Xiaojuan Cai Computational Thinking 1 Lecture 2 Simple Programs Xiaojuan Cai （蔡小娟）...

Xiaojuan Cai Computational Thinking 1

Lecture 2

Simple Programs

Xiaojuan Cai（蔡小娟）

[email protected]

Fall, 2015

mailto:[email protected]


Objectives

• To be able to understand and write

Python statements：• to output information to the screen,

• assign values to variables,

• get numeric information entered, and

• perform a counted loop


Roadmap

• The software development

• Elements of program

• Names

• expressions

• Output statements

• Assignment statements

• Definite loop


Software development• Analyze the problem

• Determine specifications

• Create a design (algorithm)

• Implement the design

• Test/Debug the program

• Maintain the program


Example

Susan刚刚来到中国，她什么都喜欢，唯独不喜欢中国的天气预报，因为她习惯华氏度（ Fahrenheit），不习惯摄氏度（Celsius）。她请求你的帮助，她告诉你，华氏和摄氏之间的转换是线性的，而且 0摄氏度=32华氏度， 100摄氏度=212华氏度。


Roadmap

• The software development

• Elements of program

• Names

• Expressions

• Output statements

• Assignment statements

• Definite loop


Names

• File (module) has name, e.g.

converter

• Function has name, e.g. main

• Variable has name, e.g. Celsius

• Names are called identifier.


Identifier

• begin with a letter or underscore _

• followed by any sequence of letters,

digits, or underscores.

• Case sensitive

• All these are legal identifiers:

x, celsius, Spam, SPAM, x2, x_2, _x_2_, …


Reserved words


Expressions• The fragments of code that produce or

calculate new data values are called

expressions, including

• literals, e.g. 3.9, 32

• identifier, e.g. celsius, x

• combining simple ones with operators, e.g.

3.9*x*(1-x)

• function calls (later in this course)

• …


Output statementprint

print <expr>

print <expr>, <expr>, …, <expr>

print <expr>, <expr>, …, <expr>,

• Syntax: <> is slots for other fragments, alternative

• … indicate any number of <expr>

• Semantics: evaluate <expr>, output the value


Assignment statement• Syntax: <var> = <expr>

• E.g., x = 3.9 * x * (1-x), x = 2

• A variable can be assigned many times, with

any values (of any types)

• Semantics:x = x + 110x

11

x 10

“x”

x = “x”


Assigning input• Syntax:

<variable> = input(<prompt>)

• <prompt>: prompt the user for input

• E.g., x = input("Enter a number ")

• Semantics:

evaluate <promt>, wait for an input and assign

the input to <variable>


Simultaneous assignment• Syntax:

<var1>,…,<varn> = <expr1>, …,<exprn>

• Semantics:

evaluate all the <expr>’s, assign the values

to corresponding <var>’s.

• E.g., x,y = 3,4; x,y = y,x

x,y = input(“…”)


Definite loop -- syntaxfor <var> in <sequence>:

<body>

• <var> is called loop index

• <body> is any sequence of statements,

indenting under the heading

• <sequence> consists of list of values


Loops

• A definite loop will execute definite

times.

• The most common definite loop is

counted loop.

• for <var> in range(<expr>): is

one of counted loops


Definite loop -- semantics


Example

• One more example – Future

values

• Susan在中国银行存了一笔钱，根据中国银行的年利率，她想知道 10年后她能取出多少钱。


Formalize

•Program Future value

• Inputs:

• principal the amount of money

initially

• apr the annual percentage rate

•Output: the value after 10 years


Algorithm

Print an introduction

Input principal and apr

Repeat 10 times:

principal = principal * (1 + apr)

Output the principal


Conclusiondef main():

print "This program illustrates a chaotic

function"

x = input("Enter a number between 0 and 1:

")

for i in range (10):

x = 3.9 * x * (1-x)

print i,x

main()

• Circle each identifier.

• Underline each expression.

• indicate the type of statement

on each line

Xiaojuan Cai Computational Thinking 1 Lecture 2 Simple Programs Xiaojuan Cai （蔡小娟）...

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