Programming Languages Part 2. Copyright 2017 by Robert M. Dondero, Ph.D. Princeton University

Transcription

1 Programming Languages Part 2 Copyright 2017 by Robert M. Dondero, Ph.D. Princeton University 1

2 Objectives You will learn/review: In Java and Python... Data types and operators Terminal input/output Statements Multi-file programs 2

3 Circle Programs Illustrate: Primitive data types Operators Terminal I/O 3

4 Circle in Java See Circle.java Data types: int, double Assignment statement Operators: =, *, +, cast Terminal input via Scanner class Exception handling via try...catch statement Generalizing Data types, operators, and terminal I/O... 4

5 Java Primitive Data Types Data Type Size Example Literals boolean 1 bit false, true char 2 bytes '\u0000', 'uffff', 'a', '\n', '\t', '\0' byte 1 byte (byte)-128, (byte)127 short 2 bytes (short)-32768, (short)32767 int 4 bytes , long 8 bytes L, L float 4 bytes F double 8 bytes

6 Java Operators Operators method(params) a[i] obj.field, obj.method() class.field, class.method() i++, ++i, i--, --i +x, -x ~i,!b new Class() (type) x*y, x/y, i%j (Precedence) Meaning (1) Method call (1) Array element selector (1) Object member selector (1) Class member selector (2) Increment, decrement (2) Unary plus, unary minus (2) Bitwise NOT, logical NOT (3) Object creation (3) Typecast (4) Multiplication, division, remainder 6

7 Java Operators Operators x+y, x-y i<<j, i>>j, i>>>j x<y, x>y, x<=y, x>=y obj instanceof Class x==y, x!=y i&j i^j i j b1&&b2 b1 b2 (Precedence) Meaning (5) Addition, subtraction (6) Left shift, right shift, right shift zero fill (7) Relational (7) instanceof (8) relational (9) Bitwise AND (10) Bitwise exclusive OR (11) Bitwise OR (12) Logical AND (13) Logical OR 7

8 Java Operators Operators b?x:y x=y x+=y, x-=y, x*=y, x/=y, x%=y i&=j, i^=j, i =j i<<=j, i>>=j, i>>>=j (Precedence) Meaning (14) Conditional expression (15) Assignment (15) Assignment (15) Assignment (15) Assignment 8

9 Java Terminal I/O Reading from stdin: import java.util.scanner;... Scanner scanner = new Scanner(System.in); int i = scanner.nextint(); double d = scanner.nextdouble(); String s = scanner.nextline(); 9

10 Java Terminal I/O Writing to stdout: System.out.print(i); System.out.println(d); System.out.println(s); Writing to stderr: System.err.print(i); System.err.println(d); System.err.println(s); 10

11 Circle in Python See circle.py No variable declarations Data types: int, float Type conversions via int() and float() functions Assignment statement String literals: ' ' or " " Operators: * (multiplication), % (string formatting) Terminal input via raw_input() Exception handling via try...except statement 11

12 Circle in Python Generalizing Conversion functions: int(obj), float(obj), str(obj), bool(obj) String formatting: conversion specifications are as in C Data types, operators, and terminal I/O... 12

13 Python Built-In Data Types Data Type Example Literals int 1, 23, 3493, 01, 027, 06645, 0x1, 0x17, 0xDA5 long 1L, 23L, L, 01L, 027L, L, 0x1L, 0x17L, 0x17486CBC75L (no theoretical size limit) float 0., 0.0,.0, 1.0, 1e0, 1.0e0 (correspond to C doubles) bool nonetype False, True None 13

14 Python Operators Operators (Priority) Meaning {key:expr,...} (1) Dictionary creation [expr,...] (2) List creation (expr,...) (3) Tuple creation, or just parentheses f(expr,...) (4) Function call x[startindex:stopindex] (5) Slicing (for sequences) x[index] (6) Indexing (for containers) x.attr (7) Attribute reference x**y (8) Exponentiation ~x (9) Bitwise NOT +x, -x (10) Unary plus, unary minus x*y, x/y, x//y, x%y (11) Mult, div, truncating div, remainder x+y, x-y (12) Addition, subtraction 14

15 Python Operators Operator x<<i, x>>i x&y x^y x y x<y, x<=y, x>y, x>=y x==y, x!=y x is y, x is not y x in y, x not in y not x x and y x or y (Priority) Meaning (13) Left-shift, right-shift (14) Bitwise AND (15) Bitwise XOR (16) Bitwise OR (17) Relational (17) Relational (18) Identity tests (19) Membership tests (20) Logical NOT (21) Logical AND (22) Logical OR 15

16 Aside: Object Identity & Equality $ python >>> x = >>> y = >>> x == y True >>> x is y False >>> x = >>> y = x >>> x == y True >>> x is y True >>> In Python, ints are objects In Python, longs, floats, and bools also are objects 16

17 Python Terminal I/O Reading from stdin: str = raw_input() str = raw_input(promptstr) from sys import stdin str = stdin.readline() 17

