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A BASIC Interpreter - Program like it's 1979!

Introduction

A simple interactive BASIC interpreter written in Python 3. It is based heavily on material in the excellent book Writing Interpreters and Compilers for the Raspberry Pi Using Python by Anthony J. Dos Reis. However, I have had to adapt the Python interpreter presented in the book, both to work with the BASIC programming language and to produce an interactive command line interface. The interpreter therefore adopts the key techniques for interpreter and compiler writing, the use of a lexical analysis stage followed by a recursive descent parser which implements the context free grammar representing the target programming language.

The interpreter is a homage to the home computers of the early 1980s, and when executed, presents an interactive prompt ('>') typical of such a home computer. Commands to run, list, save and load BASIC programs can be entered at the prompt as well as program statements themselves.

The BASIC dialect that has been implemented is slightly simplified, and naturally avoids machine specific instructions, such as those concerned with sound and graphics for example.

There is reasonably comprehensive error checking. Syntax errors will be picked up and reported on by the lexical analyser as statements are entered. Runtime errors will highlight the cause and the line number of the offending statement.

The interpreter can be invoked as follows:

$ python interpreter.py

Although this started of as a personal project, it has been enhanced considerably by some other Github users. You can see them in the list of contributors! It's very much a group endeavour now.

Operators

A limited range of arithmetic expressions are provided. Addition and subtraction have the lowest precedence, but this can be changed with parentheses.

> 10 PRINT 2 * 3
> 20 PRINT 20 / 10
> 30 PRINT 10 + 10
> 40 PRINT 10 - 10
> 50 PRINT 15 MOD 10
> RUN
6
2.0
20
0
5
>

Additional numerical operations may be performed using numeric functions (see below).

Not also that '+' does extra duty as a string concatenation operator, while '*' can be used to repeat strings.

Commands

Programs may be listed using the LIST command:

> LIST
10 LET I = 10
20 PRINT I
>

The list command can take arguments to refine the line selection listed

LIST 50 Lists only line 50.

LIST 50-100 Lists lines 50 through 100 inclusive.

LIST 50 100 Also Lists lines 50 through 100 inclusive, almost any delimiter works here.

LIST -100 Lists from the start of the program through line 100 inclusive.

LIST 50- Lists from line 50 to the end of the program.

A program is executed using the RUN command:

> RUN
10
>

A program may be saved to disk using the SAVE command. Note that the full path must be specified within double quotes:

> SAVE "C:\path\to\my\file"
Program written to file
>

The program may be re-loaded from disk using the LOAD command, again specifying the full path using double quotes:

> LOAD "C:\path\to\my\file"
Program read from file
>

When loading or saving, the .bas extension is assumed if not provided. If you are loading a simple name (alpha/numbers only) and in the working dir, quotes can be omitted:

> LOAD regression

will load regression.bas from the current working directory.

Individual program statements may be deleted by entering their line number only:

> 10 PRINT "Hello"
> 20 PRINT "Goodbye"
> LIST
10 PRINT "Hello"
20 PRINT "Goodbye"
> 10
> LIST
20 PRINT "Goodbye"
>

The program may be erased entirely from memory using the NEW command:

> 10 LET I = 10
> LIST
10 LET I = 10
> NEW
> LIST
>

Finally, it is possible to terminate the interpreter by issuing the EXIT command:

> EXIT
c:\

On occasion, it might be necessary to force termination of a program and return to the interpreter, for example, because it is caught in an infinite loop. This can be achieved by using Ctrl-C to force the program to stop:

> 10 PRINT "Hello"
> 20 GOTO 10
> RUN
"Hello"
"Hello"
"Hello"
...
...
<Ctrl-C>
Program terminated
> LIST
10 PRINT "Hello"
20 GOTO 10
>

Programming language constructs

Statement structure

As per usual in old school BASIC, all program statements must be prefixed with a line number which indicates the order in which the statements may be executed. There is no renumber command to allow all line numbers to be modified. A statement may be modified or replaced by re-entering a statement with the same line number:

> 10 LET I = 10
> LIST
10 LET I = 10
> 10 LET I = 200
> LIST
10 LET I = 200
>

Multiple statements may appear on one line separated by a colon:

> 10 LET X = 10: PRINT X

NOTE: Currently inline loops are NOT supported

10 FOR I = 1 to 10: PRINT I: NEXT

will need to be decomposed to individual lines.

Variables

Variable types follow the typical BASIC convention. Simple variables may contain either strings or numbers (the latter may be integers or floating point numbers). Likewise array variables may contain arrays of either strings or numbers, but they cannot be mixed in the same array.

