Unit1(计算机科学导论笔记)

Notes Of Introduction Of Computer Science
Unit 1
1. Backus-Naur Form
2. Python Grammer for Arithmetic Expressions
Expression---Expression Operator Expression
Expression---number
Operator---+,-,*......
Number---1,2......
3.Grace Hopper
Grace Hopper wrote one of the first languages(COBOL).
4.Variables
4.1 Assignment statement:
Name = Expression
(Any sequence
of letters,numbers
and underscores,
as long as it starts
with a letter or
an underscore)
4.2 Variables can vary
= means assignment
ment
Start a comment with a hash
6.strings
If we start the string with a single quote, it has to end with a single quote,
and if we start the string with a double quote, it has to end with a double quote.
6.1 strings and numbers
Can't add strings and numbers together, but we can multiply strings .
'!' + 5 →error
'!' * 5 →’!!!!!'
6.2 Indexing strings
<string>[<expression>]
↓
Can be anything that evaluates to a number
'udacity'[3] →'c'
0123456
'udacity'[1+2]→'c'
'udacity'[-1] →'y'
6.3 Selecting sub-sequences from strings
<string>[<expression>:<expression>]
↓↓
start stop
word='assume'
print [3:3] →(black)
print [3:4] →u
print [4:6] →me
print [4:] →me
print [:4] →assu
print [:] →assume
* review the exercises
6.4 Finding strings in strings
<string>.find(<string>)
→number that give the first position
in searching string where the target
string appears.
→if target string is not found,the output
will be the -1
<string>.find(<string>,<number>)
→number that give the first position
in searching string where the target
string appears[at or after <number>].
danton = "De l'audace, encore de l'audace, toujours de l'audace."
print danton.find('audace',0) →5
print danton.find('audace',5) →5
print danton.find('audace',6) →25
7.Extracting Links
...<a href="<url>">...
href = hyperlink reference
8.str function
str(<number>) →<string>
str(<89>) →'89'。