18 Python Terminal I/O Writing to stdout: print str print str, Writing to stderr: print >>stderr, str print >>stderr, str, 18

19 Euclid Programs Illustrate: Control flow statements 19

20 Euclid in Java See Euclid.java Control flow statements: if, while, return Generalizing Statements... 20

21 Java Statements Expression statement expr; Declaration statement modifiers datatype var [= expr] [, var [= expr]]...; Common modifiers: const, static Compound statement { statement; statement;...} 21

22 Java Statements if statement if (booleanexpr) statement else statement; switch statement switch (intexpr) { case (intvalue1): statement;...; break; case (intvalue2): statement;...; break;... default: statement;...; } 22

23 Java Statements while statement while (booleanexpr)statement; do statement do statement while (booleanexpr); for statement for (initexpr; booleanexpr; increxpr) statement; Foreach statement for (type var : IterableOrArrayObject) statement; 23

24 Java Statements break statement break; continue statement continue; return statement return; return expr; 24

25 Java Statements try...catch statement try { statement; statement; ; } catch (ExceptionType e) { statement; statement; ; } throw statement throw object; 25

26 Try? try { } catch (Exception e) { } Do or do not do; there is no try. 26

27 Euclid in Python See euclid.py Integer division operator Unpacking assignment statement Control flow statements: if, while, return Generalizing Statements... 27

28 Python Statements Assignment statements var = expr var += expr var -= expr var *= expr var /= expr var //= expr var %= expr var **= expr var &= expr var = expr var ^= expr var >>= expr var <<= expr 28

29 Python Statements Unpacking assignment statement var1,var2,... = iterable No-op statement pass assert statement assert expr, message 29

30 Python Statements print statement print expr,... print expr,..., hack Function call statement f(expr, name=expr,...) return statement return return expr 30

31 Python Statements if statement if expr: statement(s) elif expr: statement(s) else: statement(s) False, 0, None, '', "", [], (), and {} indicate logical FALSE Any other value indicates logical TRUE 31

32 Python Statements while statement while expr: statement(s) False, 0, None, '', "", [], (), and {} indicate logical FALSE Any other value indicates logical TRUE 32

33 Python Statements for statements for var in range(startint, stopint): statements for var in range(stopint): statements for var in iterable: statements for key,value in mapping.iteritems(): statements break statement break continue statement continue 33

34 Python Statements try...except statement try: statement(s) except [ExceptionType[, var]]: statement(s) raise statement raise ExceptionType(str) 34

35 IntMath Modules Illustrate: Multi-file programs... Design to facilitate reuse of functions in multiple programs Building and running multi-file programs 35

36 IntMath in Java See IntMath.java, TestIntMath.java TestIntMath.java is a client of the IntMath module 36

37 IntMath in Java Building and running $ javac TestIntMath.java Automatically builds IntMath.java Directory containing IntMath.java must be in CLASSPATH env var "." is in CLASSPATH by default $ java TestIntMath 37

38 IntMath in Python See intmath.py, testintmath.py testintmath.py is a client of the IntMath module testintmath.py must import from intmath.py 38

39 IntMath in Python Building and running $ python testintmath.py Automatically compiles intmath.py Directory containing intmath.py must be in PYTHONPATH env var "." is in PYTHONPATH by default 39

40 IntMath in Python See intmath2.py Parameter type validation Which is better, intmath1.py or intmath2.py? 40

41 Aside: Python Duck Typing Observation: Python doesn t care about the types of objects given to a function/method Python cares only that the given objects respond to the messages sent by the function/ method to the given objects 41

42 Aside: Python Duck Typing Observation: In other words Python uses duck typing When I see a bird that walks like a duck and swims like a duck and quacks like a duck I call that bird a duck. -- J. W. Riley 42

43 Aside: Python Duck Typing Style 1: Don t validate parameter types Rationale: Validating parameter types prohibits fortuitous use of functions/methods on parameters of unanticipated types Rationale: Validating parameter types consumes run-time In our example: intmath2.py works on ints only intmath1.py works on ints, and fortuitously, on longs! So intmath1.py is better 43

44 Aside: Python Duck Typing Style 2: Validate parameter types Rationale: When given a parameter of an unanticipated type, function/method may yield: Errors at low call levels rather than immediately No errors and incorrect results!!! In our example: intmath2.py safeguards against str or float arguments intmath1.py does not So intmath2.py is better 44

45 Aside: Python Duck Typing Controversy! Generally style 1 (do not validate parameter types) is considered more Pythonic So that s what we ll use in COS

46 Aside: Python Duck Typing Commentary Application-dependent code Maybe need not validate parameter types Application-independent code Maybe should validate parameter types The negative of accidental incorrect results outweighs the positive of fortuitous use of duck typing!!! 46

47 Aside: Python Duck Typing Commentary Small projects: Maybe need not validate parameter types Large projects: Maybe should validate parameter types But if you feel the need to validate parameter types, should you use Python anyway??? 47

