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

Programming Languages:
Part 1
Robert M. Dondero, Ph.D.
Princeton University
1
Objectives
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You will learn/review:
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Subsets of C, Java, and Python...
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That are appropriate for COS 333...
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Through example programs
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Example 1 in C, Java, Python
Example 2 in C, Java, Python
…
2
Objectives
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Note:
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Comprehensive coverage is impossible!
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Please supplement with reading
3
Objectives
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Specifically:
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Overview
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Program structure
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Building and running
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Multiple functions/methods
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Using library code
4
C Overview
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Who:
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AT&T Bell Labs
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Dennis Ritchie
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When: 1972
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Why: Build Unix
5
C Overview
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Characteristics:
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Low level
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Strongly typed
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Minimal standard library
Two-word description:
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Fast: Close to the OS and hardware
Eccentric: Unless you know the underlying
assembly/machine lang
6
Why Study C?
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Why study C?
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The world's most popular/influential lang
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Illustrates some of the course's material
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Provides strong contrast with other langs
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I know it
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You know it from COS 217
We'll do a quick review
7
Java Overview
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Who:
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Sun Microsystems
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Team led by...
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James Gosling
8
Java Overview
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When: 1990
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Why:
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Embedded systems ("Oak"), and then...
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"The language of the Web", and then...
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Robust portable server-side software
9
Java Overview
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Characteristics:
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Medium level
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Strongly typed
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Large standard library
Two-word description:
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Rich: Large language and standard library
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Verbose: I/O is especially so
10
Why Study Java?
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Why study Java?
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Elegant and popular
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Illustrates most of the course's material
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I know it
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You know it from COS 126/226
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But maybe not some of the advanced
features
We'll do a quick overview
11
Python Overview
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Who:
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Guido Van Rossum
12
Separated at Birth?
13
Python Overview
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When: ~1990
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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
14
Python Overview
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Characteristics:
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High level
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Weakly typed
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No variable declaration statements
"Scripting language"
Rich standard library
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But inconsistent programming conventions
15
Python Overview
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Two-word description:
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Expressive: Can accomplish much work with
little code
Immature: Still evolving; ill-documented
16
Why Study Python?
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Why study Python?
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Reasonably elegant and getting more popular
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Illustrates most of the course's material
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Esp. vs. Perl
Python/Django is popular
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I'm new to it; I want to learn it
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You also are new to it???
Thus the first part of the course won't be
entirely review???
17
Why Study that Combination?
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Among “high level” languages:
Good for
computers
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C
Java
Python
Good for
pgmmers
The three languages cover the spectrum
well
18
Our Approach
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Approach: sequence of example programs
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For each example:
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Examine/run code
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Note important features
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Generalize
Incidentally...
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No function/method comments
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Bad style but saves paper
19
"Hello World" Programs
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Illustrate
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Program structure
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Building and running
20
"Hello World" in C
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See hello.c
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Overall structure
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Comments
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#include preprocessor directive
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Definition of main() function
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printf() function
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String literal ("…")
21
"Hello World" in C
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Generalizations:
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A C program consists of functions
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Exactly one must be named "main"
22
"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
23
"Hello World" in C
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Long procedure:
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File: hello.c
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Preprocess
$ gcc -Wall -ansi -pedantic -E
hello.c > hello.i
File: hello.i
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Compile
$ gcc -Wall -ansi -pedantic -S
hello.i
24
"Hello World" in C
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File: hello.s
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File: hello.o
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Assemble
$ gcc -Wall -ansi -pedantic -c
hello.s
Link
$ gcc -Wall -ansi -pedantic
hello.o -o hello
File: hello
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Run
$ hello
25
"Hello World" in C
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Short procedure:
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File: hello.c
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Preprocess, compile, assemble, link
$ gcc -Wall -ansi -pedantic
hello.c -o hello
File: hello
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Run
$ hello
26
"Hello World" in Java
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See Hello.java
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Overall structure
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Comments
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Class definition
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Definition of main() method
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"static" => class level, not object level
System.out.println() method
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String literal ("…")
27
"Hello World" in Java
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Generalizations:
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A program consists of classes
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Define one public class per file
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Can define other non-public classes in
