1. Circular nesting and understanding of lists

1. The implementation principle of loop nesting ： External circulation once , Complete internal circulation

for x in range(5):
    for y in range(2, 5):
        print(x, y)


x Value range ：0,1,2,3,4
 The first 1 Time   x=0:   Execute the corresponding for loop , y The value range is : 2,3,4
             The first 1 Time  y=2: print(x, y) -> print(0, 2)
             The first 2 Time  y=3: print(x, y) -> print(0, 3)
             The first 3 Time  y=4: print(x, y) -> print(0, 4)
             End of internal circulation 
 The first 2 Time   x=1:   Execute the corresponding for loop , y The value range is : 2,3,4
             The first 1 Time  y=2: print(x, y) -> print(1, 2)
             The first 2 Time  y=3: print(x, y) -> print(1, 3)
             The first 3 Time  y=4: print(x, y) -> print(1, 4)
             End of internal circulation 

2. What is a list (list)

1) A list is a container data type （ Sequence ）, take []z As a sign of the container , Multiple elements are separated by commas ：[ Elements 1,  Elements 2,  Elements 3,...]
2) The list is variable ( Number of elements 、 Values and order are variable ) -  increase 、 Delete 、 Change ; The list is ordered  -  Subscript operation is supported 
3) List requirements for elements ： No requirement ( No matter what type of data can be used as elements of the list )
'''

1) An empty list 
list1 = []
list2 = [ ]
print(type(list1), type(list2))     # <class 'list'> <class 'list'>
print(bool(list1), bool(list2))     # False False
print(len(list1), len(list2))       # 0 0

2)  The list can save multiple data at the same time 
list3 = [89, 90, 76, 99, 58]
print(list3)

list4 = [' Tan ', 3, 100, True]
print(list4)

3. check - Get elements

There are three situations ： Get a single element 、 section 、 Traverse （ One by one ）

1） Get a single element

 grammar ：  list [ Subscript ]
 function ： Get the element corresponding to the specified subscript in the list 
 explain ：
 list       -        Any expression that results in a list , such as ： Save the variables of the list 、 Specific list values, etc 
[]       -         Fixed writing 
 Subscript       -        A subscript is also called an index , Is the position information of elements in an ordered sequence .
                  python Each element in the ordered sequence in has two sets of subscript values , Namely ： From front to back 0 The subscript value that starts to increment ; From back to front -1 Subscript value that begins to decrease 
 Be careful ： Subscript cannot be out of bounds 

list3 = [89, 90, 76, 99, 58]
list3[ Subscript ]
[89, 90, 76, 99, 58][ Subscript ]

names = [' Diana Spencer ', ' Childcare cord ', ' Primary school hate ', ' Obama ', ' Kalma ', ' Laches ']
print(names[0])			#  Diana Spencer 
print(names[-1])		#  Laches 
print(names[-2])		#  Kalma 
print(names[-5])		#  Childcare cord 
print(names[10])   		#  Report errors 

2） Traverse

The way 1 - Get each element in the list directly

for  Variable  in  list ：
     The loop body 


 The way 2 -  First get the subscript value of each element , Then get the element by subscript 
for  Subscript  in range(len( list )):
     The loop body 

for  Subscript  in range(-1, -len( list )-1, -1):
     The loop body 
    
range(len( list )) == range( The number of elements in the list )


 The way 3 -  At the same time, get the subscript corresponding to each element in the list 
for  Subscript , Elements  in enumerate( list )：
     The loop body 
    
    
print('---------------- Gorgeous divider --------------')
for index in range(len(names)):
    print(index, names[index])

print('---------------- Gorgeous divider --------------')
for index, item in enumerate(names):
    print(index, item)

practice 1: Count the number of people who failed

scores = [89, 67, 56, 90, 98, 30, 78, 51, 99]
count = 0
for x in scores:
    if x < 60:
        count += 1
print(' The number of failures ：', count)

practice 2： Count the number of integers in the list

list7 = [89, 9.9, 'abc', True, 'abc', '10', 81, 90, 23]
count = 0
for x in list7:
    if type(x) == int:
        count += 1
print(count)

practice 3： seek nums The sum of all even numbers in

nums = [89, 67, 56, 90, 98, 30, 78, 51, 99]
sum1 = 0
for x in nums:
    if x % 2 == 0:
        sum1 += x
print(' The sum of all even numbers ：', sum1)

