Any simple or complex algorithm can be constructed by sequential structure 、 The three basic structures of selection structure and cycle structure are combined
The program executes code sequentially from top to bottom , There is no judgment or jump in the middle , Until the end of the program
Python Everything is the object , All objects have a Boolean value
The Boolean values of the following objects are False
print(bool(False))
print(bool(0))
print(bool(0.0))
print(bool(None))
print(bool(''))
print(bool(""))
print(bool([])) # An empty list
print(bool(list())) # An empty list
print(bool(())) # An empty tuple
print(bool({
})) # An empty dictionary
print(bool(dict())) # An empty dictionary
print(bool(set())) # Empty set
print('------------ Boolean values for other objects are True-----------------')
print(bool(18))
print(bool("0"))
Output ：
False
False
False
False
False
False
False
False
False
False
False
False
------------ Boolean values for other objects are True-----------------
True
True
The program selectively executes part of the code according to the Boolean value of the judgment condition
Clearly let the computer until under what conditions , What to do
Chinese semantics ： If … Just …
If it rains , Take an umbrella
Grammatical structure ：
if Conditional expression ：
Conditional executors
# Judge whether the balance is sufficient
if money >=s:
money=money-s
print(' Successful withdrawals , The balance is ：',money)
Input 100, Output ：
Please enter the withdrawal amount ：100
Successful withdrawals , The balance is ： 900
Input 1200, Don't go ahead
Chinese semantics ：
If … dissatisfaction … Just …
If you win the prize , If you don't win, you won't receive
Grammatical structure ：
if Conditional expression :
Conditional executors 1
else:
Conditional executors 2
# Two branch structure if...else... Choose one of the two
num=int(input(' Please enter an integer ：'))
# conditional
if num%2==0:
print(num,' It's even ')
else:
print(num,' Is odd ')
Input 9, Output ：
Please enter an integer ：9
9 Is odd
Input 10, Output ：
Please enter an integer ：10
10 It's even
Chinese semantics
The result is 90 Above ？ No
The result is 80 To 90 Between points ？ No
The result is 70 To 80 Between points ？ No
The result is 60 To 70 Between points ？ No
The result is 60 Is it below ？ yes
Grammatical structure ：
if Conditional expression 1:
Conditional executors 1
elif Conditional expression 2:
Conditional executors 2
elif Conditional expression N:
Conditional executors N
[else:]
Conditional executors N+1
Square brackets indicate whether to write or not
Code example ：
''' Multi branch structure , Select one more to execute 90-100 A 80-89 B 70-79 C 60-69 D 0 -59 E Less than 0 Or greater than 100 Is an illegal value '''
score= int(input(' Please enter a score ：'))
# Judge
if score >=90 and score <=100:
print('A level ')
elif score >=80 and score <= 89:
print('B level ')
elif score >= 70 and score <= 79:
print('C level ')
elif score >=60 and score <= 69:
print('D level ')
elif score >=0 and score <=59:
print('E level ')
else:
print(' I'm sorry , Wrong grades , Not within the valid range of the grade ')
Please enter a score ：99
A level
Please enter a score ：66
D level
Please enter a score ：122
I'm sorry , Wrong grades , Not within the valid range of the grade
Writing transformation
''' Multi branch structure , Select one more to execute 90-100 A 80-89 B 70-79 C 60-69 D 0 -59 E Less than 0 Or greater than 100 Is an illegal value '''
score= int(input(' Please enter a score ：'))
# Judge
if 90 <=score <=100:
print('A level ')
elif 80<=score <= 89:
print('B level ')
elif 70<=score <= 79:
print('C level ')
elif 60<=score <= 69:
print('D level ')
elif 0<=score <=59:
print('E level ')
else:
print(' I'm sorry , Wrong grades , Not within the valid range of the grade ')
Output ：
Please enter a score ：100
A level
Grammatical structure ：
if Conditional expression 1:
if Inner conditional expression :
Inner conditional actuator 1
else:
Inner conditional actuator 2
else:
Conditional executors
Code example
''' members >= 200 8 fold >= 100 9 fold Non members >= 9.5 fold No discount '''
answer= input(' Are you a member ？y/n')
money=float(input(' Please enter your purchase amount ：'))
# If you are a member
if answer=='y':
if money>=200:
print(' hit 8 fold , The amount of payment is ：',money*0.8)
elif money>=100:
print(' hit 9 fold , The amount of payment is ：',money*0.9)
else:
print(' No discount , The amount of payment is ：',money) # Although a member , But the amount is too small to discount
else: # Non members
if money >=200:
print(' hit 9.5 fold , The amount of payment is ：',money*0.95)
else:
print(' No discount , The amount of payment is ：',money)
Output ：
Are you a member ？y/ny
Please enter your purchase amount ：2000
hit 8 fold , The amount of payment is ： 1600.0
Are you a member ？y/ny
Please enter your purchase amount ：100
hit 9 fold , The amount of payment is ： 90.0
#pass sentence , Don't do anything? , Just a placeholder , Use where you need to write a statement
answer = input(' Are you a member ？y/n')
# Judge whether you are a member
if answer=='y':
pass
else:
pass
Preemption , Post completion
age = int(input(' Please enter your age :'))
if age: # No need to interpret , Use the Boolean value of the object directly
print(age)
else:
print(' Age is ：',age)
Output ：
Please enter your age :18
18
