2、 Operators and branches

Operators and branches

Operators are divided into ： Numeric operators 、 Comparison operator 、 Logical operators 、 Assignment operator

One 、 Mathematical operators ：+（ Add ）、-（ reduce ）、*（ ride ）、/( except )、%（ Mod 、 modulus ）、//（ to be divisible by ）、**（ Power operation )

1. +、-、*、/ And in mathematics +、-、×、÷ The function is as like as two peas

print(5 + 2)		# 7
print(5 - 2)		# 3
print(5 * 2)		# 10
print(5 / 2)		# 2.5

2. % - Mod 、 modulus
   x % y — seek x Divide y The remainder of

application 1： Judge whether a number can be divided by another number （ See if the remainder is 0）

print(5 % 2)		# 1
print(16 % 2)		# 0
print(15 % 5)		# 0

application 2： Take the lower digits ( Yes 10 perhaps 10 To the power of )

num = 2398
print(num % 10)    # 8
print(num % 100)   # 98
num = 4621
print(num % 2)            # 1
print(num % 10)           # 1
print(num % 20)           # 1
print(num % 100)          # 21

3. // - to be divisible by
   x // y - x Divide y The quotient of is rounded to the small

print(5 // 2)		# 2; 2 2.5 3
print(1.8 // 2)		# 0.0; 0 0.9 1
print(-5 // 2)		# -3; -3 -2.5 -2

application ： Positive integer to low

num = 23489
print(num // 10)    	# 2348
print(num // 100)		# 234

# practice 2： Gets the tens of the specified positive integer 
num = 3945
print(num // 10 % 10)    # 4
print(num % 100 // 10)   # 4
# practice 3： Get the hundreds of the specified positive integer 
num = 3945
print(num // 100 % 10)		# 9
print(num % 1000 // 100)	# 9

4. ** - Power operation
   x ** y - seek x Of y Power

print(2 ** 3)           # 8
print(5 ** 3)           # 125
print(16 ** 0.5)        # 4.0
print(27 ** (1/3))      # 3.0
print(2.2 ** 2)         # 2.2 * 2.2 = 4.84
print(2.0 ** 2)         # 4.0

Two 、 Comparison operator ：==（ be equal to ）、！=（ It's not equal to ）

Important conclusions ： The result of all comparison operations is Boolean

print(10 > 20)      # False ( wrong )
print(10 >= 10)     # True ( Yes )
print(20 != 10)     # True ( Yes )

Be careful ：python The comparison operator in can express the range through concatenation like data

age = 16
print(18 <= age <= 28)

3、 ... and 、 Logical operators ：and（ Logic and operators ）、or（ Logic or operation ）、not（ Logic is not ）

1.and - Logic and operators

（1） Application scenarios ： Equivalent to life and , Used to connect multiple conditions that need to be met at the same time
（2） Operational rules ： All two are True The result is True, As long as one is False The result is False

# Conditions 1 and  Conditions 2
True and True       -   True
True and False      -   False
False and True      -   False
False and False     -   False

practice ： Write a number that can be simultaneously 3 and 7 The condition of division

num = 67
# Conditions 1： Can be 3 to be divisible by  - num % 3 == 0
# Conditions 2： Can be 7 to be divisible by  - num % 7 == 0
print(' Whether it can be 3 and 7 to be divisible by :'num % 7==0) and print(num % 3==0)  # False
print(' Whether it can be 3 and 7 to be divisible by :'num % 21==0)					   # False

2.or - Logic or operation

（1） Application scenarios ： Equivalent to or in life , It is used when one of the conditions needs to be met

（2） Operational rules ： As long as one is True The result is True, All two are False The result is False

# Conditions 1 and  Conditions 2
True or True       -   True
True or False      -   True
False or True      -   True
False or False     -   False

gpa = 4.0
score = 95
#  Conditions for scholarship ： Grade point higher than 4 Or the score of gymnastics is not lower than 95
#  Grade point higher than 4: gpa > 4
#  The score of gymnastics is not lower than 95: score >= 95
print(' Can I get a scholarship :', gpa > 4 or score >= 95)	 # True

  practice ： Judge whether the specified year is a leap year 
#  Common leap year ： Can be 4 Divisible but not by 100 to be divisible by : year % 4 == 0 and year % 100 != 0
#  Leap year of the century ： Can be 400 to be divisible by : year % 400 == 0
#  Can be 4 to be divisible by : year % 4 == 0
#  Can not be 100 to be divisible by : year % 100 != 0
#  Can be 400 to be divisible by : year % 400 == 0
year = int(input(' Please enter the year ：'))  #  perhaps year = 2020
print(' Is it a leap year :',(year % 4 == 0 and year % 100 != 0) or year % 400 == 0)

3. not - Logic is not
   not Conditions - A negative condition
   （1） Application scenarios : Negate the specified condition （ Take the opposite ). Generally used for positive conditions, there are many complex situations , When the reverse condition is simple , Conditions are written backwards and then added not
   （2） Operational rules : True change False, False change True

not True    -  False
not False   -  True

#  Older than 18： age > 18
#  Be no older than 18： age <= 18、 not age > 18
num = 18
print(num == 18)
print(not num == 18, num != 18)

