Lecture 1
Introduction
Xiaojuan Cai（蔡小娟）
cxj@sjtu.edu.cn
Fall, 2015
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Before we start
• Pre-course questionnaire
• TA:
• 李春淼 lichunmiao@sjtu.edu.cn
• Course website:
• http://basics.sjtu.edu.cn/~xiaojuan/ct15/
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Course policy
• Final score =
• Homework (20%) + PA (20%) + Lab (10%)
+ Final exam (50%)
• Practice: Dianxin building 4-313
• Lab sample: percolation
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Textbook and References
• 程序设计思想与方法 陆朝俊 高等教育出版社
• References:
• [R1] Python Programming: An Introduction to
Computer Science, John Zelle.
• [R2] How to Think Like a Computer Scientist, Allen
Downey, Jeff Elkner and Chris Meyers
• http://basics.sjtu.edu.cn/~xiaojuan/ct15/files/
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Why this course?
• Fundamental part of computer science
• Helps you become a more intelligent user of
computers
• It can be fun!
• Helps the development of problem solving skills
• Programmers are in great demand!
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Have fun!
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Objective
• To learn what computer scientists study and the
techniques that they use.
• To understand computer programming languages.
• To begin using the Python programming language.
• To learn about chaotic models.
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Roadmap
• What is computer science?
• Programming languages
• Introduction to Python
• A chaotic model
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Computing and computer
Computing is any goal-oriented activity
requiring, benefiting from, or creating
algorithmic processes.
A computer is a general-purpose device that
can be programmed to carry out a set of
arithmetic or logical operations automatically.
--- Definitions from wikipedia
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Computing and computer
•
Computing can be traced back to 9th century.
•
A first general-purpose computing device
(computer) is invented by Charles Babbage, in early
19th century.
•
A first digital programmable computer is Colossus
(around 1943).
•
“Computers are to computer science what telescopes are
to astronomy.”
– E. Dijkstra
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Computers
Colossus(1943), the first electronic
digital programmable computing
device.
Picture sourced from Wikipedia.
•
IBM’s Blue Gene/P supercomputer
(2011) runs over 250,000
processors.
Picture sourced from Wikipedia.
In 2013, NUDT Tianhe-2 is ranked as the fastest supercomputer.
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Computers
• Software (programs) rule the hardware
(the physical machine).
• The process of creating software is called
programming.
• Before programming, we need to design
how to create (the algorithm).
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Hardware basics
• CPU
• the heart of computers
• “simple” instructions, fast speed
• Storage
• primary – also called RAM, fast but volatile
• secondary – permanently, NVRAM, ROM
• I/O device
display,
Xiaojuan Cai • Mouse, keyboard,
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Computer science
• It is not the study of computers!
It studies “What can be computed?”
• Design – Analysis – Experimentation
• Design = algorithm
• Some problems are unsolvable
• Experimentation: some problems are intractable
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Problem solving
• The most important skill
• formulate problems,
• think creatively about solutions, and
• express a solution clearly and accurately
• One level: learn to program
• Another level: learn how to use programs
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Roadmap
• What is computer science?
• Programming languages
• Introduction to Python
• A chaotic model
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Programming languages
• Natural language has ambiguity
• Programming languages: unambiguous and precise.
• Every structure in a pl has a precise form: syntax
• And it has a precise meaning: semantics.
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Languages of computers
• Machine language
• Computer’s language
• Impossible for human beings
• Assembly language
• Possible but still unnatural
• Still useful for designing efficient programs
• High level language
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High-level to low-level
• Interpreter
• Compiler
• Hybrid compiler/interpreting process
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GCD in pseudo code
function gcd(a, b)
while a ≠ b
if a > b
a := a - b
else
b := b - a
return a
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GCD in x86 machine language
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GCD in x86 assembly language
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GCD in C
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GCD in Prolog
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GCD in Scheme
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GCD in Python
def gcd(a,b):
while a != b:
if a > b:
a = a - b
else:
b = b - a
return a
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Roadmap
• What is computer science?
• Programming languages
• Introduction to Python
• A chaotic model
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Python
• Designed by Guido van Rossum
• Predecessor：ABC
• Name origin: Guido is a big fan of
Monty Python's Flying Circus
Picture sourced from Wikipedia.
• Here are some interesting slides from one of my
students in SE352 (programming languages).
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Python
• Python is an interpreted language
• Command-line mode
• File mode
• “Hello World!” in Python
print “Hello World!”
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Program
• A program is a sequence of instructions that
specifies how to perform a computation.
• Instructions include
• input, output, math, conditional execution,
repetition (iteration, recursion), …
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What is debugging
• Programming errors are called bugs.
• The process of tracking them down and
correcting them is called debugging.
• Errors
• Syntax error
• Semantics error: Run, but not the expected output
• Runtime error: Divide by zero
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• Let’s play around.
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Roadmap
• What is computer science?
• Programming languages
• Introduction to Python
• A chaotic model
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Chaos and computing
• Two features:
• For any input, returns 10 seemingly random
numbers between 0 and 1. (chaotic)
• Very small differences in initial value can have
large differences in the output. (butterfly effect)
• The function computed by program has the
general form kx(1-x) where k is 3.9
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•
•
•
•
•
•
•
•
•
•
•
Input:
0.25
0.73125
0.76644140625
0.698135010439
0.82189581879
0.570894019197
0.955398748364
0.166186721954
0.540417912062
0.9686289303
0.118509010176
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• Input:
0.26
0.75036
0.73054749456
0.767706625733
0.6954993339
0.825942040734
0.560670965721
0.960644232282
0.147446875935
0.490254549376
0.974629602149
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Lessons from chaos
• This program has import implications.
• Many phenomena in real world exhibit this kind
of chaotic behavior.
• Butterfly effect
• Weather forecast
• Computers may give incorrect answers if there
are very small errors in models or programs.
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Summary
• A computer is a universal information-processing
machine.
• Computer science is the study of what can be
computed.
• A computer comprises a CPU, main/secondary
memory, and I/O devices.
• Programs are written using a programming
language.
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Summary (cont’d)
• Python is an interpreted language.
• A Python program is a sequence of statements
• A mathematical model is called chaotic if very
small changes in the input lead to large changes
in the random results.
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Python
“优雅”、“明确”、“简单”
周丞
上海交通大学
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Python娘
简介
• 父亲是吉多·范罗苏姆（Guido van
Rossum）
• 出生于1991年。
• 女大十八变，已经历了
幼女期（1.X版本）和
萝莉期（2.X版本）
目前进入御姐期（3.X版本）
还在健康成长着……
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Python娘
培养理念（设计哲♂学）
“优雅”、“明确”、“简单”
因为家长们不喜欢邻家男孩Perl的教育
方式
“There’s More Than One Way
To Do It.”
（不只一种方法来做一件事。）
所以Python娘受到的教育是
“用一种方法，最好是只有一种方法来
做一件事”
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Python娘
特点
• 活泼（动态语言）
• 灵活（“脚本语言”、“胶水语言”）
• 抽象思维（面向对象语言，支持函数式
编程）
• 优雅美丽（语法简捷和清晰）
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• Python娘很傲骄，御姐状态和萝莉状
态变化很大。
（3.X版本不兼容2.X版本）
• 御姐是未来和希望，但是人们现在更
加喜欢萝莉。
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