4. increase - Additive elements

1. Add a single element
   list .append( Elements ) - Add an element at the end of the list
   list .insert( Subscript , Elements ) - Inserts the specified element before the element corresponding to the specified subscript

movies = [' Fifty six degrees grey ', ' Godzilla vs. King Kong ', ' Peach blossom Man vs chrysanthemum monster ']
print(movies)       # [' Fifty six degrees grey ', ' Godzilla vs. King Kong ', ' Peach blossom Man vs chrysanthemum monster ']

movies.append(' Shawshank's salvation ')
print(movies)       # [' Fifty six degrees grey ', ' Godzilla vs. King Kong ', ' Peach blossom Man vs chrysanthemum monster ', ' Shawshank's salvation ']

movies.insert(2, ' Silent lamb ')
print(movies)       # [' Fifty six degrees grey ', ' Godzilla vs. King Kong ', ' Silent lamb ', ' Peach blossom Man vs chrysanthemum monster ', ' Shawshank's salvation ']

2） Batch addition

 list 1.extend( list 2)  -  Will list 2 All elements are added to the list 1 Behind 
movies.extend([' Let the bullets fly ', ' Touch and ', 'V For vendetta '])
print(movies)       # [' Fifty six degrees grey ', ' Godzilla vs. King Kong ', ' Silent lamb ', ' Peach blossom Man vs chrysanthemum monster ', ' Shawshank's salvation ', ' Let the bullets fly ', ' Touch and ', 'V For vendetta ']

practice ： take scores Extract all the passing scores in

scores = [89, 67, 56, 90, 98, 30, 78, 51, 99]
scores2 = []
for x in scores:
    if x >= 60:
        scores2.append(x)
print(scores2)

First week homework

One 、 choice question

1. In the following variable names illegal Yes. ？（C）

   A. abc

   B. Npc

   C. 1name

   D ab_cd

2. In the following options Do not belong to The key word is ？（B）

   A. and

   B. print

   C. True

   D. in

3. Which of the following options corresponds to the correct code writing ？（C）

   A.

   print('Python')
     print(' Novice Village ')			#  Incorrect indentation 
   

   B.

   print('Python') print(' Novice Village ')	#  useless , separate 
   

   C.

   print('Python')
   print(' Novice Village ')
   

   D.

   print('Python'' Novice Village ')		#  String is useless , separate 
   

4. The following options can print 50 Yes. ？(B)

   A.

   print('100 - 50')		# 100-50
   

   B.

   print(100 - 50)			
   

5. About Quotes , The correct choice used in the following options is ？(D)

   A.

   print('hello)
   

   B.

   print("hello')
   

   C.

   print(“hello”)
   

   D.

   print("hello")
   

Two 、 Programming questions

1. Write code and print on the console good good study, day day up!

   a = 'good good study,day day up!'
   print(a)
   

2. Write code and print on the console 5 Time you see see, one day day!

   a = 'you see see, one day day!'
   b = 5
   while b > 0:    
   	print(a)    
   	b -= 1
   

3. Write code, print numbers 11、12、13、… 21

   for x in range (11, 22):    
   	print(x)
   

4. Write code, print numbers 11,13,15,17,…99

   for x in range (11, 100, 2):   
   	print(x)
   

5. Write code, print numbers ：10、9、8、7、6、5

   for x in range(10, 4, -1):    
   	print(x)
   

6. Write code calculation ：1+2+3+4+…+20 And

   sum1 = 0for x in range(1, 21):    
   	sum1 += xprint(sum1)					# 210
   

7. Write code calculation 100 The sum of all even numbers within

   sum2 = 0for x in range(2, 100, 2):    
   	sum2 += x
   	print(sum2)					# 2450
   

8. Write code statistics 100~200 The median is 3 The number of

   a = 0for x in range(103, 201, 10):    
   	a += 1
   	print(a)					# 10
   

9. Write code calculation 2*3*4*5*...*9 Result

   b = 1
   for x in range(2, 10):    
   	b *= x
   	print(b)					# 362880
   

10. Enter a number , If the number entered is even, print even numbers Otherwise print Odd number

    c = int(input(' Please enter ：'))
    if c % 2 == 0:    
    	print(' even numbers ')
    else:    
    	print(' Odd number ')
    

11. Statistics 1000 Endogenic quilt 3 Divisible but not by 5 The number of integers .

    d = 0
    for x in range(0, 1000, 3):    
    	if x % 5 != 0:        
    		d += 1
    		print(d)			# 267