Note that all keywords and variable names are case insensitive (and will be converted to upper case internally by the lexical analyser). String literals will retain their case however. There is no inherent limit on the length of variable names or string literals, this will be dictated by the limitations of Python. The range of numeric values is also dependent upon the underlying Python implementation.

Note that variable names may only consist of alphanumeric characters and underscores. However, they must all begin with an alphabetic character. For example:

are all valid variable names, whereas:

are all invalid.

Numeric variables have no suffix, whereas string variables are always suffixed by '$'. Note that 'I' and 'I$' are considered to be separate variables. Note that string literals must always be enclosed within double quotes (not single quotes). Using no quotes will result in a syntax error. In addition, for floating point numbers less than one (e.g. 0.67), the decimal point must always be prefixed by a zero (e.g. .67 will be flagged as a syntax error).

Array variables are defined using the DIM statement, which explicitly lists how many dimensions the array has, and the sizes of those dimensions:

> REM DEFINE A THREE DIMENSIONAL NUMERIC ARRAY
> 10 DIM A(3, 3, 3)

Note that the index of each dimension always starts at zero, but for compatibility with some basic dialects the bounds of each dimension will be expanded by one to enable element access including the len. So in the above example, valid index values for array A will be 0, 1, 2 or 3 for each dimension. Arrays may have a maximum of three dimensions. Numeric arrays will be initialised with each element set to zero, while string arrays will be initialised with each element set to the empty string "".

As for simple variables, a string array has its name suffixed by a '$' character, while a numeric array does not carry a suffix. An attempt to assign a string value to a numeric array or vice versa will generate an error.

Array variables with the same name but different dimensionality are treated as the same. For example, using a DIM statement to define I(5) and then a second DIM statement to define I(5, 5) will result in the second definition (the two dimensional array) overwriting the first definition (the one dimensional array).

Array values may be used within any expression, such as in a PRINT statement for string values, or in any numerical expression for number values. However, you must be specific about which array element you are referencing, using the correct number of in-range indexes. If that particular array value has not yet been assigned, then an error message will be printed.

> 10 DIM MYARRAY(2, 2, 2)
> 20 LET MYARRAY(0, 1, 0) = 56
> 30 PRINT MYARRAY(0, 1, 0)
> RUN
56
> 30 PRINT MYARRAY(0, 0, 0)
> RUN
Empty array value returned in line 30
>

As in all implementations of BASIC, there is no garbage collection (not surprising since all variables have global scope)!

Program constants

Constants may be defined through the use of the DATA statement. They may consist of numeric or string values and are declared in a comma separated list:

> 10 DATA 56, "Hello", 78

These values can then later be assigned to variables using the READ statement. Note that the type of the value (string or numeric) must match the type of the variable, otherwise an error message will be triggered. Therefore, attention should be paid to the relative ordering of constants and variables. Once the constants on a DATA statement are used by a READ statement, the next READ statement will move to the DATA statement with the next higher line number, if there are no more DATA statements before the end of the program an error will be displayed. This is to ensure that the program is not left in a state where a variable has not been assigned a value, but nevertheless an attempt to use that variable is made later on in the program.

Normally each DATA statement is consumed sequently by READ statements however, the RESTORE statement can be used to override this order and set the line number of the DATA statement that will be used by the next READ statement. If the line-number used in a RESTORE statement does not refer to a DATA statement an error will be displayed.

The constants defined in the DATA statement may be consumed using several READ statements or several DATA statements may be consumed by a single READ statement.:

> 10 DATA 56, "Hello", 78
> 20 READ FIRSTNUM, S$
> 30 PRINT FIRSTNUM, " ", S$
> 40 READ SECONDNUM
> 50 PRINT SECONDNUM
> 60 DATA "Another "
> 70 DATA "Line "
> 80 DATA "of "
> 90 DATA "Data"
> 100 RESTORE 10
> 110 READ FIRSTNUM, S$, SECONDNUM, A$, B$, C$, D$
> 120 PRINT S$," ",A$,B$,C$,D$
> RUN
56 Hello
78
Hello Another Line of Data
>

It is a limitation of this BASIC dialect that it is not possible to assign constants directly to array variables within a READ statement, only simple variables.

Comments

The REM statement is used to indicate a comment, and occupies an entire statement. It has no effect on execution:

> 10 REM THIS IS A COMMENT

Stopping a program

The STOP statement may be used to cease program execution. The command END has the same effect.

