Programming Languages Tucker and Noonan Chapter 12 Imperative

Programming Languages Tucker and Noonan Chapter 12: Imperative Programming 12. 1 12. 2 12. 3 12. 4 12. 5 12. 6 12. 7 What Makes a Language Imperative? Procedural Abstraction Expressions and Assignment Library Support for Data Structures C Ada Perl

Imperative Programming • Oldest and most well-developed paradigm • Mirrors computer architecture • Typical Languages – Fortran, Pascal – C, Clite – Ada 83 – Perl

What Makes Languages Imperative? • In a von Neumann machine memory holds: – Instructions – Data – Intellectual heart: assignment statement – Others: • Conditional branching • Unconditional branch (goto)

Flowchart

Procedural Abstraction • Procedural abstraction allows the programmer to be concerned mainly with a function interface, ignoring the details of how it is computed. • The process of stepwise refinement utilizes procedural abstraction to develop an algorithm starting with a general form and ending with an implementation. • Ex: sort(list, len)

Expressions and Assignment • Assignment statement is fundamental: – target = expression • Copy semantics: Expression is evaluated to a value, which is copied to the target; used by imperative languages • Reference semantics: Expression is evaluated to an object, whose pointer is copied to the target; used by objectoriented languages.

Library Procedures/Functions • There exist vast libraries of functions for most imperative languages. • Partially accounts for the longevity of languages like Fortran, Cobol, and C. • Typical libraries for data structures – – – Iterators Vectors Lists Stacks, queues, deques, priority queues Sets and bags Maps

Supports • Turing Completeness – – Assignment Sequence Conditional statement: If Loop statement: • Goto • Structured programming revolution of 1970 s replace the goto with while loops. • Other supports – Integer variables, values, operations – Input/output, error/exception handling, library support

C • “C was originally designed for and implemented on the UNIX • Operating system on the DEC PDP-11, by Dennis Ritchie. • The operating system, the C compiler, and essentially all • UNIX applications programs (including all of the software • Used to prepare this book) are written in C. . C is not tied to • Any particular hardware or system, however, and it is easy to • Write programs that will run without change on any machine that supports C. ”

Influences • • Multics, PL/I Application: typesetting documentation PDP-11: 16 -bit minicomputer; 32 KB memory BCPL: typeless Portability: big-endian vs. little-endian machines Good code from a non-optimizing compiler Hardware support for: ++, --, +=, etc.

General Characteristics • • Relatively low level language Macro facility Conditional compilation Lacks: iterators, generics, exception handling, overloading • Assignments are expression – ex: strcpy

Dynamic Allocation int *a; . . . a = malloc(sizeof(int) *size); /* ANSI C: a = (int *) malloc(sizeof(int) *size); C++: a = new int[size]; */ /* deallocation left to programmer */

Ex: Grep • Grep is a Unix utility to search for strings in a text file • #include libraries • Two functions – main processes command line arguments – find • Forward reference – First signature/prototype, second definition • Procedure – – reads file search each line write line if match fgets

Ex: Average • Compute min, max, average of a set of numbers • Formatting: scanf, printf • Conditional assignment • 2 nd argument to scanf must be an address – caught by some compilers – segment violation at run time

Average C Code #include <stdio. h> Int main(int argc, char *argv[]) { int ct, number, min, max, sum; sum = ct = 0; printf(“Enter number: “); while (scanf(“%d”, &number) != EOF) { if (ct=0) min = max = number; ct++; sum += number; min = number < min? number : min; max = number > max? number : max; printf(“Enter number: “); } printf(“%d numbers readn”, ct); if (ct>0) printf(“Average: t%dn”, sum / ct); printf(“Maximum: t%dn”, max); printf(“Minimum: t%dn”, min); } }

Ada • • • Developed in late 1970’s by Do. D spending billions of dollars on software Over 450 languages in use Solution: standardize on one language Higher Order Language Working Group Ada 83 – problem: size of language/compiler – no subsets rule • Hard times during 1990 s – use of COTS • Renewed interest – COTS proved problematic – development of Spark Ada – NYU GNAT (Ada) compiler

General Characteristics • • Influences: Algol, Pascal Large language; case insensitive Unlike C, array indexing errors trapped Type safe Union Generics Exception handling -- strictly control

Tagged Union type union(b: boolean) is = record case b is when true => i : integer; when false => r : float; end case end record; tagged : union; begin tagged : = (b => false, r => 3. 375); put(tagged. i); -- error case tagged(b) is when true => put(tagged. i); when false => put(tagged. r); end case

