Programming Languages: Part 4. Robert M. Dondero, Ph.D. Princeton University

Transcription

1 Programming Languages: Part 4 Robert M. Dondero, Ph.D. Princeton University 1

2 Objectives You will learn/review: In C, Java, and Python... File I/O Data structures Arrays Strings Associative arrays 2

3 "LineSort" Programs The job: Read lines from stdin or file File name specified as command-line arg Write lines to stdout In sorted (lexicographical) order 3

4 "LineSort" Programs Illustrate: File I/O Sequences C: array, DynArray (from COS 217) Java: array, ArrayList Python: list 4

5 "LineSort" in C See linesort1.c Fixed-size arrays Reading via fgets() Sorting via qsort() Writing via puts() Simple, but unrealistic Must eliminate trailing '\n' Requires callback compare function Appends trailing '\n' CPU time consumed 5

6 "LineSort" in C See linesort2.c Dynamically expanding array Handles any number of lines (Max line length still constant) CPU time consumed 6

7 "LineSort" in C See linesort3.c DynArray object From COS 217 Similar to Java ArrayList (Max line length still constant) CPU time consumed Generalizing File I/O... 7

8 C File I/O Open/close FILE *psfile; psfile = fopen("filename", "mode"); /* mode: r, w, a, r+, w+, a+ */ fclose(psfile); Write/read a char istatus = fputc(i, psfile); istatus = putc(i, psfile); istatus = putchar(i); i = fgetc(psfile); i = getc(psfile); i = getchar(); 8

9 C File I/O Write/read a string istatus = fputs(pcstr, psfile); istatus = puts(pcstr); pcstatus = fgets(pcstr, ibufsize, psfile); pcstatus = gets(pcstr); Write/read formatted data icharswritten = fprintf(psfile, "%d", i); icharswritten = printf("%d", i); ivaluesread = fscanf(psfile, "%d", &i); ivaluesread = scanf("%d", &i); 9

10 "LineSort" in Java See LineSort1.java Fixed-size array Simple, but unrealistic Reading via Scanner object Sorting via Arrays.sort() Writing via System.out.println() CPU time consumed 10

11 "LineSort" in Java See LineSort2.java Generic ArrayList class Handles any number of lines Reading via Scanner object Writing via System.out.println() Sorting via Collections.sort() "Foreach" statement CPU time consumed 11

12 "LineSort" in Java See LineSort3.java Input buffering Output buffering Beware: Must flush buffered output!!! Verbosity!!! CPU time consumed!!! Generalizing File I/O classes... 12

13 Java I/O Classes Stream objects write/read bytes Memory File OutputStream object byte byte InputStream object For manipulating machine viewable files 13

14 Java I/O Classes Writer objects write chars Reader objects read chars Memory File Writer object (uses specified encoding UTF-8, UTF-6,...) char byte(s) Reader object (uses specified encoding UTF-8, UTF-16,...) For manipulating human viewable files 14

15 Java I/O Classes Special cases: System.out and System.err OutputStream objects More specifically, PrintStream objects Should be Writer objects Use default encoding for particular host 15

16 Java I/O Classes To read/write bytes: Stream object (decorator) Stream object (decorator) Stream object (source/sink) To read/write chars: Reader/Writer object (decorator) Reader/Writer object (decorator) Reader/Writer object (bridge) Stream object (source/sink) 16

17 Java I/O Classes A few Java I/O classes (more than 60 of them!!!): InputStream: reads bytes FileInputStream: source; reads bytes from a file BufferedInputStream: decorator; adds buffering OutputStream: writes bytes FileOutputStream: sink; writes bytes to a file BufferedOutputStream: decorator; adds buffering PrintStream: decorator; adds C-style formatting Reader: reads characters; respects local character encoding InputStreamReader: bridge BufferedReader: decorator; adds buffering, readline() Writer: writes characters; respects local character encoding OutputStreamWriter: bridge BufferedWriter: decorator; adds buffering PrintWriter: bridge, decorator; adds C-style formatting All are in package java.io System.in is of class InputStream System.out and System.err are of class PrintStream 17

18 Related to Asgt 1 For Assignment 1: RegExp object should use String objects and chars Don't be concerned with encoding schemes And so... Use Reader object (or Scanner object) Use Writer object or System.out 18

19 "LineSort" in Python See linesort.py List: variable sized array Has sort() method Can iterate over a file or list CPU time consumed Generalizing File I/O... 19