> 10 PRINT "one"
> 20 STOP
> 30 PRINT "two"
> RUN
one
>

A program will automatically cease execution when it reaches the final statement, so a STOP may not be necessary. However a STOP will be required if subroutines have been defined at the end of the program, otherwise execution will continue through to those subroutines without a corresponding subroutine call. This will cause an error when the RETURN statement is processed and the interpreter attempts to return control back to the caller.

Assignment

Assignment may be made to numeric simple variables (which can contain either integers or floating point numbers) and string simple variables (string variables are distinguished by their dollar suffix). The interpreter will enforce this division between the two types:

> 10 LET I = 10
> 20 LET I$ = "Hello"

The LET keyword is also optional:

> 10 I = 10

Array variables may also have values assigned to them. The indexes can be derived from numeric expressions:

> 10 DIM NUMS(3, 3)
> 20 DIM STRS$(3, 3)
> 30 LET INDEX = 0
> 40 LET NUMS(INDEX, INDEX) = 55
> 50 LET STRS$(INDEX, INDEX) = "hello"

Attempts to assign the wrong type (number or string) to a numeric or string array, attempts to assign a value to an array by specifying the wrong number of dimensions, and attempts to assign to an array using an out of range index, will all result in an error.

Printing to standard output

The PRINT statement is used to print to the screen (or to a file, see File I/O below):

> 10 PRINT 2 * 4
> RUN
8
> 10 PRINT "Hello"
> RUN
Hello
>

Multiple items may be printed by providing a semicolon separated list. The items in the list will be printed immediately after one another, so spaces must be inserted if these are required:

> 10 PRINT 345; " hello "; 678
> RUN
345 hello 678
>

A blank line may be printed by using the PRINT statement without arguments:

> 10 PRINT "Here is a blank line:"
> 20 PRINT
> 30 PRINT "There it was"
> RUN
Here is a blank line:

There it was
>

A print statement terminated by a semicolon will not include a CR/LF.

Unconditional branching

Like it or loath it, the GOTO statement is an integral part of BASIC, and is used to transfer control to the statement with the specified line number:

> 10 PRINT "Hello"
> 20 GOTO 10
> RUN
Hello
Hello
Hello
...

Subroutine calls

The GOSUB statement is used to generate a subroutine call. Control is passed back to the program at the next statement after the call by a RETURN statement at the end of the subroutine:

> 10 GOSUB 100
> 20 PRINT "This happens after the subroutine"
> 30 STOP
> 100 REM HERE IS THE SUBROUTINE
> 110 PRINT "This happens in the subroutine"
> 120 RETURN
> RUN
This happens in the subroutine
This happens after the subroutine
>

Note that without use of the STOP statement, execution will run past the last statement of the main program (line 30) and will re-execute the subroutine again (at line 100).

Subroutines may be nested, that is, a subroutine call may be made within another subroutine.

A subroutine may also be called using the ON-GOSUB statement (see Conditional branching below).

Loops

Bounded loops are achieved through the use of FOR-NEXT statements. The loop is controlled by a numeric loop variable that is incremented or decremented from a start value to an end value. The loop terminates when the loop variable reaches the end value. The loop variable must also be specified in the NEXT statement at the end of the loop.

> 10 FOR I = 1 TO 3
> 20 PRINT "hello"
> 30 NEXT I
> RUN
hello
hello
hello
>

Loops may be nested within one another.

The STEP statement allows the loop variable to be incremented or decremented by a specified amount. For example, to count down from 5 in steps of -1:

> 10 FOR I = 5 TO 1 STEP -1
> 20 PRINT I
> 30 NEXT I
> RUN
5
4
3
2
1
>

Note that the start value, end value and step value need not be integers, but can be floating point numbers as well. If the loop variable was previously assigned in the program, its value will be replaced by the start value, it will not be evaluated.

After the completion of the loop, the loop variable value will be the end value + step value (unless the loop is exited using a GOTO statement).

Conditionals

Conditionals are implemented using the IF-THEN-ELSE statement. The expression is evaluated and the appropriate statements executed depending upon the result of the evaluation. If a positive integer is supplied as the THEN or the ELSE statement, a branch will be performed to the indicated line number.

Note that the ELSE clause is optional and may be omitted. In this case, the THEN branch is taken if the expression evaluates to true, otherwise the next statement is executed.