#  practice ： Write one that cannot be 3 and 7 The condition of division 
#  Can be 3 Division cannot be 7 to be divisible by 
#  Can be 7 Division cannot be 3 to be divisible by 
#  Can neither be 3 It can't be divided 7 to be divisible by 
print(not num % 21 == 0)

4. Short circuit operation

#  Conditions 1 and  Conditions 2 ---  If the condition 1 yes False, So the conditions 2 Not execute 
#  Conditions 1 or  Conditions 2 ---  If the condition 1 yes True, So the conditions 2 Not execute 
True and print(' And the condition of operation 2')		 #  Both conditions are met 
False and print(' And the condition of operation 2')       #  Conditions 1 dissatisfaction , Direct negation 
False or print(' Or the condition of operation 2')        #  Conditions 1 dissatisfaction , Conditions 2 Satisfy , Choose the conditions 2（ If the condition 1 Satisfy , You don't have to look at the conditions 2 Is it true ）

5. Extension of logical operators - The operand is not a Boolean

data 1 and data 2 - If data 1 The Boolean value of is True, The result is data 2, Otherwise, the result is data 1

data 1 or data 2 - If data 1 The Boolean value of is True, The result is data 1, Otherwise, the result is data 2

print(8 and 7)     # 7
print(10 and 7)     # 7
print(0 and 7)      # 0
print(False and 7)  # False


print(8 or 7)       # 8
print(0 or 7)       # 7

6. Boolean value of data - The result of converting data to Boolean ( Of particular importance )
   python All types of data in have Boolean values .
   All Boolean values corresponding to zero and null values are False, The Boolean values of other data are True.
   Common zero and null values : 0、0.0、0.00、’’、""、[]、()、{}、None.

print(bool(0), bool(0.0), bool(''))			# False False False
print(bool(12), bool(-9.454), bool('ban'))	# True True True

Four 、 Assignment operator ：=、+=、*=、/=、%=、//=、**=

（ important ！） Important conclusions ： The function of all assignment operations is to assign values to variables , Will not produce a result .（ The expression of assignment operation has no result ）
The left side of all assignment operators must be variables

a = 10
10 > 20
# print(a = 10) #  Report errors 
print(10 > 20)

1. =
   Use when defining variables or re assigning values to variables

name = 20
num = 10 * 20

2.=、+=、*=、/=、%=、//=、**=
requirement ： The variable before the assignment symbol must be a defined variable

 Variable  +=  data      ---   Variable  =  Variable  +  data 
b = 10
b += 2
print(b)        # 12

b -= 3
print(b)        # 9

b **= 2
print(b)        # 81

b %= 2
print(b)        # 1

3. Operator precedence
Mathematical operators > Comparison operator > Logical operators > Assignment operator

#  Mathematical operators : ** > * , / , % , // > + , -
#  If there are brackets, count the... In the brackets first 
b = 10 + 30
c = 10 < 5 * 3      #  First calculate  c = 10<15  Count again  c = True
print(c)			# True

Branching structure

1. Process control
1) Sequential structure ( Default ) - The code is executed from top to bottom , Each statement is executed once ;
2) Branching structure - The execution of the code depends on whether the condition is true (if sentence );
3) Loop structure - Let the code repeat (for,while).

print('1')			# 1
print('2')			# 2
print('3')			# 3

2.if Branching structure
（1) if Single branch structure - Perform an operation when conditions are met , If this condition is not satisfied, the operation will not be executed .
a. grammar :
if Conditional statements :
Code segment

b. explain
if - keyword , Fixed writing .
Conditional statements - Any expression with a result , for example : data , Operation expression ( Except assignment ), Function call expression, etc .
: - Fixed writing .
Code segment - That's right. if One or more statements that hold an indent ( At least one ); It is necessary to meet the conditions before executing the code .

c. Execution process
First, judge whether the conditional statement is True( If the result of a conditional statement is not Boolean , Just turn it into Boolean and see ), If True Just execute the code snippet , Otherwise, the code segment will not execute .

a = 100
if a > 10:
    print(' Code 1')		#  Code 1
    print(' Code 2')		#  Code 2
    print(' Code 3')		#  Code 3
print(' Code 4')			#  Code 4

#  practice : If the specified number is even, print ' even numbers '
num = 23
if num % 2 == 0:
    print(' even numbers ')
if not num % 2:
    print(' even numbers ')
#  If the specified number is odd, print ' Odd number '
if num % 2 == 1:
    print('646464')		# 646464

num = 153
a = num % 10
b = num // 10 % 10
c = num // 100
if a**3 + b**3 + c**3 == num:
    print(' This number is narcissus number ')	#  This number is narcissus number 

