Programming Languages Tucker and Noonan Chapter 12: Imperative Programming 12.1 What Makes a Language Imperative? 12.2 Procedural Abstraction 12.3 Expressions and Assignment 12.4 Library Support for Data Structures 12.5 C 12.6 Ada 12.7 Perl CSC321: Programming Languages

Imperative Programming Oldest and most well-developed paradigm Mirrors computer architecture Typical Languages Fortran, Pascal C, Clite Ada 83 Perl CSC321: Programming Languages

What Makes Languages Imperative? In a von Neumann machine memory holds: Instructions Data Intellectual heart: assignment statement Others: Conditional branching Unconditional branch (goto) CSC321: Programming Languages

CSC321: Programming Languages Flowchart CSC321: Programming Languages

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) CSC321: Programming Languages

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 object-oriented languages. CSC321: Programming 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 CSC321: Programming Languages

CSC321: Programming Languages Supports Turing Completeness Assignment Sequence Conditional statement: If Loop statement: Goto Structured programming revolution of 1970s replace the goto with while loops. Other supports Integer variables, values, operations Input/output, error/exception handling, library support CSC321: Programming Languages

CSC321: Programming Languages “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.” CSC321: Programming Languages

CSC321: Programming Languages 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. CSC321: Programming Languages

General Characteristics Relatively low level language Macro facility Conditional compilation Lacks: iterators, generics, exception handling, overloading Assignments are expression ex: strcpy CSC321: Programming Languages

CSC321: Programming Languages 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 */ CSC321: Programming Languages

CSC321: Programming Languages 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 CSC321: Programming Languages

CSC321: Programming Languages Ex: Average Compute min, max, average of a set of numbers Formatting: scanf, printf Conditional assignment 2nd argument to scanf must be an address caught by some compilers segment violation at run time CSC321: Programming Languages

CSC321: Programming Languages 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(“%d numbers read\n”, ct); if (ct>0) printf(“Average: \t%d\n”, sum / ct); printf(“Maximum:\t%d\n”, max); printf(“Minimum: \t%d\n”, min); CSC321: Programming Languages

CSC321: Programming Languages Ada Developed in late 1970’s by DoD DoD 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 1990s use of COTS Renewed interest COTS proved problematic development of Spark Ada NYU GNAT (Ada) compiler CSC321: Programming Languages

General Characteristics Influences: Algol, Pascal Large language; case insensitive Unlike C, array indexing errors trapped Type safe Union Generics Exception handling -- strictly control CSC321: Programming Languages

CSC321: Programming Languages 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 put(tagged.i); put(tagged.r); CSC321: Programming Languages

CSC321: Programming Languages 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; CSC321: Programming Languages

CSC321: Programming Languages 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; t := a(i); a(i) := a(j); a(j) := t; end; end if; end loop end loop; end sort; end sort_pck; CSC321: Programming Languages

CSC321: Programming Languages Ada: Average Comparable to C Infinite loop; exit on end of file via exception Inner loop to catch errors caused by non-numeric data Exception handling Wordy than C CSC321: Programming Languages

CSC321: Programming Languages 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 elsif Number > Max then 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 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.Put(“Masimum: “); Ada.Integer_Text_IO.Put(Max); Ada.Text_IO.Put(“Minimum: “); Ada.Integer_Text_IO.Put(Min); end if End Average; CSC321: Programming Languages

CSC321: Programming Languages 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 CSC321: Programming Languages

CSC321: Programming Languages 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 CSC321: Programming Languages

CSC321: Programming Languages 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 CSC321: Programming Languages

CSC321: Programming Languages Perl: Grep #! /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; CSC321: Programming Languages

CSC321: Programming Languages 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 $_ CSC321: Programming Languages

CSC321: Programming Languages Strings Double quotes: special characters interpreted ex: “$a \n” forms: “ “, qq{ }, qq/ / Single quotes: special characters uninterpreted forms: ‘ ‘, q{ }, q/ / Comparison 10 < 2 # false - numeric 10 < "2" # false "10" lt "2" # true - string 10 lt "2" # true CSC321: Programming Languages

CSC321: Programming Languages 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 CSC321: Programming Languages

CSC321: Programming Languages Alternative Code #! /usr/bin/perl if (@ARGV < 1) { die "Usage mygrep string \n" ; } use strict; my $string = shift(@ARGV); my $ct = 0; my $line; while ($line = <STDIN>) { $ct++; if ($line =~ m/$string/) { print STDOUT $ct, ":\t", $line; } exit; CSC321: Programming Languages