Conditional branching example:

> 10 REM PRINT THE GREATEST NUMBER
> 20 LET I = 10
> 30 LET J = 20
> 40 IF I > J THEN 50 ELSE 70
> 50 PRINT I
> 60 GOTO 80
> 70 PRINT J
> 80 REM FINISHED
> RUN
20
>

The following code segment is equivalent to the segment above:

> 10 REM PRINT THE GREATEST NUMBER
> 20 LET I = 10
> 30 LET J = 20
> 40 IF I > J THEN PRINT I ELSE PRINT J
> 80 REM FINISHED
> RUN
20
>

A THEN or ELSE can be supplied multiple statements if they are separated by a colon ":".

> 10 REM PRINT THE GREATEST NUMBER
> 20 LET I = 10
> 30 LET J = 20
> 40 IF I > J THEN LET L = I:PRINT I ELSE LET L = J:PRINT J
> 50 PRINT L
> 80 REM FINISHED
> RUN
20
20
>

Note that should an IF-THEN-ELSE stmt be used in a THEN code block or multiple IF-THEN-ELSE statements are used in either a single THEN or ELSE code block, the block grouping is ambiguous and logical processing may not function as expected. There is no ambiguity when single IF-THEN-ELSE statements are placed within ELSE blocks.

Ambiguous:

> 100 IF I > J THEN IF J >= 100 THEN PRINT "I > 100" else PRINT "Not clear which **IF** this belongs to"

Not Ambiguous:

> 100 IF I < J THEN PRINT "I is less than J" ELSE IF J > 100 THEN PRINT "I > 100" ELSE PRINT "J <= 100"

Allowable relational operators are:

The logical operators AND and OR are also provided to allow you to join two or more expressions. The NOT operator can also be given before an expression.

= and <> can also be considered logical operators. However, unlike AND or OR they can't be used to join more than two expressions.

InputsANDOR=<> / !=
FALSEFALSEFALSEFALSETRUEFALSE
TRUEFALSEFALSETRUEFALSETRUE
TRUETRUETRUETRUETRUEFALSE
InputNOT
TRUEFALSE
FALSETRUE

Example:

> 10 a = 10
> 20 b = 20
> 30 IF NOT a > b AND b = 20 OR a >= 5 THEN 60
> 40 PRINT "Test failed!"
> 50 STOP
> 60 PRINT "Test passed!"
> RUN
Test passed!

Expressions can be inside brackets to change the order of evaluation. Compare the output when line 30 is changed:

> 30 IF NOT a > b AND (b = 20 OR a >= 5) THEN 60
> RUN
Test failed!

ON GOTO, ON GOSUB

The ON GOTO|GOSUB expr line1,line2,... statement will call a subroutine or branch to a line number in the list of line numbers corresponding to the ordinal value of the evaluated expr. The first line number corresponds with an expr value of 1. expr must evaluate to an integer value. If expr evaluates to less than 1 or greater than the number of provided line numbers execution continues on the next statement without making a subroutine call or branch:

> 20 LET J = 2
> 30 ON J GOSUB 100,200,300
> 40 STOP
> 100 REM THE 1ST SUBROUTINE
> 110 PRINT "J is ONE"
> 120 RETURN
> 200 REM THE 2ND SUBROUTINE
> 210 PRINT "J is TWO"
> 220 RETURN
> 300 REM THE 3RD SUBROUTINE
> 310 PRINT "J is THREE"
> 320 RETURN
> RUN
J is TWO
>

It is also possible to call a subroutine depending upon the result of a conditional expression using the IFF function (see Ternary Functions below). In the example below, if the expression evaluates to true, IFF returns a 1 and the subroutine is called, otherwise IFF returns a 0 and execution continues to the next statement without making the call:

> 10 LET I = 10
> 20 LET J = 5
> 30 ON IFF (I > J, 1, 0) GOSUB 100
> 40 STOP
> 100 REM THE SUBROUTINE
> 110 PRINT "I is greater than J"
> 120 RETURN
> RUN
I is greater than J
>

Ternary Functions

As an alternative to branching, Ternary functions are provided.

> 10 LET I = 10
> 20 LET J = 5
> 30 PRINT IF$(I > J, "I is greater than J", "I is not greater than J")
> 40 K = IFF(I > J, 20, 30)
> 50 PRINT K
> RUN
I is greater than J
20

User input

The INPUT statement is used to solicit input from the user (or read input from a file, see File I/O below):

> 10 INPUT A
> 20 PRINT A
> RUN
? 22
22
>

The default input prompt of '? ' may be changed by inserting a prompt string, which must be terminated by a semicolon, thus:

> 10 INPUT "Input a number - "; A
> 20 PRINT A
> RUN
Input a number - 22
22
>

Multiple items may be input by supplying a comma separated list. Input variables will be assigned to as many input values as supplied at run time. If there are more input values supplied than input variables, excess commas will be left in place. Conversely, if not enough input values are supplied, an error message will be printed and the user will be asked to re-input the values again.