same file
A class consists of methods (and fields)
28
"Hello World" in Java
Building and executing:
Hello.java
Java compiler
OS
At
run-time
Java interpreter (JVM)
Hello.class
(bytecode)
Source code is portable
Bytecode is portable
29
"Hello World" in Java
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Procedure:
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File: Hello.java
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Compile
$ javac Hello.java
File: Hello.class
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Run
$ java Hello
30
"Hello World" in Python
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See hello1.py
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Overall structure (minimal!)
#!/usr/bin/env python
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Commands OS to...
Use the Python interpreter
Search the PATH env var to find it
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Comments
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print statement
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String literal ('…')
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No semicolons!!!
31
"Hello World" in Python
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Generalizations:
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Program consists of statements (and other
things; stay tuned)
String literal: '…' or "…"
32
"Hello World" in Python
Building and executing:
OS
At
run-time
Python interpreter
hello1.py
Python compiler
hello1.pyc
(bytecode)
Source code is portable
Bytecode is portable
33
"Hello World" in Python
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Procedure 1 (works with/without #! line)
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$ python hello1.py
Procedure 2 (works only with #! line)
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$ chmod 700 hello1.py
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$ hello.py
Procedure 3 (works with/without #! line)
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$ python
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>>> import hello1
Procedure 3 is valuable for learning
34
"Hello World" in Python
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See hello2.py
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Function definitions and calls
__name__ == '__main__'
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True iff this module is the "main" module
True iff this module is compiled/interpreted
directly from the command-line
False iff this module is loaded into
interpreter by another module
Indentation matters!!!
35
"Hello World" in Python
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Generalizations:
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Program consists of statements and functions
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Indentation indicates nesting
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Better to avoid global code
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Global code can hide errors in local code
36
"Hello World" in Python
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Procedure 1 (works with/without #! line)
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python hello2.py
Procedure 2 (works only with #! line)
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chmod 700 hello2.py
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hello2.py
37
"Hello World" in Python
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Procedure 3 (works with/without #! line)
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$ python
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>>> import hello2
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Makes all names defined in the hello2
module accessible within this (the main)
module
>>> hello2.main()
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Can use those names using "hello2." prefix
38
"Hello World" in Python
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Procedure 4 (works with/without #! line)
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$ python
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>>> from hello2 import main
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Imports "main" from the hello2 module into
this (the main) module
>>> main()
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Can use "main" as if it were defined in this
module
39
"Square" Programs
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Illustrate:
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Multi-function/method programs
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Function parameters
40
"Square" in C
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See square1.c
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One source code file defines multiple
functions
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Parameters are passed by value
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Compiler does one pass only
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Compiler must see function definition (or
declaration) before function call
41
"Square" in C
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See square2.c
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Function declaration
Compiler must see function definition or
declaration before function call
42
"Square" in Java
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See Square.java
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One source code file (one class) defines
multiple methods
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Parameters are passed by value
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Compiler does two passes
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Can define methods in any order
43
"Square" in Python
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See square1.py
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One source code file defines multiple
functions
Weakly typed
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Parameters are typeless
Compiler/interpreter does one pass only
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Compiler/interpreter must see function def
before function call
44
"Square" in Python
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However...
Python does not compile/interpret code
until necessary
45
"Square" in Python
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See square2.py
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OK to define sqr() after main()
See square3bad.py
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Compiler/interpreter attempts to interpret call
of sqr() before seeing def of sqr()
Error
"Square" in Python
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See square4bad.py
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Compiler/interpreter does not attempt to
interpret call of non-existing function
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No error detected!
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Dangerous!!!
"SquareRoot" Programs
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Illustrate:
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Using library code
48
"SquareRoot" in C
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See squareroot.c
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#include <math.h>
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Provides compiler with declaration of sqrt()
Allows compiler to check calls of sqrt()
49
"SquareRoot" in C
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To build:
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$ gcc -Wall -ansi -pedantic
squareroot.c -o squareroot -lm
-lm commands linker to search libm.a for
function defs
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Required on many/most systems, but not
on ours
Experiment:
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Remove #include directive
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What happens?
50
"SquareRoot" in Java
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See SquareRoot1.java
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Uses full class name in code
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Fine, but sometimes awkward
See SquareRoot2.java
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import java.lang.Math;
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Commands compiler to resolve "Math" to
"java.lang.Math"
Actually, not necessary: "java.lang.*" is
imported by default
51
"SquareRoot" in Python
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See squareroot1.py
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import math
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Makes all names defined in the math
module accessible within this (the main)
module
Can use those names using "math." prefix
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Bad: Sometimes awkward to use prefix
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Good: No conflicts with names in this module
52
"SquareRoot" in Python
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See squareroot2.py
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from math import sqrt
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Imports "sqrt" from the math module into
this module
Can use "sqrt" as if it were defined in this
module
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Good: No need to specify prefixes
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Bad: Potential name conflicts
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New def overwrites old def
53
Summary
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We have covered:

In Java, C, and Python...

Program structure

Building and running

Multiple functions/methods

Using library code
54