多个if和elif之间的区别？

Question

在python中,说是否有区别:

if text == 'sometext':
    print(text)
if text == 'nottext':
    print("notanytext")

和

 if text == 'sometext':
        print(text)
 elif text == 'nottext':
        print("notanytext")

只是想知道多个ifs是否会导致任何不必要的问题,以及是否更好的做法使用elifs.

Answer 1

多个if表示你的代码会检查所有if条件,如同elif一样,如果条件满足,则不检查其他条件.

Answer 2

另一个简单的方法来看看使用if和elif之间的区别就是这个例子:

def analyzeAge( age ):
   if age < 21:
       print "You are a child"
   if age >= 21: #Greater than or equal to
       print "You are an adult"
   else:   #Handle all cases where 'age' is negative 
       print "The age must be a positive integer!"

analyzeAge( 18 )  #Calling the function
>You are a child
>The age must be a positive integer!

在这里你可以看到,当18用作输入时,答案是(令人惊讶的)2个句子.那是错的.它应该只是第一句话.

那是因为BOTH if语句正在评估中.计算机将它们视为两个单独的语句:

• 第一个是真的18,所以"你是一个孩子"印刷.
• 第二个if语句为false,因此执行else部分打印"age必须是一个正整数".

该ELIF解决这个问题并且让两个if语句"粘在一起"作为一个:

def analyzeAge( age ):
   if age < 21:
       print "You are a child"
   elif age > 21:
       print "You are an adult"
   else:   #Handle all cases where 'age' is negative 
       print "The age must be a positive integer!"

analyzeAge( 18 )  #Calling the function
>You are a child

Answer 3

def multipleif(text):
    if text == 'sometext':
        print(text)
    if text == 'nottext':
        print("notanytext")

def eliftest(text):
    if text == 'sometext':
        print(text)
    elif text == 'nottext':
        print("notanytext")

text = "sometext"

timeit multipleif(text)
100000 loops, best of 3: 5.22 us per loop

timeit eliftest(text)
100000 loops, best of 3: 5.13 us per loop

你可以看到elif稍快一些.如果有更多的ifs和更多的elifs,这将更加明显.

Answer 4

这是另一种思考方式：

假设您有两个特定条件，if/else catchall 结构是不够的：

例子：

我有一个 3 X 3 井字棋盘，我想打印两条对角线的坐标，而不是其余方块的坐标。

我决定改用 if/elif 结构......

for row in range(3):
    for col in range(3):
        if row == col:
            print('diagonal1', '(%s, %s)' % (i, j))
        elif col == 2 - row:
            print('\t' * 6 + 'diagonal2', '(%s, %s)' % (i, j))

输出是：

diagonal1 (0, 0)
                        diagonal2 (0, 2)
diagonal1 (1, 1)
                        diagonal2 (2, 0)
diagonal1 (2, 2)

可是等等！我想包括 diagonal2 的所有三个坐标，因为 (1, 1) 也是对角线 2 的一部分。

The 'elif' caused a dependency with the 'if' so that if the original 'if' was satisfied the 'elif' will not initiate even if the 'elif' logic satisfied the condition as well.

Let's change the second 'elif' to an 'if' instead.

for row in range(3):
    for col in range(3):
        if row == col:
            print('diagonal1', '(%s, %s)' % (i, j))
        if col == 2 - row:
            print('\t' * 6 + 'diagonal2', '(%s, %s)' % (i, j))

I now get the output that I wanted because the two 'if' statements are mutually exclusive.

diagonal1 (0, 0)
                        diagonal2 (0, 2)
diagonal1 (1, 1)
                        diagonal2 (1, 1)
                        diagonal2 (2, 0)
diagonal1 (2, 2)

Ultimately knowing what kind or result you want to achieve will determine what type of conditional relationship/structure you code.

Answer 5

elif只是一种奇特的表达方式else: if，

多个if测试后执行多个分支，而elif互斥，测试后实际执行一个分支。

以user2333594为例

    def analyzeAge( age ):
       if age < 21:
           print "You are a child"
       elif age > 21:
           print "You are an adult"
       else:   #Handle all cases were 'age' is negative 
           print "The age must be a positive integer!"

可以改写为：

    def analyzeAge( age ):
       if age < 21:
           print "You are a child"
       else:
             if age > 21:
                 print "You are an adult"
             else:   #Handle all cases were 'age' is negative 
                 print "The age must be a positive integer!"

另一个例子可能是：

def analyzeAge( age ):
       if age < 21:
           print "You are a child"
       else: pass #the if end
       if age > 21:
           print "You are an adult"
       else:   #Handle all cases were 'age' is negative 
           print "The age must be a positive integer!"