MinimalBasicInterpreter

A Minimal Basic Interpreter implemented in C++ and Qt.

Features

• Code display
• Code loading
• Code editing
• Abstract Syntax Tree(AST) display
• Program input & output
• Console commands
• Step execute debugging
• Variable state display
• Syntax error highlight

Supported Statements

REM

REM This is a comment.

This statement is used for comments. Any text on the line after the keyword REM is ignored.

LET

LET var = 1

This statement is BASIC’s assignment statement. The LET keyword is followed by a variable name, an equal sign, and an expression. As in C++, the effect of this statement is to assign the value of the expression to the variable, replacing any previous value. In BASIC, assignment is not an operator and may not be nested inside other expressions.

The value can only be a string or an integer, string value should not contain " or '.

PRINT

In minimal BASIC, the PRINT statement has the form: PRINT exp where exp is an expression. The effect of this statement is to print the value of the expression on the console and then print a newline character so that the output from the next PRINT statement begins on a new line.

PRINTF

PRINT "Hello, {}", "WORLD"

PRINTF is used for formatted output. It accepts 1 to N arguments, the first argument should be a format string which contains 0 or N-1 "{}"s as placeholder(s), corresponding to the following N-1 arguments. The format string must not contain a single "{" or "}".

INPUT

In the minimal version of the BASIC interpreter, the INPUT statement has the form: INPUT var where var is a variable read in from the user. The effect of this statement is to print a prompt consisting of the string "?" and then to read in a value to be stored in the variable. (The string "?" will be displayed in the command input editbox in GUI.)

Only integer input is supported.

INPUTS

INPUTS var

Similar to INPUT, but accepts a string as input.

GOTO

This statement has the syntax GOTO n and forces an unconditional change in the control flow of the program. When the program hits this statement, the program continues from line n instead of continuing with the next statement. An error will be thrown if line n does not exist.

IF

This statement provides conditional control. The syntax for this statement is: IF exp1 op exp2 THEN n where exp1 and exp2 are expressions and op is one of the conditional operators =, <, or >. If the condition holds, the program should continue from line n just as in the GOTO statement. If not, the program continues on to the next line.

Note that the conditional operators (=, <, >) are not parts of expressions.

END

Marks the end of the program. Execution halts when this line is reached. This statement is usually optional in BASIC programs because execution also stops if the program continues past the last numbered line.

Supported Commands

RUN

This command starts program execution beginning at the lowest-numbered line. Unless the flow is changed by GOTO and IF commands, statements are executed in line-number order. Execution ends when the program hits the END statement or continues past the last statement in the program.

LOAD

This command loads a file containing statements and commands. Statements and commands will be stored (also displayed in GUI) and executed respectively, as if they were entered into input box in order. A prompt window will be displayed when this command is entered to ask the user to choose a file to load.

DEBUG

Enter debug mode.

STEP

Step execute, only available in debug mode.

CLEAR

This command deletes the program so the user can start entering a new one.

HELP

This command displays a simple help message.

QUIT

Typing QUIT exits from the BASIC interpreter.

Code editing

1. Add a new line by typing a command starting with a line number following a statement, e.g. 35 INPUT n3

2. This above statement is inserted into the program between line 30 and line 40. To update line 40, type 40 LET total = n1 + n2 + n3

3. In classical implementations of BASIC, the standard mechanism for deleting lines was to type in a line number with nothing after it on the line. e.g. 40 deletes line 40

The LET, PRINT, and INPUT statements can be executed directly by typing them without a line number, in which case they are evaluated immediately

Supported arithmetics

• Addition: +
• Subtraction: -
• Multiplication: *
• Division: /
• Exponentiation: **