QBasic Project

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QBasic Project Help Document

Welcome to the QBasic project, a minimal BASIC language interpreter. This project allows you to learn about expression trees and class inheritance, and gain a deeper understanding of how programming languages work. You can adapt existing programs to solve different but related tasks, experiencing the process of modifying existing systems.

Features Overview:

• Supports basic BASIC statements such as REM, LET, PRINT, INPUT, GOTO, IF, END.
• Supports expression parsing and syntax tree display.
• Provides a debug mode that allows setting breakpoints and viewing variable values.
• Supports loading, editing, and running programs.

Usage:

• Load and edit BASIC programs through the input box or LOAD button.
• Execute the program using the RUN button.
• In debug mode, manage breakpoints using ADD and DELETE commands.

Notes:

1. Supported commands in the command window:

• Built-in commands: QUIT to exit, HELP to display the help document
• ADD to add breakpoints, DELETE to remove breakpoints
• Commands corresponding to buttons, such as LOAD corresponding to the LOAD command, but it is not recommended to use
• Edit commands: Commands starting with a line number, such as 10 PRINT "HELLO", will insert the command at line number 10. If there is already a command on that line, it will be replaced

2. Function of the input window:

• During program execution, there may be situations where INPUT waits for input. The input window will display a "waiting for input" icon. At this time, you can enter content in the input window and press Enter to continue the program

3. Symbols and syntax tree:

• Displayed in postfix notation, for example, 1 + 2 * 3 will be parsed as 1 2 3 * + and form a tree structure with different lengths of whitespace prefixes
• (This is actually ugly, but the lab requires it. You can swap the two in interpreter.cpp to display in BNF format

  // astOutput(stmts[status.next_line]->toString());
  auto strs = stmts[status.next_line]->toTabbedString();
  for(const auto& s: strs) {
  astOutput(s);
  }

4. Save and load:

• Code edited through commands will not be automatically saved
• Use ctrl+s to save the current code, which will bring up the save file dialog
• The program cannot be run directly before loading, please save the code first

5. Other windows: No special instructions
6. Example programs: Please refer to the example programs in the programs folder