Process control

Process control is the control process, specifically refers to the execution process of the control program, and the execution process of the program is divided into three structures:

• Sequential structure: run from top to bottom (the code we wrote before is sequential structure)

• Branch structure: different processes may be executed according to different conditions during operation (if judgment is used)

• Loop structure: some code needs to be executed repeatedly during operation (using while and for)

 

 

Branch structure: if judgment

Must know must know:

• 1. Conditions are converted to Boolean values, which determine whether the code is executed
• 2. Indents are used in python to represent dependencies of code
• 3. Not all code can have subcodes
• 4. Multiple lines of subcode belonging to one of your codes must be indented the same amount
• 5. In python, it is recommended to use the same indentation (four spaces represent an indentation) to identify a group of code blocks (if the end of the previous line of code is a colon, the next line of code must be indented)

What is a branch structure

The branch structure is to execute the sub code corresponding to different branches according to the true or false conditions

Why use a branch structure

In order to make the computer do different things according to the judgment results of conditions like people

Implement branch structure with if keyword

if Condition 1:  
    ...
    Sub code block executed after condition 1 is true

elif Condition 2: 
    ...
    The sub code block executed after condition 1 is not true and condition 2 is true
    
else: 
    ...
    If the above conditions are not met, go this section

be careful:

• The condition can be any expression, but the execution result must be Boolean. In if judgment, all data types will be automatically converted to Boolean

  • None, 0, null (empty string, empty list, empty dictionary, etc.) the Boolean value converted to is False
  • The rest are True

Single if branch structure

if condition:
    ...
    Sub code block executed after the condition is established
    print("end...")
 
# Case: young man under 22
age = 20
if age < 22: 
    print("A guy with no hair")

 

 

 

if..else (dual branch)

if condition:
    ...
    Sub code block executed after the condition is established
else:
    ...
    if If the conditions are not established, go this section

# Case: guess age
age = 22
if age < 20:
    print("Hello, young man")
else:
    print("Stupid")

 

 

 

 

if...elif...else (multi branch)

 

if Condition 1:
    ...
    Sub code block executed after condition 1 is true
elif Condition 2:
    ...
    The sub code block executed after condition 1 is not true and condition 2 is true
elif Condition 3:
    ...
    The sub code block executed after condition 1 and 2 are not true and condition 3 is true
else:
    ...
    If none of the above conditions holds, go this paragraph
    
# Case:
# If achievement >= 90: excellent
# If achievement >= 80 And< 90: good
# If achievement >= 70 And< 80: ordinary
# Other conditions: very poor

score = 83
if score >= 90:
    print("excellent")
elif score >= 80:
    print("good")
elif score >= 70:
    print("ordinary")
else:
    print("Very bad")

 

 

Nesting of if judgment

# We must combine pictures and texts to digest, understand and absorb
# Case: continue on the basis of blind date. If hand in hand succeeds, then: together, otherwise...
age = 26
height = 165
weight = 99
is_success = True
if age < 28 and height > 160 and weight < 100:
    print('Can my little sister hold hands')
    # Judge whether the little sister will hold hands
    if is_success:
        print('It's dark at dinner and movies...')
    else:
        print('Fuck your sister's pervert!')
else:
    print('it's a pity')

Loop structure: while loop

• What is a while loop

  • Loop is to do something repeatedly. while loop is the first loop mechanism provided by Python, which is called conditional loop

• Why is there a while loop

  • In order to control computers, they can do something repeatedly like people

 

 

 

Basic syntax of while loop

while condition:
    ...
    The sub code block executed by the loop after the condition is established
""" 
while Operation steps:
    1,If the condition is true, the sub code blocks are executed in turn
    2,Judge the condition again after execution. If the condition is True The sub code block is executed again if the condition is False，Then the cycle ends
"""   
while True:
    # 1.Get the user name and password entered by the user
    username = input('enter one user name>>>:')
    password = input('Please input a password>>>:')
    # 2.Determine whether the user name and password are correct
    if username == 'jack' and password == '123':
        print('Login succeeded!')
    else:
        print('Login failed, please re-enter!')
        
