Day3: branch structure

Branching structure

Application scenarios

so far , We wrote Python The code is executed one by one , This kind of code structure is usually called sequential structure . However, only sequential structure can not solve all problems , For example, we design a game , The first pass condition of the game is that the player obtains 1000 branch , So after finishing this game , We have to decide whether to enter the second level according to the score obtained by the player , Or tell the player “Game Over”, There will be two branches , And only one of these two branches will be executed . There are many similar scenes , We call this structure “ Branching structure ” or “ Selection structure ”. Give everyone a minute , You should be able to think of at least 5 More than one such example , Give it a try .

if Use of statements

stay Python in , To construct a branching structure, you can use if、elif and else keyword . So-called keyword Is a word with a special meaning , image if and else It's a key word used to construct a branching structure , Obviously you can't use it as a variable name （ in fact , You can't use it as any other identifier ）. The following example demonstrates how to construct a branch structure .

"""
 User authentication 

"""
username = input(' Please enter a user name : ')
password = input(' please input password : ')
#  User name is admin And the code is 123456 Then the authentication succeeds or the authentication fails 
if username == 'admin' and password == '123456':
    print(' Authentication succeeded !')
else:
    print(' Authentication failed !')

It should be noted that and C/C++、Java Different languages ,Python Instead of using curly braces to construct code blocks, instead Indentation is used to represent the hierarchy of code , If if If the condition holds, multiple statements need to be executed , Just keep multiple statements with the same indentation . let me put it another way If continuous code keeps the same indentation, they belong to the same code block , It is equivalent to an executive whole . Indent You can use any number of spaces , but Usually use 4 A space , recommend Do not use tab keys perhaps Set your code editing tool to automatically change the tab key to 4 A space .

Of course, if you want to construct more branches , have access to if...elif...else... Structured or nested if...else... structure , The following code demonstrates how to use multi branch structure to realize piecewise function evaluation .

"""
 Piecewise function evaluation 

        3x - 5  (x > 1)
f(x) =  x + 2   (-1 <= x <= 1)
        5x + 3  (x < -1)

"""

x = float(input('x = '))
if x > 1:
    y = 3 * x - 5
elif x >= -1:
    y = x + 2
else:
    y = 5 * x + 3
print('f(%.2f) = %.2f' % (x, y))

Of course, according to the actual development needs , Branch structures can be nested , For example, after judging whether to pass the customs, you should give a level to your performance according to the number of treasures or props you get （ Like lighting up two or three stars ）, Then we need to be in if A new branch structure is constructed inside of , Empathy elif and else We can also construct new branches in , We call it a nested branch structure , In other words, the above code can also be written as the following .

"""
 Piecewise function evaluation 
		3x - 5	(x > 1)
f(x) =	x + 2	(-1 <= x <= 1)
		5x + 3	(x < -1)

"""

x = float(input('x = '))
if x > 1:
    y = 3 * x - 5
else:
    if x >= -1:
        y = x + 2
    else:
        y = 5 * x + 3
print('f(%.2f) = %.2f' % (x, y))

explain ： You can feel which of these two writing methods is better . As we mentioned earlier Python There is a saying in Zen “Flat is better than nested.”, The reason why code is advocated “ flat ” Because the nesting level of the nesting structure will seriously affect the readability of the code , So don't use nesting when you can use flat structures .

practice

practice 1： The English unit inch is interchangeable with the metric unit centimeter .

Refer to the answer ：

"""
 English units inches and metric units centimeters are interchangeable 


"""
value = float(input(' Please enter the length : '))
unit = input(' Please enter the unit : ')
if unit == 'in' or unit == ' Inch ':
    print('%f Inch  = %f centimeter ' % (value, value * 2.54))
elif unit == 'cm' or unit == ' centimeter ':
    print('%f centimeter  = %f Inch ' % (value, value / 2.54))
else:
    print(' Please enter a valid unit ')

practice 2： Conversion of a 100% grade to a graded grade .

requirement ： If the score entered is in 90 More than （ contain 90 branch ） Output A;80 branch -90 branch （ Not included 90 branch ） Output B;70 branch -80 branch （ Not included 80 branch ） Output C;60 branch -70 branch （ Not included 70 branch ） Output D;60 The output is as follows E.

Refer to the answer ：

"""
 Conversion of a 100% grade to a graded grade 

"""
score = float(input(' Please enter the grade : '))
if score >= 90:
    grade = 'A'
elif score >= 80:
    grade = 'B'
elif score >= 70:
    grade = 'C'
elif score >= 60:
    grade = 'D'
else:
    grade = 'E'
print(' The corresponding level is :', grade)

practice 3： Enter three side lengths , If you can form a triangle, calculate the perimeter and area .

Refer to the answer ：

"""
 Judge whether the input side length can form a triangle , If so, calculate the perimeter and area of the triangle 

"""
a = float(input('a = '))
b = float(input('b = '))
c = float(input('c = '))
if a + b > c and a + c > b and b + c > a:
    print(' Perimeter : %f' % (a + b + c))
    p = (a + b + c) / 2
    area = (p * (p - a) * (p - b) * (p - c)) ** 0.5
    print(' area : %f' % (area))
else:
    print(' Can't make a triangle ')

explain ： The formula used above to calculate the area of a triangle by its side length is called Helen's formula .

Relevant code has been uploaded to https://download.csdn.net/download/weixin_43510203/86242452