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

Transcription

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

2 For Your Amusement Java is C++ without the guns, knives, and clubs. James Gosling Python is the most powerful language you can still read. Paul Dubois 2

3 Objectives You will learn/review: Subsets of Java and Python... That are appropriate for COS Through example programs Example 1 in Java, Python Example 2 in Java, Python 3

4 Objectives Note: Comprehensive coverage is impossible! Please supplement with reading 4

5 Objectives Specifically: Overview Program structure Building and running Multiple functions/methods Using library code Interactive Python 5

6 Java Overview Who: Sun Microsystems Team led by... James Gosling 6

7 Java Overview When: 1990 Why: Embedded systems (Oak), and then... The language of the web, and then... Fast, robust, portable server-side software 7

8 Java Overview Characteristics: Medium level Strongly typed Objects and object references have types Rich standard library Well documented Not particularly expressive Not particularly subsettable 8

9 Why Study Java? Why study Java? Elegant and popular Illustrates most of the course s material I know it You know it from COS 126/226 But maybe not some of the advanced features? We ll do a quick overview Mostly for the sake of contrast with Python 9

10 Python Overview Who: Guido Van Rossum 10

11 Separated at Birth? Separated at Birth? 11

12 Python Overview When: ~1990 Why: I was looking for a hobby programming project that would keep me occupied during the week around Christmas. My office would be closed, but I had a home computer, and not much else on my hands. I decided to write an interpreter for the new scripting language I had been thinking about lately: a descendant of ABC that would appeal to Unix/C hackers. I chose Python as a working title for the project, being in a slightly irreverent mood (and a big fan of Monty Python s Flying Circus). Van Rossum 12

13 Python Overview Characteristics: High level Weakly typed Objects have types; object references do not Scripting language Rich standard library Often poorly documented Often with inconsistent pgmming conventions Expressive Subsettable 13

14 Why Study Python? Why study Python? Reasonably elegant and popular Esp. vs. Perl Illustrates most of the course s material You are new to it??? Thus the first part of the course won t be entirely review??? We ll use Python version

15 Our Approach Approach: sequence of example programs For each example: Examine/run code Note important features Generalize Incidentally... No function/method comments Bad style but saves paper 15

16 Hello World Programs Illustrate Program structure Building and running 16

17 Hello World in Java See Hello.java Overall structure Comments Class definition Definition of main() method static => class level, not object level System.out.println() method String literal (" ") 17

18 Hello World in Java Generalizations: A program consists of classes Define one public class per file Can define other non-public classes in same file A class consists of methods (and fields) 18

19 Hello World in Java Building and executing: File: Hello.java Compile $ javac Hello.java File: Hello.class Run $ java Hello 19

20 Hello World in Java Building and executing: Hello.java (source code) Java compiler At run-time OS Java interpreter (JVM) Hello.class (bytecode) Source code is portable Bytecode is portable 20

21 Hello World in Python See hello1.py Overall structure (minimal!) #!/usr/bin/env python Commands OS to... Use the python program Search the PATH env var to find it Meaningful on Unix(-like) systems Meaningless (but harmless) on MS Windows systems 21

22 Hello World in Python Comments print statement String literal (' ') No semicolons!!! 22

23 Hello World in Python Generalizations: Program consists of statements (and other things; stay tuned) String literal: ' ' or " " 23

24 Hello World in Python Building and executing Procedure 1 (Unix/Win, with/without #!) $ python hello1.py 24

25 Hello World in Python Building and executing: At run-time OS Python interpreter hello1.py (source code) hello1.pyc (bytecode) Python compiler Source code is portable Bytecode is portable; often discarded 25

26 Hello World in Python Building and executing (cont.) Procedure 2 (Unix, with #!) $ chmod 700 hello1.py $./hello1.py Procedure 3 (Win, with/without #!) C:\>hello1.py 26

27 Hello World in Python See hello2.py Function definitions and calls name == ' main ' True if this module is the main module True if this module is compiled/interpreted directly from the command-line False if this module is loaded into interpreter by another module Indentation matters!!! 27