Further, numeric input values must be valid numbers (integers or floating point).

> 10 INPUT "Num, Str, Num: ": A, B$, C
> 20 PRINT A, B$, C
> RUN
Num, Str, Num: 22, hello!, 33
22 hello!33
>

A mismatch between the input value and input variable type will trigger an error, and the user will be asked to re-input the values again.

It is a limitation of this BASIC dialect that it is not possible to assign constants directly to array variables within an INPUT statement, only simple variables.

File Input/Output

Data can be read from or written to files using the OPEN, FSEEK, INPUT, PRINT and CLOSE statements.

When a file is opened using the syntax OPEN "filename" FOR INPUT|OUTPUT|APPEND AS #filenum [ELSE linenum] a file number (#filenum) is assigned to the file, which if specified as the first argument of an INPUT or PRINT statement, will direct the input or output to the file.

If there is an error opening a file and the optional ELSE option has been specified, program control will branch to the specified line number, if the ELSE has not been provided an error message will be displayed.

If a file is opened for OUTPUT which does not exist, the file will be created, if the file does exist, its contents will be erased and any new PRINT output will replace it. If a file is opened for APPEND an error will occur if the file doesn't exist (or the ELSE branch will occur if specified). If the file does exist, any PRINT statements will add to the end of the file.

If an input prompt is specified on an INPUT statement being used for file I/O (i.e. #filenum is specified) an error will be displayed.

The FSEEK #filenum,filepos statement will position the file pointer for the next INPUT statement.

The CLOSE #filenum statement will close the file.

> 10 OPEN "FILE.TXT" FOR OUTPUT AS #1
> 20 PRINT #1,"0123456789Hello World!"
> 30 CLOSE #1
> 40 OPEN "FILE.TXT" FOR INPUT AS #2
> 50 FSEEK #2,10
> 60 INPUT #2,A$
> 70 PRINT A$
> RUN
Hello World!
>

Numeric functions

Selected numeric functions are provided, and may be used with any numeric expression. For example, the square root function, SQR, can be applied expressions consisting of both literals and variables:

> 10 LET I = 6
> 20 PRINT SQR(I - 2)
> RUN
2.0
>

Allowable numeric functions are:

> 10 PRINT MAX(-2, 0, 1.5, 4)
> 20 PRINT MIN(-2, 0, 1.5, 4)
> RUN
> 4
> -2

> 10 RANDOMIZE 100
> 20 PRINT RND(1)
> RUN
0.1456692551041303
>

Random integers can be generated by combining RND and INT: e.g.

> 10 PRINT INT(RND(1) * 6) + 1
> RUN
3
> RUN
6
>

Seeds may not produce the same result on another platform.

String functions

Some functions are provided to help you manipulate strings. Functions that return a string have a '$' suffix like string variables.

NOTE For compatibility with older basic dialetcs, all string indexes are 1 based.

The functions are:

Examples for ASC, CHR$ and STR$

> 10 I = 65
> 20 J$ = CHR$(I) + " - " + STR$(I)
> 30 PRINT J$
> 40 PRINT ASC("Z")
RUN
A - 65
90

Strings may also be concatenated using the '+' operator:

> 10 PRINT "Hello" + " there"
> RUN
Hello there

Strings may be repeated using the '*' operator:

> 10 PRINT "Hello " * 5
> RUN
Hello Hello Hello Hello Hello

Example programs

A number of example BASIC programs have been supplied in the repository, in the examples directory:

Informal grammar definition

ABS(numerical-expression) - Calculates the absolute value of the result of numerical-expression

ASC(string-expression) - Returns the character code of the result of string-expression.

ATN(numerical-expression) - Calculates the arctangent value of the result of numerical-expression

CHR$(numerical-expression) - Returns the character specified by character code of the result of numerical-expression.

