Python programming from entry to practice Chapter 5: if statement and conditional test

1. Condition test

1.1 Detect multiple conditions (and / or)

keywords used

1. and : both conditions are true
2. or : at least one condition is met

example

age_o=20
age_1=22
print(age_o>=21 and age_1<=22,age_o>=21 or age_1<=22)

1.2 Check if a specific value is contained in a list

keywords used

1. Determine if a specific value is in a column: in
2. Judging that a specific value is not in the list: not in

example

color = ['green','red','blue','yellow','pink']
if 'pink' in color:
    print('yes!')
if 'dark' not in color:
    print('sorry!')

1.3 if statement structure

1. single if statement
2. if-else statement
3. if-elif-else statement

Two, if statement processing list

2.1 Determine whether the list is empty

Three ways to judge:

• Directly use the conditional expression if + list, if the list is not empty, the if list expression returns true, otherwise returns false
• Check whether the list == []
• Judging by column length

list_1 = []
if list_1:
    print('This is not a empty list!')
else:
    print('This is a empty list!')

if list_1==[]:
    print('This is a empty list!')
else:
    print('This is not a empty list!')

if len(list_1)==0:
    print('This is a empty list!')
else:
    print('This is not a empty list!'

2.2 Exercises

the code

print('5-8')
usernames = ['root','admin','jack','peter','diane']
for username in usernames:
    if username == 'admin':
        print('Hello admin, would you like to see a status report?')
    else:
        print('Hello Jaden, thank you for logging in again.')

print('5-9')
usernames.clear()
if usernames:
    for username in usernames:
        if username == 'admin':
            print('Hello admin, would you like to see a status report?')
        else:
            print('Hello Jaden, thank you for logging in again.')
else:
    print('We need to find some users!')

print('5-10')
current_users = ['jack','peter','will','admin','diane']
new_users = ['mary','peter','will','lili','frank']
for new_user in new_users:
    for current_user in current_users:
        if new_user == current_user:
            print(f'Dear {
      
      new_user.title()}, you need to change a username!')
            break;
    else:
        print(f'Dear {
      
      new_user.title()}, this username is ready to use!')
else:
    print('All the usernames are finished checked!')

print('5-11')
numbers = list(range(1,10))
for number in numbers:
    if number==1:
        print('first')
    elif number == 2:
        print('second')
    elif number==3:
        print('third')
    else:
        print(f'{
      
      number}th')

output

Note: How to clear the list

1. Use the clear method
2. Let the list == [] directly
3. Use the del statement: del [:], without specifying the start and end indexes, delete all elements and clear
   the list