20 "ASCII I/O" in Python To read/write ordinary strings containing 1-byte chars: Open/Close file = open('filename', mode='somemode') # somemode: r, w, a, r+, w+, a+ file.close() # beware of truncation Read str = file.readline() list = file.readlines() Write print >>file, str print >>file, number file.write(str) file.writelines(list) 20

21 "ASCII I/O" in Python Common pattern: infile = open('infilename', mode='r') outfile = open('outfilename', mode='w') for str in infile: print >>outfile, str outfile.close() # beware of truncation infile.close() 21

22 Unicode I/O in Python To read/write Unicode strings containing 2-byte or 4-byte chars: A common pattern: infile = open('infilename', mode='r') outfile = open('outfilename', mode='w') for str in infile: unicodestr = str.decode('scheme') # scheme: utf-8, utf-16, etc. str = unicodestr.encode('scheme') print >>outfile, str outfile.close() # beware of truncation infile.close() 22

23 Related to Asgt 1 For Assignment 1: Don't be concerned with encoding schemes And so... OK to use "ASCII I/O" in Python code 23

24 "Concordance" Programs The job: Read words from stdin or file Write to stdout File name specified as command-line arg Illustrate: Each word and occurrence count Associative arrays 24

25 Associative Arrays Associative array An array whose indices are not necessarily integers C: (None) And so whose elements are not necessarily ordered But recall SymTable from COS 217 Java: HashMap object Python: dictionary object 25

26 "Concordance" in C See concord.c Lack of regular expressions Awkward (incorrect!) use of strtok() Lack of associative arrays Must create your own Recall SymTable module from COS 217 CPU time consumed 26

27 "Concordance" in Java See Concord1.java Associative array = HashMap object Regular expressions CPU time consumed 27

28 "Concordance" in Java See Concord2.java Autoboxing concordance.put(word, 1); Autounboxing Compiler automatically generates: concordance.put(word, new Integer(1)); int count = concordance.get(word); Compiler automatically generates: int count = concordance.get(word).intvalue(); CPU time consumed 28

29 "Concordance" in Java Generalizing Autoboxing and autounboxing... Java container interfaces and classes... 29

30 Java Autoboxing Autoboxing and autounboxing Bridge the gap between native types and wrappers Integer I = 123; // no need for new Integer() int i = I; // no need for I.intValue() Same for Long, Float, Double, Short, Byte, Character, Boolean 30

31 Java Autoboxing Pitfalls What does this print (from Josh Bloch via Brian Kernighan) public class Box { public static void main(string[] args) { cmp(new Integer(42), new Integer(42)); } private static void cmp(integer first, Integer second) { if (first < second) System.out.println("first is less than second"); else if (first == second) System.out.println("first equals second"); else if (first > second) System.out.println("first is greater than second"); } } 31

32 Java Container Interfaces Some Java container interfaces: Iterable: an object over which one can iterate Collection: the root interface List: an ordered collection Set: a collection that contains no duplicates SortedSet: a set that orders its elements Map: an object that maps keys to values SortedMap: a map that orders its keys All are in package java.util 32

33 Java Container Classes Some Java container classes: ArrayList: a List implemented as an array LinkedList: a List implemented as a linked list HashSet: a Set implemented as a hash table TreeSet: a SortedSet implemented as a red-black tree HashMap: a Map implemented as a hash table TreeMap: a SortedMap implemented as a red-black tree Arrays: static methods related to arrays Collections: static methods related to Collection objects All are in package java.util 33

34 "Concordance" in Python See concord.py Regular expressions Associative array = dictionary object "in" operator Iteration using "for" statement CPU time consumed Generalizing... Python container classes 34

35 Python Container Classes Iterable Container Mapping Dictionary: a hash table Sequence String: a character sequence Tuple List Set: a collection of unique items File Note: A string is a sequence Strings and tuples are immutable 35

36 Python Container Objects String strobj1 = 'hi' strobj2 = "hi" strobj3 = r'hi' # raw string; \ not interpreted Tuple tupleobj1 = (obj1, obj2,...) tupleobj2 = (obj1, ) hack List listobj = [obj1, obj2,...] Set setobj = set(listobj) Dictionary dictobj = {keyobj1:valueobj1, keyobj2:valueobj2,...} File fileobj = open('filename', mode='somemode') # somemode: r, w, a, r+, w+, a+ 36

37 Using Python Containers Nonmutating Operator on Iterables bool = obj in iterable # bool = iterable. contains (obj) Nonmutating Function on Containers i = len(container) # i = container. len () 37

