); if(!token.equals(")")) handleErr(UNBALPARENS); getToken(); }
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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 &&
// 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; } C h a p t e r 3 : I m p l e m e n t i n g L a n g u a g e I n t e r p r e t e r s i n J a v a
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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;
}
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.