CLOSE #filenum - Closes an open file

COS(numerical-expression) - Calculates the cosine value of the result of numerical-expression

DATA(expression-list) - Defines a list of string or numerical values

DIM array-variable(dimensions) - Defines a new array variable

EXIT - Exits the interpreter

EXP(numerical-expression) - Calculates the exponential value of the result of numerical-expression

FOR loop-variable = start-value TO end-value [STEP increment] - Bounded loop

FSEEK #filenum,filepos - Positions the file input pointer to the specified location within the open file, the next INPUT #filenum will read starting at file position filepos

GOSUB line-number - Subroutine call

GOTO line-number - Unconditional branch

IF expression THEN line-number|basic-statement(s) [ELSE line-number|basic-statement(s)] - Conditional

IFF(expression, numeric-expression, numeric-expression) - Evaluates expression and returns the value of the result of the first numeric-expression if true, or the second if false.

IF$(expression, string-expression, string-expression) - Evaluates expression and returns the value of the result of the first string-expression if true, or the second if false.

INPUT [#filenum,|input-prompt;] simple-variable-list - Processes user or file input presented as a comma separated list

INSTR(hackstack-string-expression, needle-string-expression[, start-numeric-expression[, end-numeric-expression]]) - Returns position of first needle-string-expression inside first hackstack-string-expression, optionally start searching at position given by start-numeric-expression and optionally ending at position given by end-numeric-expression. Returns -1 if no match found.

LEFT$(string-expression, char-count) - Takes the result of string-expression and returns the left-most char-count characters. If char-count exceeds string length the entire string is returned.

LEN(string-expression) - Returns the length of the result of string-expression

[LET] variable = numeric-expression | string-expression - Assigns a value to a simple variable or array variable

LIST - Lists the program

LOAD filename - Loads a program from disk

LOWER$(string-expression) - Returns a lower-case version of the result of string-expression.

LOG(numerical-expression) - Calculates the natural logarithm value of the result of numerical-expression

NEW - Clears the program from memory

NEXT loop-variable - See FOR statement

MAX(expression-list) - Returns the highest value in expression-list

MID$(string-expression, start-position[, end-position]) - Takes the result of string-expression and returns part of it, starting at position start-position, and ending at end-position. end-position can be omitted to get the rest of the string. If start-position or end-position are negative, the position is counted backwards from the end of the string.

MIN(expression-list) - Returns the lowest value in expression-list

ON expression GOSUB|GOTO line-number1,line-number2,... - Conditional subroutine call|branch - Program flow will be transferred either through a GOSUB subroutine call or a GOTO branch to the line number in the list of line numbers corresponding to the ordinal value of the evaluated expr. The first line number corresponds with an expr value of 1. expr must evaluate to an integer value.

OPEN "filename" FOR INPUT|OUTPUT|APPEND AS #filenum [ELSE linenum] - Opens the specified file. Program control is transferred to linenum if an error occurs otherwise continues on the next line.

PI - Returns the value of pi

POW(base, exponent) - Calculates the result of raising the base to the power of the exponent

PRINT [#filenum,]print-list - Prints a semicolon separated list of literals or variables to the screen or to a file. Included CR/LF by default, but this can be suppressed by ending the statement with a semicolon.

RANDOMIZE [numeric-expression] - Resets random number generator to an unpredictable sequence. With optional seed (numeric expression), the sequence is predictable.

READ simple-variable-list - Reads a set of constants into the list of variables.

REM comment - Internal program documentation

RETURN - Return from a subroutine

RESTORE line-number - sets the line number that the next READ will start loading constants from. line-number must refer to a DATA statement

RIGHT$(string-expression, char-count) - Takes the result of string-expression and returns the right-most char-count characters. If char-count exceeds string length, the entire string is returned.

RND(mode) - For mode values >= 0 generates a pseudo random number N, where 0 <= N < 1. For values < 0 reseeds the PRNG

RNDINT(lo-numerical-expression, hi-numerical-expression) - Generates a pseudo random integer N, where lo-numerical-expression <= N <= hi-numerical-expression

ROUND(numerical-expression) - Rounds numerical-expression to the nearest integer

RUN - Runs the program

SAVE filename - Saves a program to disk

SIN(numerical-expression) - Calculates the sine value of the result of numerical-expression

SQR(numerical-expression) - Calculates the square root of the expression

STOP - Terminates a program

STR$(numerical-expression) - Returns a string representation of the result of numerical-expression

TAN(numerical-expression) - Calculates the tangent value of the result of numerical-expression

UPPER$(string-expression) - Returns an upper-case version of the result of string-expression

VAL(string-expression) - Attempts to convert the result of string-expression to a numeric value. If it is not numeric, returns 0.

Architecture

The interpreter is implemented using the following Python classes:

Open issues

License

PyBasic is made available under the GNU General Public License, version 3.0 or later (GPL-3.0-or-later).