python Introduction learning :6. User input and while loop

Key points : Input 、while loop

6.1 function input() working principle 6.2 while Introduction to cycle 6.3 Use while Loop through dictionaries and lists

6.1 function input() working principle

function input() Pause the program , Wait for the user to enter some text . function input() Take a parameter : That is, the prompt or instruction to be displayed to the user , Let the user know what to do .

1message = input("Tell me something, and I will repeat it back to you:")
2print(message)   # Returns the value entered

6.1.1 Use int() Get numeric input
Using functions input() when ,python Interpret user input as a string .

 1age = input("How old are you? ")
2print(age)                        # Input age = '21'
3if age > 18 :                     # Strings cannot be compared with integers
4    print("you are old")
5else:
6    print("you are too young")
7
8How old are you? 23
923
10Traceback (most recent call last):
11  File "chapter6.py", line 6, in <module>
12    if age > 18 :
13TypeError: unorderable types: str() > int()

adopt int() Function to convert the obtained string to an integer

 1age = input("How old are you? ")
2age = int(age)
3print(age)                        
4if age > 18 :                    
5    print("you are old")
6else:
7    print("you are too young")
8How old are you? 23
923
10you are old

6.1.2 Modulo operators
Modulo operators (%) It's a very useful tool , It divides two numbers and returns the remainder :

14 % 3
21     # Remainder is 1
35 % 3
42     # Remainder is 2
56 % 3
60     # Remainder is 0

6.2 while Introduction to cycle

6.2.1 Use while loop
for Loop a block of code for each element in the collection , and while The cycle goes on and on , Until the specified conditions are met .

1current_number = 1
2while current_number <=5:
3    print(current_number)
4    current_number +=1

6.2.2 Let the user choose when to quit
You can use while Loop allows the program to run continuously when the user wants to , Control exit through user input

1prompt = "\nTell me something,and I will repeat it back to you:"
2prompt +="\nEnter 'quit' to end the program.\n"
3message = ""
4while message != 'quit':
5    message = input(prompt)
6    if message != 'quit':
7        print(message)

6.2.3 Use logo
occasionally , Many conditions are required to be met before the program continues to run , You can define a variable , Used to determine whether a program is active , This variable is called the flag , Act as traffic lights for the program .

1active = True
2while active:
3    message = input(prompt)
4    if message == 'quit'
5        active = False
6    else:
7        print(message)

6.2.4 Use break Exit loop
Use keywords break You can end the current loop immediately

1prompt = "\nPlease enter the name of a city you have visited:"
2prompt+= "\n(Enter 'quit' when you are finished.) "
3while True:
4    city = input(prompt)
5    if city == 'quit'
6        break;
7    else:
8        print("I'd love to go to " + city.title() +"!")

6.2.5 Use... In a loop continue
Use continue End this cycle , Returns when the loop starts .

1current_number = 0;
2while current_number < 10:
3    current_number +=1
4    if current_number %2 == 0:
5        continue
6    print(current_number)

6.3 Use while Loop through dictionaries and lists

for Loops are an effective way to traverse lists , But in for The list should not be modified in the loop , Otherwise, it will lead to python It's hard to track its elements . To modify the list while traversing , You can use while loop .
6.3.1 Move elements between lists
Suppose there is a list , It contains newly registered but not yet verified website users ; After verifying the user , How to transfer them to another verified user list ?

 1unconfirmed_users = ['alice','brain','candace']
2confirmed_users = []
3while unconfirmed_users:
4    current_user = unconfirmed_users.pop()
5    print("Verfying user: "+ current_user.title())
6    confirmed_users.append(current_user)
7# Show all authenticated users
8print("\nThe following users have been confirmed:")
9for confirmed_user in confirmed_users:
10    print(confirmed_user.title())

6.3.2 Delete all list elements that contain specific values
To delete multiple duplicate elements in a list, you can use while loop

1pet = ['dog','cat','dog','goldfish','cat','rabbit','cat']
2print(pets)
3while 'cat' in pets:
4    pets.remove('cat')
5print(pets)

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