48 ReadNum Modules The job: Prompt for and read an int/double from stdin Re-prompt and re-read if user enters bad data Illustrate Multi-file programs (as previous example) Unit testing 48

49 ReadNum in Java See ReadNum.java, Circle2.java Consumes entire line Otherwise bad characters would affect subsequent reads Calls Integer.parseInt() and Double.parseDouble() to convert String to int/double Uses exceptions to handle errors Defines main() method in ReadNum.java Common technique for unit testing 49

50 ReadNum in Python See readnum.py, circle2.py Consumes entire line Calls int() and float() to convert string to int/double Uses exceptions to handle errors Defines conditionally-executed test code Common technique for unit testing Uses leading underscore (_test) to suggest private function 50

51 Summary We have covered: In Java and Python... Data types and operators Terminal input/output Statements Multi-file programs 51

52 Appendix: C Programming 52

53 Circle in C See circle.c Data types: int, double Operators: =, *, +, cast Terminal input via scanf() Lack of exception handling Must check function return value instead Generalizing Data types, operators, and terminal I/O... 53

54 C Primitive Data Types Data Type Size Example Literals char 1 byte (char)-128, (char)127, 'a', '\n', '\t', '\0' unsigned char 1 byte (unsigned char) 0, (unsigned char)256 short 2 bytes? (short)-32768, (short)32767 unsigned short 2 bytes? (short)0, (short)65535 int 4 bytes? , unsigned int 4 bytes? 0U, U, 0123, 0x123 long 4 bytes? L, L unsigned long 4 bytes? 0UL, UL, 0123L, 0x123L float 4 bytes? F double 8 bytes? long double 12 bytes? L 54

55 C Operators Operator fun(params) a[i] s.f ps->f (type) *p, &x i++, ++i, i--, --i +x, -x (Precedence) Meaning (1) Function call (1) Array element selector (1) Structure field selector (1) Structure dereference and field selector (2) Typecast (2) Dereference, address of (2) Increment, decrement (2) Unary positive, unary negative ~i (2) Bitwise NOT!i (2) Logical NOT sizeof(type) (2) sizeof 55

56 C Operators Operator x*y, x/y, i%j x+y, x-y i<<j, i>>j x,y, x>y, x<=y, x>=y x==y, x!=y i&j i^j i j i&&j i j i?x:y (Precedence) Meaning (3) Multiplication, division, remainder (4) Addition, subtraction (5) Left-shift, right-shift (6) Relational (7) Relational (8) Bitwise AND (9) Bitwise EXCLUSIVE OR (10) Bitwise OR (11) Logical AND (12) Logical OR (13) Conditional expression 56

57 C Operators Operator x=y x+=y, x-=y, x*=y, x/=y i&=j, i^=j, i =j i<<=j, i>>=j x,y (Precedence) Meaning (14) Assignment (14) Assignment (14) Assignment (14) Assignment (15) Sequence 57

58 C Terminal I/O Reading from stdin: ivaluesread = scanf("%d", &i); ivaluesread = scanf("%lf", &d); s = fgets(s, size, stdin) 58

59 C Terminal I/O Writing to stdout: printf("%d", i); printf("%f", d); fputs(s, stdout); Writing to stderr: fprintf(stderr, "%d", i); fprintf(stderr, "%f", d); fputs(s, stderr); 59

60 Euclid in C See euclid.c Control flow statements: if, while, return Generalizing Statements... 60

61 C Statements Expression statement expr; Declaration statement modifiers datatype var [= expr] [, var [= expr]]...; Common modifiers: const, static Compound statement { statement; statement;...} 61

62 C Statements If statement if (intorptrexpr) statement [else statement] 0 or NULL pointer => false; anything else => true Switch statement switch (intexpr) { case (intvalue1): statement;...; break; case (intvalue2): statement;...; break;... default: statement;...; } 62

63 C Statements While statement while (intorptrexpr) statement; Do...while statement do statement while (intorptrexpr); For statement for (initexpr; intorptrexpr; increxpr) statement; 63

64 C Statements Break statement break; Continue statement continue; Return statement: return; return expr; 64

65 IntMath in C See intmath.h, intmath.c, testintmath.c intmath.h is IntMath module interface Protection against multiple inclusion Function names begin with module name intmath.c is IntMath module implementation #include intmath.h so compiler can enforce consistency testintmath.c is IntMath module client #include intmath.h so compiler can check calls of IntMath_gcd() and IntMath_lcm() 65

66 IntMath in C Building: $ gcc intmath.c testintmath.c -o testintmath Running: $ testintmath 66

67 ReadNum in C See readnum.h, readnum.c, circle2.c Consumes entire line Otherwise bad characters would affect subsequent reads Calls strtol() and strtod() to convert string to int/double Contains lots of code for error handling 67

68 ReadNum in C Conditionally includes main() function in readnum module Common technique for unit testing To build normally: gcc circle2.c readnum.c o circle2 To build for unit testing: gcc -D TEST_READNUM readnum.c o readnum 68