08 various derivations

List comprehensions

NOTE: Use () returns the derivations generator object. You may be used tuple (generator object) cast tuple. It will be a one-time all the data read memory.

#[每一个元素或者是和元素相关的操作 for 元素 in 可迭代数据类型]    #遍历之后挨个处理

#[满足条件的元素相关的操作 for 元素 in 可迭代数据类型 if 元素相关的条件]   #筛选功能

#30以内所有能被3整除的数
ret = [i for i in range(30) if i%3 == 0]  #完整的列表推导式  

#30以内所有能被3整除的数的平方
ret = [i*i for i in (1,2,3,4) if i%3 == 0]

# 例三:找到嵌套列表中名字含有两个‘e’的所有名字
names = [['Tom', 'Billy', 'Jefferson', 'Andrew', 'Wesley', 'Steven', 'Joe'],
         ['Alice', 'Jill', 'Ana', 'Wendy', 'Jennifer', 'Sherry', 'Eva']]

# 列表推导式
ret = [name for lst in names for name in lst if name.count('e') ==2]

# =====等同于======

names_temp = []
for lst in names:
    for name in lst:
        if name.count('e') == 2:
            names_temp.append(name)
print(names_temp)

Dictionary derivations

# 例一：将一个字典的key和value对调
mcase = {'a': 10, 'b': 34}
#{10:'a' , 34:'b'}
mcase_frequency = {mcase[k]: k for k in mcase}
print(mcase_frequency)


# 例二：合并大小写对应的value值，将k统一成小写
mcase = {'a': 10, 'b': 34, 'A': 7, 'Z': 3}
#{'a':10+7,'b':34,'z':3}
mcase_frequency = {k.lower(): mcase.get(k.lower(), 0) + mcase.get(k.upper(), 0) for k in mcase}
print(mcase_frequency)

Set derivations

The results comes with deduplication feature

squared = {x**2 for x in [1, -1, 2]}
print(squared)