Tag: C++

My BASIC Interpreter

In a project in my previous job, I used a product that provides TCL as an extensible API for users. TCL is pretty good and easy to extend, but when you write a bigger script, things become terrible. Also, according to my personal experience, the biggest weakness of TCL is that it cannot evaluate expressions. You have to call ‘expr’ command to evaluate anything, which is pretty troublesome. It’s flow control is also kind of ugly. So Richard Stallman published an article “Why you should not use Tcl” long time ago, said that one should avoid TCL when choosing an extension language:

As interest builds in extensible application programs and tools, and some programmers are tempted to use Tcl, we should not forget the lessons learned … a language for extensions should not be a mere “extension language”. It should be a real programming language, designed for writing and maintaining substantial programs … Extensions are often large, complex programs in their own right, and the people who write them deserve the same facilities that other programmers rely on.

I couldn’t agree more. But I don’t like Python for it restricts you to add certain number of tabs in front of every line… It’s also a pain to integrate Python with C++ or Java code. I don’t like JavaScript (Rhino) either. Since I got some time lately, I decided to write an interpreter to use as an embedded language tool in the future… I chose BASIC as the prototype, for it was the first programming language I learned long time ago. The previously finished expression evaluator was also for this project.

After some work, I finished the main part of it. It can be downloaded here as an executable: mybasic.zip. (Please use Winzip or Winrar to unzip it. Or, here are the unzipped files: mybasic.exe, readme in English, Chinese and Japanese.

The current form of it is a command line tool to interprete BASIC programs or evaluate expressions. It writes the result to stdout and returns 0. When error occurs, it writes error message to stderr, and returns 1.

This is version 0.6, without array & datetime support yet. Since the initial purpose of this is to make an embedded language interpreter, I simplified the BASIC syntax to only keep the important parts (and also borrowed some new stuffs from Visual Basic).

This software was written in C++, with STL, without 3rd-party software (such as yacc and so on).

Usage:

    mybasic -e <expression>

or

    mybasic -f <BASIC_program_file_name>

The sample BASIC program used above (test.bas):

function mySum (byval x as integer) as integer
	if x <= 1 then
		return x
	else
		return x + mySum(x - 1)
	end if
end function

print mysum(100)

Here is the spec of it:

Expression Evaluatorr

The expression should be written in BASIC syntax. algebra, string, comparison and logic calculations are supported.

The spec of it is as follows:

Data types

  • boolean
  • integer
  • double
  • string

Operators

  • math: ^, +(unary), -(unary), *, /, \, MOD, +, –
  • string: +
  • relational: =, <>, <, <=, >, >=
  • logic: NOT, AND, OR
  • others: (, ) (to change precedences of operators)

Math Functions

abs, sign, int, sqrt, exp, log, log10, rad (change degrees to radians), deg (change radians to degrees), sin, cos, tan, asin, acos, atan, sinh, cosh, tanh, asinh, acosh, atanh, sec, csc, pi (return value of Pi), random (return random number in [0, 1])

String Functions

str(num) (convert number to string), space(n), tab(n), ltrim(str), rtrim(str), trim(str), len(str), ucase(str), lcase(str), val(str) (parse string to number), isNumeric(str), left(str, n), right(str, n), mid(str, from, n), instr(str1, str2) (return index starting from 0; if not found, return -1)

Differences from traditional BASIC language

  • some function names are different, for example, “sqrt” instead of “sqr”
  • indexes in a string starts from 0 instead of 1
  • BASIC type characters (such as “2#” for 2 of double type) not supported

BASIC Interpreter

This interpreter supports a practically simplified BASIC syntax.

Spec of ‘practically simplified BASIC syntax’ (besides things listed above):

Supported

  • comment: REM, ‘ (single quotation mark)
  • variable declarison: DIM … AS …
  • condition (block IF): IF, THEN, ELSEIF, ELSE, END IF
  • FOR loop: FOR, TO, STEP, NEXT, CONTINUE FOR, EXIT FOR
  • DO loop: DO, WHILE, UNTIL, LOOP, CONTINUE DO, EXIT DO
  • function & sub-routine: FUNCTION, END FUNCTION, SUB, END SUB, RETURN, BYVAL, BYREF
  • output: PRINT
  • others: END

