Interpreter for Mini-Basic in Java Get barcode 128 in Java Interpreter for Mini-Basic

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Example 8.3 Interpreter for Mini-Basic use tomcat barcode code 128 drawer togenerate code-128b on java Microsoft Official Website Consider a si code 128b for Java mple programming language, Mini-Basic, with the following syntax (expressed in EBNF):. Program Command . ..


. Command* Vari able = Expression read Variable write Variable go Label if Expression Relational-Op Expression go Label. (8.5) (8.6a) code 128 barcode for Java (8.

6b) (8.6~) (8.6d) (8.

6e) (8.60 (8.7a) (8.

7b). ( ( (. Expression stop primary-Expression Expression Arithmetic-Op primary-Expression . ..

-. Interpretation 3 15 primary-Expression Numeral Varia jvm barcode code 128 ble ( Expression ). Arithmetic-Op Relational-Op Variable Label + [ - [ * I /. (8.9a-d). 1 ...

1. (8.1 1a-z) (8.12).

. ..

-. Digit Digit*. A Mini-Basic Code 128 Code Set B for Java program is just a sequence of commands. The commands are implicitly labeled 0, 1, 2, etc., and these labels may be referenced in g o and i f commands.

The program may use up to twenty-six variables, which are predeclared.. The semantics of Mini-Basic programs should be intuitively clear. All values are real numbers. The program shown in Figure 8.

3 reads a number (into variable a ) , computes its square root accurate to two decimal places (in variable b), and writes the square root. It is easy to imagine a Mini-Basic abstract machine. The Mini-Basic program is loaded into a code store, with successive commands at addresses 0, 1, 2, etc.

The code pointer, CP, contains the address of the command due to be executed next. The program"s data are held in a data store of 26 cells, one cell for each variable. Figure 8.

3 illustrates the code store and data store. Figure 8.4 shows how the abstract machine"s state would change during the first few execution steps of the square-root program, assuming that the number read is 10.

0. We must decide how to represent Mini-Basic commands in the code store. The choices, and their consequences, are as follows: (a).

Source text: swing code 128b Each command must be scanned and parsed at run-time (i.e., every time the command is fetched from the code store).

. (b) Token seq uence: Each command must be scanned at load-time, and parsed at runtime. (c) AST: All commands must be scanned and parsed at load-time. Choice (a), illustrated in Figure 8.

3, would slow the interpreter drastically. Choice (c) is better but would slow the loader somewhat. Choice (b) is a reasonable compromise, so let us adopt it here:.

class T o k e barcode code 128 for Java n { byte k i n d ; String spelling; 1 class ScannedCornrnand Token [ 1 t o k e n s ; 1. Interpretation 3 17 public abstra barcode code 128 for Java ct class Command { / / A Command object is an AST representing a Mini-Basic command. public void execute (MiniBasicState state); / / Execute the command, using state..

And similarly for expression ASTs:. public abstra USS Code 128 for Java ct class Expression { / / An Expression object is an AST representing a Mini-Basic / / expression. public float evaluate (MiniBasicState state); / / Evaluate the expression, using state, and return its result..

Later we shal l define concrete subclasses for particular forms of commands and expressions. These will implement the methods execute and evaluate,which we shall call interpreting methods. Note that we must allow the interpreting methods to access the state of the MiniBasic abstract machine, hence their argument state.

The following class will represent the abstract machine state:. public class code-128c for Java ~iniBasicState { public static final short CODESIZE public static final short DATASIZE . / / Code stor e . ..

public ScannedCommand[] code = ScannedCommand[CODESIZE]; / / Data store . . .

public float[] data / / Registers . ..

public short CP; public byte status;. = = =. 4096; 26;. new float[DATASIZEl;. public static final byte / / status values RUNNING = 0, HALTED = 1, FAILED Here the code ANSI/AIM Code 128 for Java store is represented by an array of scanned commands, code.The data store is represented by an array of real numbers, data, indexed by variable addresses (using 0 for a, 1 for b, ..

., 25 for z). The registers are represented by variables Cp and status.

. 318 Programming Language Processors in Java The following tomcat code 128 barcode class will define the Mini-Basic interpreter:. public class MiniBasicInterpreter extends Mini~asicState { gublic void load ( 1 { . . .

/ / Load the program into the code store, starting at address 0. 1 gublic void run ( ) { ..

. / / Run the program in the code store, starting at address 0. 1 public static Command parse (ScannedCommand scannedcom) { .

.. / / Parse scannedcom,and return the corresponding / / command AST.

1. Note that we Code-128 for Java need a method, here called parse, to parse a scanned command an translate it to an AST. The following method is the interpreter proper. It just fetches, analyzes, and execute the commands, one after another:.

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