Interpreters, Scripting

Traditionally:


Source Code  --->  compiler  ---> machine code

The machine code(or assembly language) runs very quickly, with no overhead.

It is targetted to a particular CPU (e.g intel for pcs) and to a particular Op.Sys

(e.g Windows - it uses OS facilities to access files, keyboard etc).

What a compiler does

splits source code lines into separate strings(e.g numbers, keywords, variable names...)
checks syntax
- for each encountered variable - has it been declared?
- for each procedure call - check the type of parameters...
generates machine-code equivalent for the source code (many to one)

What an Interpreter does

-it depends - there are different approaches (but in every case, machine code is not created)

In general, more work is done at run-time (e.g parameter checking)

Often, the source code is obeyed as it is checked:

no machine code
much easier to write an interpreter than a compiler (machine-code is very intricate to generate)
can run on any cpu
run-time overhead - may involve checks, deciding what to do, then doing it.

The speed issue is diminishing over the years.

Moore's law:
processor power (speed, memory) doubles every 18 months approx.

Scripting

not a clear definition - many PL's CAN be used to script, but it is much easier in certain ones.
Perl, Tcl, Python
main area: PL as 'glue' for pre-written components: (libraries, GUI's, programs, other scripts, O.S commands....)
Other areas can also be described as 'scripting':
- VB Script, JavaScript - in web pages
- VBA - vb for applications:
  - we can write vb code within Word, excel,...
  - in Word, every menu, button, area of text .. has a vba equivalent (is an object).
    ( rather like javascript, where the elements of the pages can be programmed as objects)
  - keystrokes can be recorded - they generate MODIFIABLE vb code.

Some features of scripting languages

String manipulation eg. - split a string of text into separate words:


input:  the cat sat on the mat.
output: an array of 6 strings - one word per element.

In vb:


dim words () as string
line=textbox1.text   '  or get it from a web form...
words=Split(line, " ")

all scripting languages can do stuff like the above.

Regular expressions

used to check chars within strings against a pattern


e.g check if a room number is of the form:
    A12 , A90, b21
      (e.g letter, digit, digit
in perl:
if room= ~/^[A-Za-z][0-9][0-9]
     print "ok"

Running other programs

in vb:


Dim result As Integer
result = Shell("notepad.exe", 1)  '2nd param: focus option

Examples

do: View source in IExplorer - it runs notepad.
Japa - behind the scenes, it runs the java compiler, and pretty printer.
VB forms can incorporate Word docs, excel sheets.

Summary- scripting

on increase
academic studies lagging (see your booklet for good article)
a range of meanings

To Script or not?

The following do not apply in every situation, but
yes if

gui
lots of complex string work
connecting existing components

no if

complex algorithms (e.g playing mp3, zipping up a file)
speed essential (games, video editing...