（2) if Two branch structure - Perform an operation when conditions are met , When this condition is not satisfied, perform another operation

 grammar :
''' if  Conditional statements :  Code segment 1 else:  Code segment 2 '''
   
 
num = 30
if 220 > num:
    print(123)			# 123
else:
    print(321)

age = 12
if age >= 18:
    print(' adult ')
else:
    print(' The child ')	  #  The child 

（3) if Multi branch structure - Do different things according to different conditions
if - elif - elif - … - else

''' if  Conditional statements 1:  Code segment 1 elif  Conditional statements :  Code segment 2 elif  Conditional statements 3:  Code segment three  . . . else:  Code segment N '''    
      
score = 98
if 60 > score:
    print(' Bad ')
elif score < 80:
    print(' commonly ')
elif score < 90:
    print(' good ')
# elif score <= 100:
else:
    print(' optimal ')			#  optimal 

choice question

1. print(100 - 25 * 3 % 4) What should be output ? （B）

   A. 1

   B. 97

   C. 25

   D. 0

2. Which of the following statements is wrong （A）.

   A. Except for dictionary types , All standard objects can ⽤ Yu Boolean test

   B. The Boolean value of an empty string is False

   C. The Boolean value of an empty list object is False

   D. The value is 0 The Boolean value of any numeric object of is False

3. The value of the following expression is True Yes. （B）.

   A. 3>2>2

   B. 1 and 2 != 1

   C. not(11 and 0 != 2)

   D. 10 < 20 and 10 < 5

4. Python No ⽀ The data types held are （A）.

   A. char

   B. int

   C. float

   D. list

5. （ multi-select ）n = 6784, You can get 7 Of ⽅ FA you （C、D）.

   A. n / 1000 % 100

   B. n % 1000 / 100

   C. n // 100 % 10

   D. n // 10 % 100 // 10

6. shipment ⾏ The following procedure , When typing from the keyboard ⼊12, shipment ⾏ The result is （A）.

   x = (input())
   print(type(x))
   

   A. <class 'str'>

   B. <class 'int'>

   C. error

   D. <class 'dict'>

7. The operation result of the following expression is (D ) .

   a = 100
   b = False
   print(a * b > -1)
   

   A. False

   B. 1

   C. 0

   D. True

Completion

1. The function name to view the type of data in the variable is （type（））.
2. It is known that x = 3 == 3 , Of board ⾏ After the end , Variable x The value of is （True）.
3. It is known that x = 3 , So clinging ⾏ sentence x += 6 after ,x The value of is （9）.
4. expression 3 ** 2 The value of is （9）, expression 3 * 2 The value of is （6）, expression 4 ** 0.5 The value of is （2.0）.

Programming questions

1. Write a judgment ⼀ Whether the number can be at the same time 2 and 5 Divisible conditional statement , And print the corresponding results .

   q = 16
   print(q % 2 == 0 and q % 5 == 0)	# False
   
   
   
   

2. Write a judgment ⼀ Whether the number can be 2 perhaps 5 to be divisible by , But not both 2 perhaps 5 Divisible conditional statement , And print the corresponding results .

   q = 35
   input((q % 2 == 0 or q % 5 == 0)and q % 10 != 0)  # True
   
   
   
   

3. Suppose today's class time is 15678 second , Program to calculate the class time today ⼩ when , How many minutes? , How many seconds ; With ‘XX

   when XX branch XX second ’ Of ⽅ Express .

   for example ：100 Seconds are expressed as 0 when 1 branch 40 second

   time=15678
   a=time // 3600
   b=(time - a*3600) // 60
   c=time - a * 3600 - b * 60
   print(a,' when ',b,' branch ',c,' second ')	# 4  when  21  branch  18  second 
   
   
   
   
   

4. Define two variables to save ⼀ individual ⼈ Body ⾼ And weight , The programming implementation judges this ⼈ Is your figure normal !

   The formula : weight (kg) / ( body ⾼(m) The level of ⽅ value ) stay 18.5 ~ 24.9 It's normal .

   Output format ： If there is something wrong ：True/False

   #  weight  = 47kg  height  = 1.65
   if 18.5 <= 47 / (1.65 ** 2) <= 24.9:
       print(' normal ')
   else:
       print(' Is not normal ')		#  Is not normal 
   
   

5. Print according to the range of grades entered pass perhaps fail,

   a = 80
   if a >=60:
       print(' pass ')	
   else:
       print(' fail, ')		#  pass  
   

6. Print according to the entered age range adult perhaps A minor , If the age is not within the normal range (0~150) Print This is not a person !.

   age = int(input(' Please enter age '))
   if 18 <= age <= 150:
   	print(' adult ')
   elif 0 < age < 18:
       print(' A minor ')
   else:
       print(' This is not a person ')
   
   

7. Enter two integers a and b, if a-b The result is an odd number , The result is output , Otherwise, the prompt message will be output a-b The result is not an odd number .

   a = 67
   b = 24
   if (a - b) % 2==1:
   	print(a - b,' Is odd ')
   else:
   	print(a-b, ' It's not an odd number ')		# 43  Is odd