Python conditional statements and loop statements
Tip: the following is the main content of this article. The following cases are for reference. Some cases come from the Internet and the copyright belongs to the original author.
1, Conditional statement
1.if
Syntax:
if boolean_expression:
statement(s)
If Boolean_ If expression (Boolean expression) returns True, the statement of the if block is executed. Otherwise, the statement is not executed, and the program continues to execute the statement (if any) after the if statement.
Example 1: conditional statement (Boolean expression) is True
a = 2 b = 5 if a<b: print(a, 'Is less than', b)
Example 2: conditional statement (Boolean expression) is False
a = 2 b = 5 if a<b: print(a, 'Is less than', b) # 2 is less than 5
The condition provided in the if statement evaluates to false, so the statement in the if block will not be executed.
Example 3: a conditional statement (Boolean expression) has multiple conditions
Use logical operators (and, or, not) to connect multiple conditions and create composite conditions.
a = 2 b = 5 c = 4 if a<b and a<c: print(a, 'Is less than', b, 'and', c) # 2 is less than 5 and 4
Example 4: Boolean expression evaluates to a number
If the expression in the if statement evaluates to a number, the statement is executed if the number is non-zero. Zero is considered false and non-zero (positive or negative) is considered true.
a = 2 if a: print(a, 'is not zero') # 2 is not zero.
Example 5: if conditional statement block contains multiple statements
a = 2
if a:
print(a, 'is not zero')
print('And this is another statement')
print('Yet another statement')
Output:
2 is not zero And this is another statement Yet another statement
Example 6: nested if
a = 2 if a!=0: print(a, 'is not zero.') if a>0: print(a, 'is positive.') if a<0: print(a, 'is negative.')
2.if else
Syntax:
if boolean_expression:
statement(s)
else:
statement(s)
Example 1: condition is True
a = 2 b = 4 if a<b: # 2 < 4 returns True, so the if block is executed print(a, 'is less than', b) else: print(a, 'is not less than', b)
Example 2: condition is False
a = 5 b = 4 if a<b: # 5 < 4 returns False, so else block is executed print(a, 'is less than', b) else: print(a, 'is not less than', b)
Example 3: nested if else
a = 2 b = 4 c = 5 if a<b: print(a, 'is less than', b) if c<b: print(c, 'is less than', b) else: print(c, 'is not less than', b) else: print(a, 'is not less than', b)
3.elif
Syntax:
if boolean_expression_1: statement(s) elif boolean_expression_2: statement(s) elif boolean_expression_3: statement(s) else statement(s)
Example 1: if elif else
a = 2 b = 4 if a<b: print(a, 'is less than', b) elif a>b: print(a, 'is greater than', b) else: print(a, 'equals', b)
Example 2: if elif elif else: with multiple elif blocks
a = 2
if a<0:
print(a, 'is negative')
elif a==0:
print('its a 0')
elif a>0 and a<5:
print(a, 'is in (0,5)')
else:
print(a, 'equals or greater than 5')
4.if and
Example 1: if conditional expression with and operator
a = 5
b = 2
# Nested if
if a==5:
if b>0:
print('a is 5 and',b,'is greater than zero.')
# Alternatively, combine multiple conditions into one expression
if a==5 and b>0:
print('a is 5 and',b,'is greater than zero.')
Example 2: if else conditional expression with and operator
a = 3
b = 2
if a==5 and b>0:# If the condition holds, execute here
print('a is 5 and',b,'is greater than zero.')
else:# If the condition does not hold, execute here
print('a is not 5 or',b,'is not greater than zero.')
Example 3: elif conditional expression with and operator
a = 8
if a<0:
print('a is less than zero.')
elif a>0 and a<8:
print('a is in (0,8)')
elif a>7 and a<15:
print('a is in (7,15)')
5.if or
Example 1: if conditional expression with or operator
today = 'Saturday'
if today=='Sunday' or today=='Saturday':
print('Today is off. Rest at home.')
Example 2: if else conditional expression with or operator
today = 'Wednesday'
if today=='Sunday' or today=='Saturday':
print('Today is off. Rest at home.')
else:
print('Go to work.')
Example 3: elif conditional expression with or operator
today = 'Sunday'
if today=='Monday':
print('Your weekend is over. Go to work.')
elif today=='Sunday' or today=='Saturday':
print('Today is off.')
else:
print('Go to work.')
6.if not
Syntax:
if not value:
statement(s)
- value is bool type:
If value yes bool Type, then NOT Acts as a negative operator. If value by False，be not value Will be True，also if-block The statement in will execute. If value by True，be not value Will be False，also if-block Statements in will not be executed.
- value is string type:
If value yes string Type, then if-block The statements in will be string Execute when empty.
- value is list type:
If value yes list Type, then if-block The statements in will be list Execute when empty. The same interpretation applies to values of other collection data types: dict,set and tuple.
Example 1: not - bool
a = False
if not a:
print('a is false.')