28 Hello World in Python Generalizations: Program consists of statements and functions Indentation indicates nesting Better to avoid global code File more reasonably can be included into others Avoid some anomalies 28

29 Hello World in Python Procedure 1 (Unix/Win, with/without #!) $ python hello2.py Procedure 2 (Unix, with #!) $ chmod 700 hello2.py $./hello2.py Procedure 3 (Win, with/without #!) C:\>hello2.py 29

30 Square Programs Illustrate: Multi-function/method programs Function parameters 30

31 Square in Java See Square.java One class can define multiple methods Parameters are passed by value 31

32 Square in Python See square.py One module can define multiple functions Weakly typed Parameters are typeless 32

33 SquareRoot Programs Illustrate: Using library code 33

34 SquareRoot in Java See SquareRoot1.java Uses full class name in code Fine, but sometimes awkward See SquareRoot2.java import java.lang.math; Commands compiler to resolve Math to java.lang.math Actually, not necessary: java.lang.* is imported by default 34

35 SquareRoot in Python See squareroot1.py import math Makes all names defined in the math module accessible within this (the main) module Can use those names using math. prefix Bad: Sometimes awkward to use prefix Good: No conflicts with names in this module 35

36 SquareRoot in Python See squareroot2.py from math import sqrt Imports sqrt from the math module into this module Can use sqrt as if it were defined in this module Good: No need to specify prefixes Bad: Potential name conflicts New def overwrites old def 36

37 Aside: Interactive Python $ python >>> print >>> >>> import math >>> print math.sqrt(5) >>> math.sqrt(5) >>> from math import sqrt >>> sqrt(5) Interactive Python is valuable for learning 37

38 Aside: Interactive Python $ python >>> import hello1 hello, world >>> import hello2 >>> hello2.main() hello, world >>> from hello2 import main >>> main() hello, world Interactive Python is valuable for testing 38

39 Summary We have covered: In Java and Python... Program structure Building and running Multiple functions/methods Using library code Interactive Python 39

40 Appendix: C Programming 40

41 C Overview Who: AT&T Bell Labs Dennis Ritchie When: 1972 Why: Build Unix 41

42 C Overview Characteristics: Low level Strongly typed Minimal standard library Two-word description: Fast: Close to the OS and hardware Eccentric: Unless you know the underlying assembly/machine lang 42

43 Why Study C? Why study C? The world's most popular/influential lang Illustrates some of the course's material Provides strong contrast with other langs I know it You know it from COS

44 Hello World in C See hello.c Overall structure Comments #include preprocessor directive Definition of main() function printf() function String literal (" ") 44

45 Hello World in C Generalizations: A C program consists of functions Exactly one must be named main() 45

46 Hello World in C Building and executing: hello.c C preprocessor/compiler/ assembler/linker At run-time OS hello (executable binary code) Source code is portable (to some extent) Executable binary code is not portable 46

47 Hello World in C Long procedure: File: hello.c Preprocess $ gcc -E hello.c > hello.i File: hello.i Compile $ gcc -S hello.i 47

48 Hello World in C File: hello.s Assemble $ gcc -c hello.s File: hello.o Link $ gcc hello.o -o hello File: hello Run $./hello 48

49 Hello World in C Short procedure: File: hello.c Preprocess, compile, assemble, link $ gcc hello.c -o hello File: hello Run $./hello 49

50 Square in C See square1.c One source code file defines multiple functions Parameters are passed by value Compiler does one pass only Compiler must see function definition (or declaration) before function call 50

51 Square in C See square2.c Function declaration Compiler must see function definition or declaration before function call 51

52 SquareRoot in C See squareroot.c #include <math.h> Provides compiler with declaration of sqrt() Allows compiler to check calls of sqrt() 52

53 SquareRoot in C To build: $ gcc squareroot.c -o squareroot -lm -lm commands linker to search libm.a for function defs Required on many/most systems, but not on ours Experiment: Remove #include directive What happens? 53