Introduction to Scripting FA16 – ICS 215 Ed Meyer What is a scripting language? 2 What is the difference between a scripting language and a system programming language? 3 System Programming Languages • From scratch – build data structures – algorithms • Strongly typed to manage complexity 4 Scripting Languages • Designed for gluing • Automate the execute of tasks • Typeless to simplify connections among components 5 System Programming: Beginning • An alternative to assembly – Very low-level – Each statement is a single machine instruction ;; To output a CR, LF crlf macro push ax push dx mov ah, 2 mov dl, 0dh int 21h mov dl, 0ah int 21h pop dx pop ax endm 6 System Programming: Beginning • An alternative to assembly – Very low-level – Each statement is a single machine instruction • Use a compiler to translate higher-level languages to binary instructions 7 System Programming vs Assembly Language Higher-Level Languages • Means many details are handled automatically – Can write less code to get the same job done • Such details are – Register allocation, handled by the compiler – Procedure calling sequences – Simple keywords such as while and if for control structures • 1 system program line ≈ 5 instructions 9 Procedure Calling Sequences 10 if-else statement Suppose we want to do something like: if (ax < bx) { X = -1; } else { X = 1; } it would look like this: mov ax, 5 ; put the value 5 in register ax mov bx, 7 ; put the value 7 in register bx cmp ax, bx jl axLess ; go to 'axLess' if ax < bx mov word [X], 1 ; This is the 'else part jmp Both ; skip the 'then' part axLess: mov word [X], -1 ; This is the 'then' part Both: 11 Read in and Compare 2 Numbers (in C) ~18 Lines of code 12 Read in and Compare 2 Numbers (Assembly) 13 ~54 Lines of code 14 Comparison 18 lines in C : 54 lines in Assembly 1 line in C : 3 lines in Assembly To run C programs, just install C Assembly is restricted to machines hardware Much easier to install software than replace hardware. 15 Typing (System Prog.) • Need to declare how each piece of information will be used String output = new String(); • Each variable must be used in ways that are appropriate for that type 16 String output = "Hello World!" / 5; 17 Typing (System Prog.) • Need to declare how each piece of information will be used String output = new String(); • Each variable must be used in ways that are appropriate for that type • Data and code are segregated • Objects have well-defined structure – With procedures or methods to manipulate them – An object of one type cannot be used in place of another 18 Advantages of Typing • Large programs more manageable – Clarifies how things are used and treated • Catch errors before runtime – Using a string value as a pointer – Compile error vs Runtime error • Improves performance – Mainly via the compiler 19 Scripting Languages Some Examples 21 Typing (Scripting) • Actually, computer are fundamentally typeless – Memory can hold an integer, floating-point, a pointer, or instruction • Meaning of information is determined by the way it is used • No declaration of variable type – Declare a variable and use it! • Easy to glue components together 22 Typing Example (Scripting) • In Java String output = new String(); • In PHP $output = "Hello World!"; $output = 5 + 3; 23 Typing Example (Scripting) • Creating a button in Tcl button .b -text Hello! -font {Times 16} -command {puts hello} 24 Typing Example (Scripting) • Creating a button in C++ CFont *fontPtr = new CFont(); fontPtr->CreateFont(16, 0, 0, 0, 700, 0, 0, 0, ANSI_CHARSET, OUT_DEFAULT_PRECIS, CLIP_DEFAULT_PRECIS, DEFAULT_QUALITY, DEFAULT_PITCH|FF_DONTCARE, "Times New Roman"); buttonPtr->SetFont(fontPtr); 25 Scripts are Interpreted • Not compiled • Rapid turnaround • Increases productivity • Generate code on the fly • Less efficient resource wise – Not really an issue 26 A comparison of various programming languages based on their level (higher-level languages execute more machine instructions for each language statement) and their degree of typing. 27 When should you use…??? 28 A Scripting Language… • Is the application’s main task to connect preexisting components? • Will the application manipulate a variety of different things? • Does the application include a GUI? • Does the application do a lot of string manipulation? • Will the application’s functions evolve rapidly over time? • Does the application need to be extensible? 29 A System Prog. Language… • Does the application implement complex algorithms or data structures? • Does the application manipulate large datasets, for example, all the pixels in an image, such that execution speed is critical? • Are the application’s functions well defined and slow to change? 30 In Comparison… Scripting System Programming • Interpreted • Best suited for complex • Higher productivity systems • Weakly typed • Code is compiled • Higher level than system • Strong typing programming • Reuse is high • Machine instructions per statement is high 31 Trend • Faster machines • Better scripting languages • Growth of Internet • Expand the applicability of scripting languages 32 Additional Readings • On Laulima – "Scripting: Higher Level Programming for the 21st Century" by John K. Ousterhout, IEEE Computer, 31(3) 1998, pp. 23-30, – "Are Scripting Languages Any Good? A Validation of Perl, Python, Rexx, and Tcl against C, C++, and Java" by Lutz Prechelt, Advances in Computers, 57 (Ed.: M. Zelkowitz), Academic Press, 2003, pp. 205-270. 33 More Recently… • ACM Blog July 7, 2014 – Python is Now the Most Popular Introductory Teaching Language at Top U.S. Universities 34.