# Note: dead cycle--->The condition is always True

 

 

Two ways to end a while loop

Method 1: Global flag bit

• Feature: it will not take effect until the body code of this cycle is run, and the judgment conditions of the next cycle

# Use of flag bits
count = 0
flag = True

while tag:
    if count == 5:
      flag = False  # Change the condition to False
    print(i)
    count += 1

Mode 2: break

• Feature: break immediately ends the while loop of this layer

count = 0
tag = True

while tag:
    if count == 5:
        # tag = False
        break
    print(i)
    count += 1

while nesting

while loop nesting + use of global flag bits

• For nested multi-layer while loops, if our purpose is to directly exit the loops of all layers at a certain layer, there is a trick. The "global flag bit" makes the conditions of all while loops use the same variable, and the initial value of the variable is True. Once the value of the variable is changed to False at a certain layer, the loops of all layers end

flag = True
while flag:  # First layer circulation
    while flag:  # Second layer circulation
      flag = False  # tag Become False，All while The conditions of the cycle become False
      
# Case:
flag = True
while flag:
    inp_user = input("enter one user name>>>: ")
    inp_pwd = input("Please input a password>>>: ")

    if inp_user == "jack" and inp_pwd == "123":
        print('Login succeeded')
        # break
        flag = False
    else:
        print('Wrong user name or password')   

Use of while loop nesting + break

• If the while loop is nested in many layers, you need to have a break in each layer of the loop to exit each layer of the loop

while True:  # First layer circulation
    while True:  # Second layer circulation
        while True:  # Third layer circulation
            break  # Terminate the third layer cycle
        break  # Terminate the second layer cycle
    break  # Terminate the first layer cycle
          
# Case:
while True:
    inp_user = input("enter one user name>>>: ")
    inp_pwd = input("Please input a password>>>: ")
    
    if inp_user == "jack" and inp_pwd == "123":
        print('Login succeeded!')
        while True:
            print("""
            0 sign out
            1 withdraw money
            2 deposit
            3 transfer accounts
            """)
            choice = input("Please enter your command number:")
            if choice == "0":
                break
            elif choice == "1":
                print("Withdrawing money...")
            elif choice == "2":
                print("Depositing...")
            elif choice == "3":
                print("Transferring...")
            else:
                print("The input instruction does not exist")
        break
    else:
        print('Wrong user name or password')

 

while+continue: skip this cycle and directly enter the next cycle

# use while Loop print out 0-5 But do not print 2 and 4
# 1.Define a starting variable
count = 0
# 2.loop
while count < 6:
    if count == 2 or count == 4:
        count += 1
        continue  # continue After the same level, don't write code, and it won't run
    # 3.Print the value of the variable
    print(count)
    # 4.Variable value increases by 1
    count += 1
    
"""
continue It will let the loop body code directly return to the condition judgment for re judgment
"""

# Case:
while True:
    inp_user = input("username>>>: ")
    inp_pwd = input("password>>>: ")
        # Determine whether the user name and password are correct
    if inp_user == "jack" and inp_pwd == "123":
        print('Login succeeded') # Enter the correct password and log in successfully
        break # End cycle
    else:
        print('Wrong user name or password')
        # continue  # Be sure not to add it to the last step

while+else: the sub code block of else will run when the while loop dies normally. It is called normal death if it is not killed by break

# Case 1: normal death and normal circulation
count = 0
while count < 5:
    if count == 2:
        count += 1
        continue
    print(count)
    count += 1
else:
    print('====>')

# Case 2: abnormal death, if it is break，It won't be executed else Statement of
count = 0
while count < 5:
    if count == 2:
        break
    print(count)
    count += 1
else:
    print('====>')

Dead loop: condition is always True

while True:
    print(1)
    
"""Dead circulation will make CPU Extremely busy and even running"""

Small exercise:

"""
Age guessing game
    General requirements
        Users can guess wrong three times. If they guess right in the process, they can exit directly
    Pull up requirements
        After three opportunities are used up, the user will be prompted whether to continue trying. If yes, another three opportunities will be given. Otherwise, it will end directly
"""
real_age = 18  # Define fixed age variables
count = 0  # Define input times variable
while True:
    # Three opportunities will be executed
    if count == 3:
        choice = input("Three opportunities have been used up! Continue?\n Please enter again'q'，Exit, please enter't'>>>:")
        if choice == 'q' or choice == 'Q':
            count = 0  # Set the number of guesses to 0
        else:
            print('The game is over'.center(35, '='))
            break
    print('Age guessing game'.center(35, '*'))
    age = int(input('Please enter age>>>: '))
    if age < real_age:
        print('Harm! Guess small, you also have:%s A chance!' % (2 - count))
    elif age > real_age:
        print('Harm! Guess big, you also have:%s Second chance,!' % (2 - count))
    else:
        print('Good guy, you guessed right. That's great!')
        break
    count += 1

Loop structure: for loop

• 1. What is a for loop

  • Loop is to do something repeatedly. for loop is the second loop mechanism provided by python

