This series of notes for learning Python for yourself, if there are errors, please correct me
Select result
The selection structure determines which branch to execute by judging whether the condition is true. There are many forms of selection structure, divided into: single branch, double branch, and multiple branches. The flow chart is as follows:
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Single branch selection structure
The grammatical form of the single branch result of the if statement is as follows:
if conditional expression:
Statement/Block
among them:
- Conditional expressions: can be logical expressions, relational expressions, arithmetic expressions, etc.
- Statement/statement block: It can be one statement or multiple statements. Multiple statements, indentation must be aligned
num = input('请输入一个数字:')
if int(num)<10:
print(num)
Detailed Explanation of Conditional Expressions
In the selection and loop structure, the value of the conditional expression is False as follows:
False 0 0.0 Null value None, empty sequence object (empty list, empty set, empty dictionary, empty string), empty range object, empty iteration object
All other cases are True.
if 3: #整数作为条件表达式
print('ok')
a = [] #列表作为表达式 空列表为False
if a:
print('空列表 False')
s = 'False' #非空字符串
if s:
print('非空字符串')
c = 9
if 3<c and c<20:
print("3<c and c<20")
if True: #布尔值
print("True")
Conditional expressions cannot have assignment operators =
Double branch selection structure
if conditional expression:
Statement block/statement
else:
Statement
num = input('请输入一个数字')
if int(num)<10:
print(num)
else:
print('数字太大')
Ternary conditional operator
Python provides a ternary operator, used in some simple double-branch assignment situations, the ternary conditional operator syntax:
The value if the condition is true if (conditional expression) else the value when the condition is false
num = input('请输入一个数字')
print(num if int(num)<10 else '数字太大')
Multi-branch selection structure
if conditional expression 1:
Statement/Block
elif conditional expression 2:
Statement/Block
…
else:
Statement/Block
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score = int(input('请输入分数'))
grade = ''
if score<60:
grade = '不及格'
elif score>=60 and score<70:
grade='及格'
elif score>=70 and score<80:
grade = '良好'
else:
grade = '优秀'
print('分数是{0}，等级是{1}'.format(score,grade))
Select structure nesting
The selection structure can be nested. When using it, you must pay attention to controlling the indentation of different levels of code blocks, because the indentation determines the subordination of the code.
if expression 1:
Statement block 1
if expression 2:
Statement block 2:
else:
Statement block
else:
Statement block
Cyclic structure
The loop structure is used to repeatedly execute one or more statements. Expression logic: If the conditions are met, the statements in the loop body are executed repeatedly. After each execution, it will be judged whether the condition is True, and if it is True, the loop will be repeated.
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The statement in the loop body should at least contain a statement that changes the conditional expression, so that the loop tends to end, otherwise, it will become an endless loop.
while loop
while 判断条件(condition)： 执行语句(statements)……
n = 100
sum = 0
counter = 1
while counter <= n:
sum = sum + counter
counter += 1
print("1 到 %d 之和为: %d" % (n,sum))
Infinite loop
We can achieve an infinite loop by setting the conditional expression to never be false
var = 1
while var == 1 : # 表达式永远为 true
num = int(input("输入一个数字 :"))
print ("你输入的数字是: ", num)
print ("Good bye!")
You can use CTRL+C to exit the current infinite loop.
The infinite loop is very useful for real-time client requests on the server.
for loop and iterable object traversal
The for loop is usually used to traverse an iterable object. The syntax format of for loop is as follows:
for variable in iterable object:
Loop body statement
Iterable object
- Sequence: the ancestor of a list of strings
- dictionary
- Iterator alignment
- Generator object
for x in list('slp'):
print(x)
d = {
'name':'slp','age':18,'address':'bj'}
for x in d: #遍历所有的key
print(x)
for x in d.keys():#遍历字典所有的key
print(x)
for x in d.values():#遍历字典所有的value
print(x)
for x in d.items():#遍历字典所有的键值对
print(x)
range object
The range object is an iterator object that uses a sequence of numbers to generate a specified range. The format is: range(start,end [,step])
sum_all=0
sum_even=0
sum_odd=0
for num in range(101):
sum_all +=num
if num%2 ==0:
sum_even+=num
else:
sum_odd+=num
print(sum_all,sum_even,sum_odd)
Nested loop
A loop can be embedded in another loop, generally called a nested loop, or multiple loops
eg: print 99 multiplication table:
for m in range(1,10):
for n in range(1,m+1):
print(m,'*',n,'=',m*n ,end='\t')
print('')
break statement
The break statement can be used in while and for loops to end the entire loop. When there is nesting, the break statement can only jump out of the nearest loop
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while True:
a = input('输入Q退出')
if a.upper() == 'Q':
print('退出')
break
else:
print('继续')
continue statement
continue is used to end this loop and continue to the next time. The nesting is also the application and the most recent loop.
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n = 5
while n > 0:
n -= 1
if n == 2:
continue
print(n)
print('循环结束。')
empNum = 0
salarySum = 0;
salarys = []
while True:
s = input('请输入员工的工资（q或Q结束）')
if s.upper() =='Q':
print('录入完成 退出')
break;
if float(s)<0:
continue;
empNum+=1
salarys.append(float(s))
salarySum +=float(s)
print('员工数{0}'.format(empNum))
print('录入薪资',salarys)
print('平均薪资{0}'.format(salarySum/empNum))
pass statement
Python pass is an empty statement to maintain the integrity of the program structure.
pass does not do anything, it is generally used as a placeholder statement,
Loop code optimization
- Minimize unnecessary calculations inside the loop
- In the nested loop, try to reduce the count of the inner loop, and lift it outward as much as possible
- Local variable query is faster, try to use local variables
Other optimization methods
- Concatenate multiple strings, use join instead of +
- Insert and delete elements in the list, try to operate at the end
Use zip() to iterate in parallel
We can use the zip() function to iterate multiple sequences in parallel, and the zip() function will stop when the shortest sequence is used up
names = ('a','b','c')
ages =(16,10)
jobs = ('teacher','programmer','drive')
for name,age,job in zip(names,ages,jobs):
print("{0}--{1}--{2}".format(name,age,job))
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