Example 2: not - string
string_1 = ''
if not string_1:
print('String is empty.')
else:
print(string_1)
Example 3: not - list
a = []
if not a:
print('List is empty.')
else:
print(a)
Example 4: not - dict
a = dict({})
if not a:
print('Dictionary is empty.')
else:
print(a)
Example 5: not - set
a = set({})
if not a:
print('Set is empty.')
else:
print(a)
Example 6: not - tuple
a = tuple()
if not a:
print('Tuple is empty.')
else:
print(a)
7. Ternary operator
https://blog.csdn.net/weixin_40458518/article/details/120477844
2, Circular statement
It is often used for loop iteration of (range, list, tuple, dictionary, set, string) sequences and sets.
1.for loop
Syntax:
for item in iterable:
statement(s)
1.1 for loop of range
for i in range(25,29): print(i)
1.2 for loop of string
mystring = 'python.org' for x in mystring: print(x)
1.3 for loop of list
mylist = ['python', 'programming', 'examples', 'programs'] for x in mylist: print(x)
1.4 for loop of tuple
mytuple = ('python', 'programming', 'examples', 'programs')
for x in mytuple:
print(x)
1.5 for loop of Dict
mydictionary = {'name':'python', 'category':'programming', 'topic':'examples'}
for x in mydictionary:
print(x, ':', mydictionary[x])
1.6 for loop of set
myset = {'python', 'programming', 'examples'}
for x in myset:
print(x)
2.while loop
Syntax:
while condition:
statement(s)
Example 1: while loop printing 1 to N
n = 4 i = 1 while i <= n: print(i) i+=1
3.break: interrupt
----------------------------- for-break -----------------------------
Example 1: in the for loop, break completely interrupts the program
for x in range(2, 10):
if(x==7):
break
print(x)
Example 2: in the for loop, break completely interrupts the program, and the subsequent else statement block is not executed, which is invalid code.
for x in range(1, 11) :
if x == 4:
break # The program is completely interrupted, and the subsequent else statement block is not executed, which is invalid code.
print(x)
else:
print('else Code in does not execute/invalid')
Example 3: list as a for loop object
myList = [1, 5, 4, 9, 7, 2]
for x in myList :
if x == 9:
break
print(x)
----------------------------- while-break -----------------------------
Example 1: in the while loop, break completely interrupts the program
a = 4 i = 0 while i<a: print(i) i+=1 if i>1: break
Example 2: in the while loop, break completely interrupts the program, and the subsequent else statement block is not executed, which is invalid code.
i = 1
while i <= 10 :
if i == 4 :
break # The program is completely interrupted, and the subsequent else statement block is not executed, which is invalid code.
print(i)
i += 1
else:
print('else Code in does not execute/invalid')
4.continue: skip
----------------------------- for-continue -----------------------------
Example 1: in the for loop, skip the loop
for x in range(2, 10):
if(x==7):
continue
print(x)
Example 2: in the for loop, skip the loop
for x in range(1, 11) :
if x%3 == 0 :
continue # The else statement block after the for loop is not affected
print(x)
else:
print('This line of code is in for Execute before the end of the cycle')
----------------------------- while-continue -----------------------------
Example 1: in a while loop, skip the loop
a = 4 i = 0 while i<a: if i==2: i+=1 # If the conditional variable increment is not added in the while loop, the loop will die and the program will run continuously continue print(i) i+=1
Example 2: in a while loop, skip the loop
i = 1
while i <= 10 :
if i == 4 or i==7 :
i += 1
continue
print(i)
i += 1
5.else: for loop or while loop, the code block executed before exiting
5.1 for-else
Example 1: use else statement block after for loop
for x in range(2, 6):
print(x)
else:
print('Out of for loop')
Example 2: break will completely interrupt the program. The else statement block is invalid and will not be executed
for x in range(1, 11) :
if x == 4:
break # The program is completely interrupted, and the subsequent else statement block is not executed, which is invalid code.
print(x)
else:
print('else Code in does not execute/invalid')
5.2 while-else
While else syntax:
while condition:
statement(s)
else:
statement(s)
Example 1: use else statement block after while loop
i = 0
while i < 5:
print(i)
i += 1
else:
print('Printing task is done.')
Example 2: use else statement block to close file handle after while loop
f = open("sample.txt", "r")
while True:
line = f.readline()
if not line:
break
print(line.strip())
else:
f.close
6. Nested loop
6.1 for nesting
for x in range(5):
for y in range(6):
print(x, end=' ')
print()
6.2 while nesting
flag = True
while flag: # Layer 1 (outer) while loop
username = input("enter one user name:")
password = input("Please enter the user password:")
if username == 'admin' and password == '123':
while flag: # Second layer (inner layer) while loop
command = input("Enter the to perform linux Code command:")
if command.lower() == 'q':
flag = False # Change the change and control the while loop of the first layer (outer layer) and the second layer (inner layer) at the same time
print("Executing command:%s"%command)
else:
print('User name or password input error')