Not supported

  • LET (most useless command. ignored by the interpreter.)
  • line number (too old)
  • GOTO (evil)
  • GUSUB…RETURN (too old)
  • DEF FN (too old)
  • label (since GOTO/GOSUB/etc are not supported, there is no need to have labels)
  • Single-line IF
  • READ…DATA (too old)
  • SELECT CASE (don’t like the syntax. don’t want to introduce something too different, such as the ‘switch/case’ in C/C++/Java. can be replaced with IF)
  • ON [ERROR] … (too old)
  • WHILE … WEND (old but not good; replaced by the more powerful & flexible DO … LOOP)
  • EXIT SUB, EXIT FUNCTION (use RETURN instead)
  • CALL (but functions can be called directly as sub-routines)
  • variant data type (decreases code quality)
  • user defined type (class)
  • PRINT USING (if it is really necessary, will consider adding it)
  • file I/O (OPEN, etc… will provide new set of file I/O API in later versions)

Differences from normal BASIC language

  • variables must be declared (with DIM) before use (similar to always declaring ‘OPTIONAL EXPLICIT’ in Visual Basic. to help improve code quality.)
  • “DIM a, b, c AS DOUBLE” is OK, but “DIM a AS INTEGER, b AS DOUBLE” not supported (to help improve code quality)
  • Boolean type not allowed to convert from & to Integer (to help improve code quality)
  • functions: BYVAL or BYREF must be specified (no default) (to help improve code quality)
  • in PRINT statement:
    • “,” means inserting a tab, instead of starting to print at a certain place (too old-fashioned and not practical any more)
    • “;” means to start right after the previous place (same as original BASIC spec), but won’t insert a space before a number (programmer should be responsible for this when necessary!)

Not yet supported (will be supported in later versions)

  • data type:
    • array (multi-dimensional)
      • array index starts from 0, cannot be specified to another number
      • in ‘DIM A(N) AS INTEGER’, N is not upperbound, but size of array
    • date
    • time
    • datetime
  • built-in functions
    • some statistical functions
    • some algorithms
    • file I/O
    • other interoperabilities (?)
  • commands
    • INPUT (?)

My Expression Evaluator

Recently I have been taking some time to work on a personal project. A sub-project of it is an expression evaluator. I just finished it today. It was written in C++ with STL, without 3rd-party software. (download “my expression evaluator”)

This small tool work as a command line application, it take an argument as the expression. After evaluating the expression, it writes the result to stdout and returns 0. When error occurs, it writes error message to stderr, and returns 1.

The expression should be written in BASIC syntax. algebra, string, comparison and logic calculations are supported. The spec of it is as follows:

Data types

  • boolean
  • integer
  • double
  • string

Operators

  • math: ^, +(unary), -(unary), *, /, \, MOD, +, –
  • string: +
  • relational: =, <>, <, <=, >, >=
  • logic: NOT, AND, OR
  • others: (, ) (to change precedences of operators)

Math Functions

abs, sign, int, sqrt, exp, log, log10, rad (change degrees to radians), deg (change radians to degrees), sin, cos, tan, asin, acos, atan, sinh, cosh, tanh, asinh, acosh, atanh, sec, csc, pi (return value of Pi), random (return random number in [0, 1])

String Functions:

str(num) (convert number to string), space(n), tab(n), ltrim(str), rtrim(str), trim(str), len(str), ucase(str), lcase(str), val(str) (parse string to number), isNumeric(str), left(str, n), right(str, n), mid(str, from, n), instr(str1, str2) (return index starting from 0; if not found, return -1)

Note that here are some differences with the traditional BASIC language:

  • some function names are different, for example, “sqrt” instead of “sqr”
  • indexes in a string starts from 0 instead of 1
  • BASIC type characters (such as “2#” for 2 of double type) not supported

This software is licensed under “HSL“, you can use it freely. When there are bugs, please let me know, I will try to fix them.

PC Awaker

Click to download it.
Click to download it.

Usually there are a lot of rules in global companies. One of them is the rule of forcing PC go into screen saving after 5 minutes, and if you want to use the PC again, you have to enter your password to unlock the PC.

I admit that this is necessary for security reasons. But sometimes it’s not so reasonable for some of the PCs. For example, if one needs to support a product in a fast paced environment, they cannot afford entering password every time they need the PC – and in fact, the PC’s screen should never be turned off during the work time at all.

The tricky part is that usually we cannot change the screen saver settings because this feature is locked to enforce this rule. In this case, this ‘PC Awaker’ comes to the rescue.

This application is a tiny Windows utility, it is designed as a ‘tray application’, that is, it usually stays in the ‘tray’ of Windows task bar, unless you bring it out. The way how it keeps PC ‘awake’ is to send simulated key strokes periodically – to press F14 key every 4 minutes, to be precise. These are hardcoded since I don’t think it’s necessary to make these configurable.

The license of this software is ‘HSL‘. Help yourself if you need it.

Download it here. It was built in Windows XP using Visual C++ 6.0. I hope that it can work in Windows 9x/ME/NT/2000/XP. Don’t know if it works in Windows Vista…