• 2. Why a for loop

  • Theoretically, the while loop can do whatever the for loop can do
  • The reason why there is a for loop is that the for loop is more concise in loop value (traversal value) than the while loop

for loop basic syntax

for Variable name in Iteratable object:  # Iteratable objects can be strings, lists, dictionaries, tuples, and collections
    ...
    for Loop body code
    
# Example 1
for i in ['a','b','c']:
    print(i)
# Operation results
a
b
c

# Refer to example 1 for Operation steps of cycle
# Step 1: from the list['a','b','c']Read out the first value and assign it to item(item='a')，Then execute the loop body code
# Step 2: from the list['a','b','c']Read out the second value and assign it to item(item='b')，Then execute the loop body code
# Step 3: Repeat the above process until the values in the list are read out    
for Basic usage of loop

# Case 1: traversing the list

names = ["kevin", "tom", "jack", "lili", "lxx", "xxx"]
for x in names:
    print(x)
for x,y in [["name","kevin"],["age",18],["gender","male"]]:  # x,y = ['name', 'kevin']
    print(x,y)


# Case 2: traversing the dictionary
dic={"k1":111,'K2':2222,"k3":333}
for x in dic:  # for The loop defaults to the dictionary key Assign to variable name k
    print(x,dic[x])


# Case 3: traversal string
for x in "hello":
    print(x)
msg="you can you up,no can no bb"
for x in msg:
    print(x)

 

• Summarize the similarities and differences between for loop and while loop

  • 1,Similarities:
    It's all a cycle, for Cycle what you can do, while Circulation can also be dry
    2,Differences:
    while Loops are called conditional loops, and the number of loops depends on when the condition becomes false
    for The cycle is called"Value cycle"，Number of cycles depends on in Number of values contained after

     

for+break: the same as the while loop

for i in [11,22,33,44,55]:
    if i == 33:
        break
    print(i)

 

for+continue: same as while loop

for i in [11,22,33,44,55]:
    if i == 33:
        continue
    print(i)

 

for+else: same as while loop

for i in [11,22,33,44,55]:
    if i == 33:
       break
    print(i)
else:
    print('++++++++>')

 

range keyword

for + range control cycle times: range()

• There are limitations in placing a data type directly after in to control the number of cycles:
  • When the number of cycles is too many, the format of the value contained in the data type needs to be increased

# range Function introduction
# First kind:A parameter starts from 0, regardless of the head and tail
for i in range(10):
     print(i)
    
# Second:Two parameters customize the starting position, regardless of the head and tail
for i in range(4, 10):
     print(i)
    
# Third:The third number of the three parameters is used to control the equal difference value
for i in range(2, 100, 10):
    print(i)

"""
Expand knowledge
    https://movie.douban.com/top250 page 1
    https://movie. douban. com/top250? Start = 25 & filter = second page
    https://movie. douban. com/top250? Start = 50 & filter = page 3
    https://movie. douban. com/top250? Start = 75 & filter = page 4
    
    https://movie. douban. com/top250? Start = 0 & filter = guess first page
"""
base_url = "https://movie.douban.com/top250?start=%s&filter="
for i in range(0, 250, 25):
    print(base_url % i)
    
""" 
range It is essentially different in different versions of the interpreter
    stay python2.X in range A list is generated directly
        stay python2.X One of them xrange It's also an iterator(Old sow)

stay python3.X in range Is an iterator(Old sow)  Save memory space
      python2.X in xrange namely python3.x Inside range
"""    
# python2 Version demo
'''   
>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> 
>>> range(1,9) # 1...8
[1, 2, 3, 4, 5, 6, 7, 8]
>>> 
>>> range(1,9,1) # 1 2 3 4 5 6 7 8 
[1, 2, 3, 4, 5, 6, 7, 8]
>>> range(1,9,2) # 1 3 5 7 
[1, 3, 5, 7]
'''


for i in range(30):
    print('===>')
username='kevin'
password='123'
for i in range(3):
    inp_name = input('Please enter your account number:')
    inp_pwd = input('Please enter your password:')

    if inp_name == username and inp_pwd == password:
        print('Login succeeded')
        break
else:
    print('Too many times of entering wrong account and password')

 

enumerate keyword

for + enumerate

i,name=(0,"kevin")
for i,name in enumerate(["kevin","tom","jack"]):  # [(0,"kevin"),(1,"tom"),(2,"jack")]
    print(i,name)


for x in enumerate({"k1":111,"k2":2222,"k3":3333}):
    print(x)

Nested use of for loops

copy# for i in range(3):
#     for j in range(5):
#         print("*", end='')
#     print()


for i in range(1, 10):
    for j in range(1, i + 1):
        print('%s*%s=%s' % (i, j, i * j), end=' ')
    print()