basic.c

A comprehensive (and small) BASIC language interpreter implementation in C that supports classic BASIC programming features and syntax.

Features

Core Language Support

• Variables: Numeric and string variables with automatic type detection
• Data Types: Numbers (double precision) and strings
• Operators: Arithmetic (+, -, *, /, ^, MOD), comparison (=, <>, <, <=, >, >=), and logical (AND, OR, NOT)
• String Operations: Concatenation and comparison

Control Flow

• Conditional Statements: IF-THEN with support for both statement execution and line jumps
• Loops: FOR-NEXT loops with optional STEP values (including negative steps)
• Jumps: GOTO for unconditional jumps to line numbers
• Subroutines: GOSUB and RETURN for subroutine calls

Built-in Functions

• Mathematical: ABS(), SIN(), COS(), TAN(), SQR(), INT(), RND()
• String Functions: LEN(), VAL(), STR$(), CHR$(), ASC()

I/O Operations

• Output: PRINT with support for expressions, separators (, for tabs, ; for no separation)
• Input: INPUT with optional prompts for user data entry
• Comments: REM for program documentation

Program Management

• Line Numbers: Traditional BASIC line numbering system
• Program Loading: Load and execute BASIC programs from files
• Interactive Mode: Direct command execution and program development

Usage

Command Line

# Run a BASIC program from file
basic.exe program.bas

# Start interactive mode
basic.exe

Interactive Mode Commands

• RUN - Execute the loaded program
• LIST - Display program lines
• VARS - Show variables in memory
• NEW - Clear program and variables
• HELP - Show available commands
• QUIT or EXIT - Exit the interpreter

BASIC Syntax Examples

Variables and Arithmetic

10 LET A = 10
20 LET B = 5
30 PRINT "A + B = "; A + B
40 PRINT "A ^ B = "; A ^ B

String Operations

10 LET NAME$ = "World"
20 PRINT "Hello " + NAME$ + "!\n"
30 PRINT "Length: "; LEN(NAME$)

Control Flow

10 FOR I = 1 TO 10 STEP 2
20   PRINT I; " ";
30 NEXT I
40 PRINT "\n"

50 IF A > B THEN PRINT "A is greater\n"
60 IF A < B THEN GOTO 100

Subroutines

10 GOSUB 1000
20 PRINT "Back from subroutine\n"
30 END

1000 PRINT "Inside subroutine\n"
1010 RETURN