heycat-svg / basic

SEP proj2

BASIC

Brief introduction of this project

This project is a mini version of BASIC made by Microsoft. It is an interpreter which provides us with serveral command and statements to make another program with the BASIC language.

Compared with many High-level languages like C++ and Java, BASIC is a relatively original language. But in this project, mini BASIC is even more simple than BASIC made by Microsoft.

Description of class

• Widget： Class Widget is declared in widget.h. It is used to hold the consolo widget.

• Consolo: Class Console is declared in console.h. Class Console is used to hold the input from user. It parses the command and dispatch the command to different member functions of class Console.

• Buffer: Class Buffer is declared in buffer.h. Class Buffer is used to store what user has input to the interpreter.

• Program Class Program is declared in program.h. Class Program is used to run the code input by user.

• Tokenizer Class Tokenizer is declared in tokenizer.h. Class Tokenizer is used to tokenize each line of code and divide them into different parts which is good for the subsequent parsing.

• EvalState: Class EvalState is declared in evalstate.h which is used to store the variables and their value.

• Parser: Class Parser is declared in parser.h which is responsible to parse the expressions and calculate them. Checking the cerrectness of code is one of its functions.

• Statement: Class Statement is declared in statement.h. Class Statement has ten subclasses, which corresponse to ten different kinds of statements.

• Expression: Class Expression is declared in expression.h. Class Expression has three sub classed, which corresponse to class ConstantExp, class IdentifierExp and class CompondExp.

How the program works

I will introduce the structure from top to bottom.

First, the whole widget is a big input box where user can type anything. The input box corresponses to class Console. Consolo is set in a blank widget which is generated by class Widget. When user finishes inputing command and press 'Enter', the command will be sent as a signal form function Console::keypressEvent() to function Console::dispatchCmd(). Then Console::dispatchCmd() will parse the command and dispatch it to corresponding member function in class Console. If what user inputs is code, Console will pass the code to Buffer which can store lines of code using linked list and sort them by line number from small to large. When user finishes inputting code and then inputs the command of RUN, Console will instantiate the class Program which starts to parser the code stored in Buffer and run these lines of code according different kinds of statements.

Next, before run lines of code, some preparation should be made. Class Tokenizer is stored in a array named Tokenizer ** Program::code as pointer. Each Tokenizer corresponds to a line of code. Tokenizer divides code into line number, statement and remanent string named QString Tokenizer::others. QString Tokenizer::others will be further processed by function Tokenizer::furtherToken() to remove needless space. Besides, each Tokenizer contains a pointer of class Expression and a pointer of class Statement. In class Program, there is a member variable named int Program::pointer which is similar to %rip in assemble code and tell Program which code should be excute next. Then Program can run code form line to line according to the value of int Program::pointer.

Then I will introduce how the class Statemnet and class Parser works.

Statement

• RemStatement: RemStatement does nothing so the program will jump over it.

• LetStatement: LetStatement calculate the expression in QString Tokenizer::others and assign the value of result to identifier. The specific process of caluculation is provided in Parser.

• InputStatement: InputStatement receive numbers from user and assign the value to identifier. It changes the value of a flag named bool Program::hang which tell Program whether program is in input mode. In this mode, Program stops execute code and wait for user to enter a number.

• PrintStatement: PrintStatement print the result of expression in QString Tokenizer::others. The specific process of caluculation is provided in Parser.

• GotoStatement: GotoStatement change int Program::pointer, making program jump to specific line of code.

• IfStatement: IfStatement first analysises QString Tokenizer::others and divides it into two parts, logic condition and target address. Calculation of logic condition is left to Parser. Jumping part is similar to GotoStatement.

• EndStatement: EndStatement change the value of a flag named bool Programm::end which tell Program to stop executing the code.

Parser

Class Parser is responsible to build expression tree, calculate the expression and check the gramma. Expression in QString Tokenizer::others is build in binary tree based on class Expression. For calculation, Parser uses recursive method to traverse the tree.

Advance function

Besides the basic function of mini BASIC, the ability to call function is added to the project. To do this, three new statement is added to class Statement.

• SubStatement: SubStatement add function name and corresponding line number to map<QString,int> Program::funList. Then change int Program::pointer behind next End.

• EndSubStatment: EndSubStatment pop line number from stack<int> Program::return_addr and jump to the line number.

• CallStatement: CallStatement search function name in map<QString,int> Program::funList, push the next line number to stack<int> Program::return_addr and jump to line number where the function is.

Something need to improve

1. Process of exception should be more specific. In the current stage, try and catch can hold most exception, pointing out where the erro happend. But it can't tell the specified place where the erro happened.

2. LET and INPUT can't be execute directely without typing line number. Because I don't konw what namespace in which these two commands will affect variables. If all the program share the same namespace, will execution of program be affected by that executed previously? I don't think so. Besides, it's not hard to realize the function, given that I have already realized the PPRINT command without typing line number.