Python Notes from scratch (i)

Author identity: Beginner python, rookie

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1. >>> main prompt - wait for input Next statement

... Secondary prompt- waits for the remainder of the current statement to be entered

2. Python Two ways: statements and Expressions

2-1 Print statement complete Hello World:

>>> print ("Hello world!")

2-2 expression does not have a keyword

Can be a mathematical operator to form an arithmetic expression

Functions called by parentheses

(Note: A function that does not specify a return value returns None equivalent to NULL)

Call ABS () to return the absolute value function:

1 >>> abs ( -4)2 43 >>> abs ( -12) + 4 5 >>> ABS (3/2)6 1.57 >>>

3. Program output

View string contents with print :

>>> myString = ' Hello string! ' >>> print (myString) Hello string!

You can also view this:

>>> myString ' Hello string! ' >>>

Note: string contents in single quotation marks

4. In the expression, the underscore _ denotes the value of the last expression, followed by:

>>> _ ' Hello string! '

5. Use of %s

>>> Print ("%s is%d"% ("Bro.young", 1)) Bro.young was 1>>> print ("%s is%d"% (7788,1)) 7788 is 1

(Note: You can pass a number to%s)

>>> print ('%s is%d '% (77.88,1)) 77.88 is 1

(Note: Floating-point type is also possible)

6. Program input (built-in function input())

Get user input data :

2.x series using raw_input ()

>>> user = Raw_input (' Please Enter Your Name: ')

(Note: The 3.x series is not available raw_input() because the 3.x series is no longer using raw_input ( ) but instead uses input () )

3.x series using input ()

>>> user = input (' Please enter Your name: ') "Please enter Your name:

You can enter your name at this time Bro.young

7. Enter a numeric string

The previous example is limited to text input, and the following is a numeric string input

>>> number = input (' Please enter Your number: ') Please enter Your number:1234>>>print (' Your number is:%d '% (int (number) *) Your number is:2468

(Note: int () converts a numeric string to an integer value)

8. View the Help documentation

Call Help (), using input as an example, as follows:

>>> Help (Input)

9. # Symbol comment (starting from # until line ends)

>>> #one Word ... print (' Single Dog ') a single dog

10. Standard Operators

+  -  *  /  //  %   **

Add, subtract, multiply, divide, and withdraw are standard operators

/- Traditional Division

- Floating point Division (rounding)

* *- exponentiation operator

>>> Print ( -1*2+3**3) 25

11. Standard comparison operator - Returns a Boolean value

< <= > >= = = = = <>

>>> 1<2true>>> 1<= 2true>>> 1>2false

(Note: <> is not equal to operator abc/pascal style, basically eliminated )

12. Logical operators can connect any expression to return a Boolean value

And Or not

>>> 1 = = 2false>>> 1!=2true>>> 1<2 and 1==2false>>> 1<2 or 1==2true>> > Not 6.5<2true>>> 1<2<3true

Python is a dynamic language - There is no need to pre-declare the variable type, and the type and value of the variable are initialized at the moment of assignment.

>>> name = ' Tom ' >>> age = 20>>> Peer = ' senior student ' >>> print ("%s is%d,%s"% (name , Age,peer)) Tom is 20,senior student>>> print ('%s was%d,%s '% (Name,age,peer)) Tom is 20,senior student

14. Support for incremental assignment - operators and equals signs are merged together

Example:n *=

>>> n=1>>> n *= 10>>>--n10>>>-n-10>>> ++n10

(Note: self-increment (+ +) self-decrement (--) is not supported,such as:--n is considered to be -(-N ) is n )

15. Five basic types of numbers

INT- signed integral type

Long- integer

BOOL- Boolean type

Float- Float type

Complex- plural

(Note 1: The long integer in Python is larger than the number represented by a long integer in the C language, and its size is limited by computer memory)

: Boolean Span style= "FONT-FAMILY:CALIBRI;" >true and false put in numeric context true 1 false 0 )

16. Use the index operator ([]) and the slice operator ([:]) to get a string rule: The index of the first character is 0, the last character is indexed -1 .

The plus sign (+) is used for string join operations asterisk (*) for string repetition

[Start:end]- access range from start to end(not including end) for all elements

>>> str = ' ABCD ' >>> str[1:2] ' B ' >>> str[1:3] ' BC ' >>> str[1:4] ' BCD ' >>> str[ -3:-1] ' BC ' >>> str2 = ' EFG ' >>> str + str2 ' ABCDEFG ' >>> str*2 ' abcdabcd ' >>> ' _ ' *10 ' ___ _______‘

(Note: [:] 0 can not write)

>>> Str[0:2] ' ab ' >>> str[:2] ' ab '

17. Lists and tuples <==> Arrays can hold any number of Python objects of any type starting from 0 to access elements

Differences from arrays: lists and tuples can hold different types of objects

18. The dictionary is a mapping data type in Python, consisting of a key-value pair (key-value). The value can be any type of Python object, and the dictionary element is wrapped with {}

>>> mydict = {' Hello ': ' Host '}>>> mydict{' hello ': ' Host '}>>> mydict[' world ' = 100>> > mydict{' World ': +, ' hello ': ' Host '}

　　

Python Notes from scratch (i)