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It takes longer to run a program under an interpreter than to run the compiled code but it can take less time to interpret it than the total time required to compile and run it. This is especially important when prototyping and testing code when an edit-interpret-debug cycle can often be much shorter than an edit-compile-run-debug cycle.

Interpreting code is slower than running the compiled code because the interpreter must analyse each statement in the program each time it is executed and then perform the desired action whereas the compiled code just performs the action. This run-time analysis is known as "interpretive overhead". Access to variables is also slower in an interpreter because the mapping of identifiers to storage locations must be done repeatedly at run-time rather than at compile time.

There are various compromises between the development speed when using an interpreter and the execution speed when using a compiler. Some systems (e.g. some LISPs ) allow interpreted and compiled code to call each other and to share variables. This means that once a routine has been tested and debugged under the interpreter it can be compiled and thus benefit from faster execution while other routines are being developed. Many interpreters do not execute the source code as it stands but convert it into some more compact internal form. For example, some BASIC interpreters replace Keywords with single byte tokens which can be used to find the instruction in a Jump Table . An interpreter might well use the same Lexical Analyzer and Parser as the compiler and then interpret the resulting Abstract Syntax Tree .

There is thus a spectrum of possibilities between interpreting and compiling, depending on the amount of analysis performed before the program is executed. For example Emacs Lisp is compiled to Bytecode which is a highly compressed and optimised representation of the Lisp source but is not machine code (and therefore not tied to any particular hardware). This "compiled" code is then executed interpreted by a Bytecode Interpreter (itself written in C ). The compiled code in this case is machine code for a Virtual Machine which is implemented not in hardware but in the byte-code interpreter. The same approach is used with the Forth code used in Open Firmware systems: the source language is compiled into "F code" (a bytecode) which is then interpreted by an architecture-independent virtual machine.

A technique that has gained attention in recent years is Just-in-time Compilation (JIT), which further blurs the distinction between interpreters, byte-code interpreters and compilation. JIT is available for both the .NET and Java platforms. The JIT technique is a few decades old, appearing in languages such as Smalltalk in the 1980's.


EXAMPLE OF A SIMPLE INTERPRETER

The following simple interpreter program is written using BASIC. When compiled using the QuickBASIC compiler it is a straightforward interpreter but when run on the QBASIC interpreter, it is an example of an interpreted interpreter.



DECLARE SUB SplitFirst (aFirst AS STRING, aRest AS STRING)

LET Q$ = "TESTPROG.TXT"

LET F = FREEFILE

OPEN Q$ FOR INPUT AS #F

DO WHILE NOT EOF(F)

  LINE INPUT #F, FileInput$

  LET FileInput$ = LTRIM$(FileInput$)

  SplitFirst KeyWord$, FileInput$

  SELECT CASE KeyWord$

  CASE "-"

    SplitFirst KeyWord$, FileInput$

    GOSUB InterpretKeyword

  END SELECT

LOOP

CLOSE #F

SYSTEM



InterpretKeyword:

  SELECT CASE UCASE$(KeyWord$)

  CASE "STORE"

    GOSUB AssignToValue

  CASE "ADD"

    GOSUB AddToValue

  CASE "MULTIPLYBY"

    GOSUB MultiplyWithValue

  CASE "PRINT"

    GOSUB PutOutput

  CASE "CLS"

    GOSUB ClearScreen

  CASE "NEWLINE"

    PRINT

  CASE ELSE

    PRINT

    PRINT "I don't know what "; KeyWord$; " "; FileInput$; " means."

  END SELECT

  RETURN



AssignToValue:

  GOSUB GetArg

  LET Value$ = Arg$

  RETURN



AddToValue:

  GOSUB GetArg

  LET Value$ = LTRIM$(STR$(VAL(Value$) + VAL(Arg$)))

  RETURN



MultiplyWithValue:

  GOSUB GetArg


    

  •  VAL(Arg$)))

    • 
  RETURN






GetArg:

  Split KeyWord$, FileInput$

  SELECT CASE UCASE$(KeyWord$)

  CASE "INPUT"

    GOSUB GetInput

    LET Arg$ = UserInput$

  CASE "VALUE"

    LET Arg$ = Value$

  CASE "TEXT"

    LET Arg$ = FileInput$

  CASE ELSE

    LET Arg$ = KeyWord$ + " " + FileInput$

  END SELECT

  RETURN



GetInput:

  LINE INPUT "", UserInput$

  RETURN



PutOutput:

  GOSUB GetArg

  PRINT Arg$; " ";

  RETURN



ClearScreen:

  CLS

  RETURN



NewLine:

  PRINT

  RETURN



SUB SplitFirst (aFirst AS STRING, aRest AS STRING)



  DIM J AS INTEGER



  LET J = INSTR(aRest + " ", " ")

  LET aFirst = LTRIM$(LEFT$(aRest, J - 1))

  LET aRest = LTRIM$(MID$(aRest, J))



END SUB


There is a program to try with this interpreter, on the Literate Programming article. If you want to do so you must save the interpreter as ''INTERP.BAS'' and then save the ''Literate programming'' article as ''TESTPROG.TXT'' in the same folder.


PUNCHED CARD INTERPRETER

The term "interpreter" often referred to a piece of Unit Record Equipment that could read Punched Cards and print the characters in human-readable form along the top edge of the card. The IBM 550 Numeric Interpreter and IBM 557 Alphabetic Interpreter are typical examples from 1930 and 1954 , respectively.


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