Python comprehension and ternary operator

列表推導式

List comprehensions simplify list looping

• 格式：表達式 for 變量 in 列表 [if 條件語句]（The content in square brackets can be omitted）
• 使用方式：

"""Quickly generate an include1-20數字的列表"""
list1 = [i for i in range(1,21)]
"""Quickly generate an include1-20之間能被3Divisible list of all numbers"""
list2 = [i for i in range(1,21) if i%3==0]
"""Quickly generate an include1-20之間能被3List of squares of all numbers that are divisible"""
list3 = [i*i for i in range(1,21) if i%3==0]

• 替代for循環和filter函數：Filter out all even numbers in the list

test = [1,2,3,4,5,6,7,8,9,10]
"""Use a normal loop"""
list1 = []
for i in test:
if i%2==0:
list1.append(i)
"""使用filter"""
list2 = list(filter(lambda i:i%2==0,test))
"""使用列表推導式"""
list3 = [i for i in test if i%2==0]

• 替代for循環和map函數：Converts all strings in the list to integers

test = ["1","2","3","4","5","6","7","8","9","10"]
"""Use a normal loop"""
list1 = []
for i in test:
list1.append(int(i))
"""使用filter"""
list2 = list(map(lambda i:int(i),test))
"""使用列表推導式"""
list3 = [int(i) for i in test]

• 注意：List comprehensions work for single-level loops,When there are two or more layers of loops, using the derivation will be counterproductive,Although the code is simplified,But it will make the content of the simplified code obscure and difficult to understand,結構也不清晰,Very inappropriate to read and modify.

"""Use a normal loop"""
list1 = []
for i in range(1,5):
for j in range(1,i):
if j%2 == 0:
list1.append((i,j))
"""使用列表推導式"""
list2 = [(i,j) for i in range(1,5) for j in range(1,i) if j%2==0]

字典推導式

Dictionary comprehensions simplify dictionary loops

• 格式：{v: k for k, v in 字典 [if 條件語句]} （The content in square brackets can be omitted）
• 舉例1：將字典中keyValues ​​are converted to uppercase

test = {
"a":5,"B":10,"c":15,"D":20}
"""Use a normal loop"""
dict1 = {
}
for k,v in test.items():
dict1[k.upper()] = v
"""使用字典推導式"""
dict2 = {
k.upper():v for k,v in test.items()}

• 舉例2：Filter out all in the dictionaryvalue是偶數的鍵值對

test = {
"a":5,"B":10,"c":15,"D":20}
"""Use a normal loop"""
dict1 = {
}
for k,v in test.items():
if v%2==0:
dict1[k] = v
"""使用字典推導式"""
dict2 = {
k:v for k,v in test.items() if v%2==0}

三目運算符

The ternary operator can simplify selection

• 格式：exp1 if 條件語句 else exp2

• 運行過程：if the conditional statement is true,just execute and returnexp1,Otherwise, execute and return exp2

"""簡化前"""
if a>b:
max = a;
else:
max = b;
"""簡化後"""
max = a if a>b else b