Automated development using Python - introduction to variables, data types and operation methods

1. Variables

Variable definition: Variables are used to store information to referrenced and manipulated in a computer program.

Used to Store the intermediate operation results of the program running

• Identification

• Storage

Variables are used in the program A variable name means

• The variable name must be a combination of uppercase and lowercase English, numbers and _, and cannot start with a number

• Sensitive to case

• Recommended camel case naming method, such as myFirstName, myLastName

• Keywords cannot be declared as variables

In Python, the equal sign = is an assignment statement, and any can be Data type is assigned to a variable. The same variable can be assigned repeatedly, and it can be a different type of variable

##myFisrtName =

"Jonathan"

myLastName =

"Ni"

##Please do not equate the equal sign of the assignment statement with the mathematical equal sign. For example, the following code:

x =

8

x = x +

2

Understand mathematically x = x +

2

is not established. In the program, the assignment statement first calculates the expression x + 2 on the right side, obtains the result 10, and then assigns it to the variable x. Since the previous value of x was

8

, after reassignment, the value of x becomes 10.

It is very important to understand the representation of variables in computer memory, as shown below in declaring variables and assigning values.

a =

"ABC"

##The Python interpreter did two things:

Created a

"ABC"## in memory
1. #String

   ; creates a variable named a in memory and points it to

   "ABC"
.

2. Constant
is a quantity that cannot be changed, usually represented by a variable name in all uppercase letters. ​

##PI = 3.14159265359

##

Summary:

Inside the computer, any data is regarded as an "object", and Variables are used in programs to point to these data objects, and assigning values ​​to variables associates data with variables.

2. Data types

A computer is a machine that can do mathematical calculations. Computer programs naturally deal with various numerical values. In addition to numerical values, computers can also process various data such as text, graphics, audio, video, web pages, etc. Different data requires different data types to be defined.

1. Numbers

Integers

: The representation method is the same as the mathematical writing method, such as 1, -100, 0, 1000, etc. Or hexadecimal representation, 0xffffff, 0xabcd, etc. Floating point numbers: that is, decimals, such as

0.99

, -1.25, 88.88, etc. Or expressed in scientific notation 1.23e8, 1.2e-8, etc. Complex numbers: composed of real part and imaginary part

grouped

, general form It is x + yj, such as (-5+4j), etc. There is no size limit for numbers. If they exceed a certain range, they are directly expressed as

inf

(Infinite)

2. String

A string is any text enclosed in single quotes ' or double quotes ", such as

'abc'

, "XYZ", etc. ' or " itself is just a representation, not part of the string.

For example, 'abc' only has three characters: a, b, and c. If ' itself is also a character, it can be enclosed by "", for example

"I'm OK"

The characters included are I, ', m, space, O, K these 6 characters. Strings containing both ' and " can be identified by the escape character \. For example,

'I\'m \"OK\"!'

means I' m "OK"! escape characters can escape many characters, such as

\n

represents a newline, \t represents a tab character, \\ represents the characters \ If there are many line breaks inside the string, you can use

'''.....'''

The format represents multi-line content.

Common string functions

#string

.strip([chars])
• method is used to remove specified characters at the beginning and end of the string (default is space)

" Jonathan ".strip()

>>>

"Jonathan"";Jonathan;;" .strip(

";") >>> "Jonathan"

Parameters: chars -- Remove the specified characters from the beginning and end of the string

Return Value: Returns the new string generated by removing the specified characters from the beginning and end of the string. The original string remains unchanged.

#len(string)

method returns the characters. String length.

• #len(

"Jonathan") > >> 8

Parameters: str -- String

Return value: String length

• string.index(str, beg=0, end=len(str))Method to detect whether the string contains the substring str

Parameters:

str -- Specify the string to be retrieved

beg -- Starting index , default is 0

end -- Ending index, defaulting to the length of the string

Return value: If a substring is included, return the starting index value, otherwise throws an exception .

• Accessing the value in the string

Python does not support the single character type , single Characters are also used as a string in Python.
To access substrings, you can use square brackets to intercept the string

##"Jonathan".index("J") >>> 0

"Jonathan".index("J",1) >>> substring not found

"Jonathan".index("an") >>> 6

"Jonathan"[ 0]                                                                                                       [0:3] >>>

"Jon"

                                                                                 ##"Jonathan"[-1]   >>> "n"         # The last one

"Jonathan"[-3:-1] >>>"ha" ​ # Pay attention to the beginning but not the end

"Jonathan"[-3:0] >>> "" # There is no interception from back to front"Jonathan

[

-3:]                                                                                                         "han" # The interpreter recognizes all characters from the third to last to the last

"Jonathan "[]                                         > "Jonathan"[:]                                                                                                                                    

##"Jonathan"[0:len("Jonathan")]

>>>

"Jonathan""Jonathan"[-len("Jonathan" )

:

] >>> "Jonathan""Jonathan"[::-2]

>>> "Jnta" # The last digit indicates the step size 2

String concatenation

"Jon"

+

"nathan"

>> >

"Joanthan"

# In case of + , the interpreter needs to re-apply for memory

• ##

  • Formatted output

  name = "Jonathan"

  print("My name is %s."% (name) >>> "My name is Jonathan."

  3. Boolean type

  The expression of Boolean value and Boolean algebra are exactly the same. A Boolean value only has True and False Two kinds of values ​​(note case), Boolean values ​​can use and, or, and not operations

  and operations are AND operation, only if all are True, the result of and operation is True

  or operation is an OR operation , as long as one of them is True, the result of the or operation is True: The

  not operation is a non-operation, it It is a unary operator, which turns True into False and False into True

  Boolean values ​​are often used in conditional judgment

  ##if age > ;= 18:

  print("adult")

  else :

  print("teenager")

  4. Null value

  Null value It is a special value in Python, represented by

  None. None cannot be understood as 0, because 0 is meaningful. None is a special null value

  5. List

  List is the most commonly used data type in Python. , the most convenient storage, modification and other operations can be performed on the data through the list Define the list names = [

  "Jon"

  ,

  "Alex"

  , "Tom ", "Catherine"]Access elements in the list through subscripts, and the subscripts start counting from 0

  names[0] >>> "Jon"

  names[-1] >>> "Catherine"

  Operation on the list

  3. Operations

  1. Arithmetic operations

  

  2. Comparison operations

  

  3. Assignment operation

  

  4. Logical operation

  

  5. Member operation

  

  6, identity operation

  

  7,bit operation

  

  ##names[0] =

  "Jonathan" # Modify element

  names.append(

  "Jim") >>> ["Jonathan", "Alex", "Tom", "Catherine", "Jim"] # Attach element at the end of the list

  names.insert(2,

  "Linda") >>> [" Jonathan", "Alex", "Linda", "Tom", "Catherine", "Jim"] # Insert the element at the subscript 2, and move other elements after

  names.remove(

  "Linda") >>> ["Jonathan", "Alex", "Tom", "Catherine ", "Jim"] #Delete the first retrieved element

  del names[4] >> ;> [

  "Jonathan", "Alex", "Tom", "Catherine"] # Press Index delete element

  names.index(

  "Alex") >>> 1 # Return index

  names .

  sort() >>> ["Alex", "Catherine", "Jonathan", "Tom"] # Arrange in positive ASCII order

  names.reverse() >>> [

  "Tom", "Jonathan", "Catherine", "Alex"] # Element inversion

  names.

  count("Tom") >>> 1 # Return the number of occurrences

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