xiaojuan cai computational thinking 1 lecture 1 introduction xiaojuan cai （蔡小娟）...

Xiaojuan Cai Computational Thinking 1

Lecture 1

Introduction

Xiaojuan Cai （蔡小娟）

[email protected]

Fall, 2015

mailto:[email protected]


Before we start

• Pre-course questionnaire

• TA:

• 李春淼 [email protected]

• Course website:

• http://basics.sjtu.edu.cn/~xiaojuan/c

t15/

mailto:[email protected]

http://basics.sjtu.edu.cn/~xiaojuan/ct15/

http://basics.sjtu.edu.cn/~xiaojuan/ct15/


Course policy

• Final score =

• Homework (20%) + PA (20%) + Lab

(10%) + Final exam (50%)

• Practice: Dianxin building 4-313

• Lab sample: percolation


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/


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!


Have fun!


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.


Roadmap

• What is computer science?

• Programming languages

• Introduction to Python

• A chaotic model


Computing and computer

A computer is a general-purpose device that can be programmed to carry out a set of arithmetic or logical operations automatically.

Computing is any goal-oriented activity requiring, benefiting from, or creating algorithmic processes.

--- Definitions from wikipedia


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


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.


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).


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• Mouse, keyboard, display, touch screen


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


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


Roadmap




• A chaotic model


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.


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


High-level to low-level

• Interpreter

• Compiler

• Hybrid compiler/interpreting process


GCD in pseudo code

function gcd(a, b) while a ≠ b if a > b

a := a - belse

b := b - a return a


GCD in x86 machine language


GCD in x86 assembly language


GCD in C


GCD in Prolog


GCD in Scheme


GCD in Pythondef gcd(a,b):

while a != b:

if a > b:

a = a - b

else:

b = b - a

return a


Roadmap




• A chaotic model


Python • Designed by Guido van Rossum

• Predecessor ： ABC

• Name origin: Guido is a big fan of

Monty Python's Flying Circus

• Here are some interesting slides from one of my

students in SE352 (programming languages).

Picture sourced from Wikipedia.


Python

• Python is an interpreted language

• Command-line mode

• File mode

• “Hello World!” in Python

print “Hello World!”


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), …


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


• Let’s play around.


Roadmap




• A chaotic model


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


• Input: 0.25• 0.73125• 0.76644140625• 0.698135010439• 0.82189581879• 0.570894019197• 0.955398748364• 0.166186721954• 0.540417912062• 0.9686289303• 0.118509010176

• Input: 0.260.750360.730547494560.7677066257330.69549933390.8259420407340.5606709657210.9606442322820.1474468759350.4902545493760.974629602149


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.


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.


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.


Python

“ 优雅”、“明确”、“简单”周丞

上海交通大学


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……

http://zh.wikipedia.org/wiki/%E5%90%89%E5%A4%9A%C2%B7%E8%8C%83%E7%BD%97%E8%8B%8F%E5%A7%86




培养理念（设计哲♂学）

“ 优雅”、“明确”、“简单”

Python 娘

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”


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）

Python 娘


• 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。 BACK

xiaojuan cai computational thinking 1 lecture 1 introduction xiaojuan cai （蔡小娟）...

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