38 Using Python Sequences Nonmutating Operators/Methods on Sequences seq1 = seq2 + seq3 # seq1 = seq2. add (seq3) obj = seq1[i] # obj = seq1. getitem (i) seq1 = seq2[i:j] # seq1 = seq2. getitem (slice(i,j)) i = seq1.count(obj) i = seq1.index(obj) The string class has many more methods; some useful ones: count, endswith, find, index, isalnum, isalpha, isdigit, islower, isspace, isupper, join, lower, lstrip, replace, rfind, rstrip, split, startswith, strip, upper 38

39 Using Python Sequences Mutating Operators/Functions/Methods on Sequences seq[i] = obj # seq. setitem (i, obj) seq1[i:j] = seq2 # seq1. setitem (slice(i,j), seq2) del(seq[i]) # seq. delitem (i) del(seq[i:j]) # seq. delitem (slice(i,j)) seq1 += seq2 # seq1. iadd (seq2) seq.append(obj) seq.extend(iterable) seq.insert(index, obj) seq.remove(obj) obj = seq.pop() obj = seq.pop(index) seq.reverse() seq.sort() seq.sort(fun) 39

40 Using Python Sets Nonmutating Methods on Sets set1 = set2.copy() set1 = set2.difference(set3) set1 = set2.intersection(set3) bool = set1.issubset(set2) bool = set1.issuperset(set2) set1 = set2.symmetric_difference(set3) set1 = set2.union(set3) Mutating Methods on Sets set1.add(set2) set.clear() set.discard(obj) obj = set1.pop() set.remove(obj) 40

41 Using Python Mappings Nonmutating Methods on Mappings map1 = map2.copy() bool = map.has_key(obj) list = map.items() # creates list of tuples list = map.keys() # creates list of keys list = map.values() # creates list of values iterable = map.iteritems() # creates iterator, not list iterable = map.iterkeys() # creates iterator, not list iterable = map.itervalues() # creates iterator, not list obj = map1.get(key) obj = map1.get(key,default) 41

42 Using Python Mappings Mutating Methods on Mappings map.clear() map1.update(map2) map.setdefault(key) map.setdefault(key,defaultobj) obj = map.pop(key) obj = map.pop(key, defaultobj) obj = map.popitem() # remove & return an arbitrary item! 42

43 An Opinion From Sun Jan 1 16:18: Date: Sun, 1 Jan :18: From: Rob 'Commander' Pike <r@google.com> To: Brian Kernighan <bwk@cs.princeton.edu> python is a very easy language. i think it's actually a good choice for some things. awk is perfect for a line or two, python for a page or two. both break down badly when used on larger examples, although python users utterly refuse to admit its weaknesses for large-scale programming, both in syntax and efficiency. -rob What do you think? 43

44 Summary We have covered: In C, Java, and Python... File I/O Data structures Arrays Strings Associative arrays 44

45 Appendix 1: C Debugging 45

46 C Debugging "gdb" debugger from GNU Integrated with Emacs editor Recall COS 217!!! To use gdb: $ gcc -Wall -ansi -pedantic -g program.c -o program $ emacs <Esc>x gdb<enter>program<enter> 46

47 C Debugging Some gdb Commands help break function break filename:linenum run list next step continue print expr where quit 47

48 C Debugging Common commands have abbreviations: h, b, r, l, n, s, c, p, q Blank line means repeat the same command 48

49 Appendix 2: Java Debugging 49

50 Java Debugging "jdb" debugger is bundled with JDK To use jdb: $ javac -g Class.java $ jdb Class 50

51 Java Debugging Some jdb Commands help stop in pkg.pkg.class.method stop in pkg.pkg.class.method(paramtype, paramtype, ) stop at pkg.pkg.class:line clear pkg.pkg.class.method clear pkg.pkg.class.method(paramtype, paramtype, ) clear pkg.pkg.class:line run run pkg.pkg.class run pkg.pkg.class args list next step cont print expr where quit 51

52 Java Debugging Type!! to repeat the last command Beware: Cannot read from stdin 52

53 Appendix 3: Python Debugging 53

54 Python Debugging "pdb" debugger is bundled with Python To use pdb: $ python -m pdb file.py 54

55 Python Debugging Some pdb Commands help break functionormethod break filename:linenum run list next step continue print expr where quit 55

56 Python Debugging Common commands have abbreviations: h, b, r, l, n, s, c, p, w, q Blank line means repeat the same command Beware: Cannot easily read from stdin 56