Generics • Generic sort – Sort various data set of different types • Code generic type element is private; type list is array(natural range <>) of element; with function ">"(a, b : element) return boolean; package sort_pck is procedure sort (in out a : list); end sort_pck;

package body sort_pck is procedure sort (in out a : list) is begin for i in a'first. . a'last - 1 loop for j in i+1. . a'last loop if a(i) > a(j) then declare t : element; begin t : = a(i); a(i) : = a(j); a(j) : = t; end if; end loop; end sort_pck;

Ada: Average • Comparable to C • Infinite loop; exit on end of file via exception • Inner loop to catch errors caused by nonnumeric data • Exception handling • Wordy than C

with Ada. Text_IO: with Ada. Integer_Text_IO; Procedure Average is sum : = 0; Ct : = 0; Ada. Text_IO. Put(“Enter number: “); loop begin Ada. Integer_Text_IO. Get(Number); if Ct = 0 then Min : = Number; Max : = Number; end if Count : = Count + 1; if Number < Min then Min : = Number; elsif Number > Max then Max : = Number; end if exception when Constraint_Error => Ada. Text_IO. Put(“Value out of range. ”); when Ada. Text_IO. Data_Error => Ada. Text_IO. Put(“Value not an integer. “); when Ada. Text_IO. End_Error => exit; end Ada. Text_IO. Put(“Enter number: “); end loop Ada: Average Code Ada. Integer_Text_IO. Put(Ct, 5); Ada. Text_IO. Put(“ numbers read”); Ada. Text. IO. New_Line; if Ct > 0 then Ada. Text_IO. Put(“Average: “); Ada. Integer_Text_IO. Put(Sum/ Ct); Ada. Text. IO. New_Line; Ada. Text_IO. Put(“Masimum: “); Ada. Integer_Text_IO. Put(Max); Ada. Text. IO. New_Line; Ada. Text_IO. Put(“Minimum: “); Ada. Integer_Text_IO. Put(Min); Ada. Text. IO. New_Line; end if End Average;

Perl • • • Widely used A scripting language (originally for Unix) Dynamically typed Encourages a variety of styles Supports regular expression pattern matching Default conversion from one type to another (vs. Python) • Result is distinct operators; ex: . for string concatenation • Types: numbers, strings, regular expressions • Dynamic arrays: indexed and associative

Scripting Languages • “glue” • Take output from one application and reformat into desired input format for a different application. • Most time is spent in the underlying applications. • Also used for Web applications

Arrays • Indexed Arrays – @a = (2, 3, 5, 7); # size is 4 –. . . – $a[7] = 17; # size is 8; – # $a[4: 6] are undef • Associative Arrays – %d = (“bob” => “ 3465”, – “allen” => “ 3131”, – “rebecca” => “ 2912”); – print $d{“bob”}; # prints 3465

Perl: Grep 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. #! /usr/bin/perl die "Usage mygrep string n" if @ARGV < 1; use strict; my $string = shift; my $ct = 0; while (<>) { $ct++; print "$ct: t$_" if /$string/; } exit;

Comments on Grep • • • Scalar variables start with a $ Indexed arrays with an @ Hash arrays with % Otherwise: bare word syntax error use strict forces declaration of variables – local : dynamic scoping – my : static scoping – NB: only 1 $_

Strings • Double quotes: special characters interpreted – ex: “$a n” – forms: “ “, qq{ }, qq/ / • Single quotes: special characters uninterpreted – forms: ‘ ‘, q{ }, q/ / • Comparison – – 10 < 2 10 < "2" "10" lt "2" 10 lt "2" # false - numeric # false # true - string # true

Loops and Patterns • while (<>) {. . . }is same as: while ($_ = <STDIN>) {. . . } where <> is read a line • returns undef at end of file; undef interpreted as false • no subject: $_ • if $_ =~ m/pattern/ # implied subject, operator

Alternative Code 1. #! /usr/bin/perl 2. if (@ARGV < 1) { die "Usage mygrep string n" ; } 3. use strict; 4. my $string = shift(@ARGV); 5. my $ct = 0; 6. my $line; 7. while ($line = <STDIN>) { 8. $ct++; 9. if ($line =~ m/$string/) { 10. 11. print STDOUT $ct, ": t", $line; } 12. } 13. exit;