Python 列表生成式

列表生成式是一种基于其他 iterable（如集合、元组、其他列表等）创建列表的方法。它还可以用更简单、更吸引人的语法表示 for 和 if 循环

语法格式如：list = [expression for iterm in iterable (if conditional)]

• (if conditional) 中的 if 语句可有可无，但是前面的 expression 和 for 循环表达式必须存在
• 注意细节
  • for 循环末尾的表达式放在列表的开头

1、实例如下：

words = ['data','science','machine','learning']
# 计算出 words 列表中每个字符串的长度

a = []
for word in words:
    a.append(len(word))
print(a)

# 列表生成式做法
b = [len(word) for word in words]
print(b)

# 结果如下
[4, 7, 7, 8]
[4, 7, 7, 8]

  2、列表生成式中带有 if 语句实例如下：

words = ['data', 'science', 'machine', 'learning']
# 取出 words 列表中长度大于5的字符串长度
a = []
for word in words:
    if len(word) > 5:
        a.append(word)
print(a)

# 列表生成式做法
b = [word for word in words if len(word) > 5]
print(b)

# 结果如下
['science', 'machine', 'learning']
['science', 'machine', 'learning']

•   在列表生成式中带有多个 for 循环，表示 for 循环嵌套

words = ['data', 'science', 'machine', 'learning']
# 找出列表中所有字符串中的所有元音字母

a = []
for word in words:
    for letter in word:
        if letter in ["a","e","i","o","u"]:
            a.append(letter)
print(a)

# 列表生成式做法
b = [letter for word in words for letter in word if letter in ["a","e","i","o","u"]]
print(b)

words = ['data', 'science', 'machine', 'learning']
# 找出列表中所有字符串中的所有元音字母,并将所有的元音字母进行大写

b = [letter.upper() for word in words for letter in word if letter in ["a","e","i","o","u"]]
print(b)

# 结果如下
['A', 'A', 'I', 'E', 'E', 'A', 'I', 'E', 'E', 'A', 'I']

words = ['data', 'science', 'machine', 'learning']
# 找出 machine 中 c 的索引值

li = []
for word in words:
    if word == "machine":
        for letter in word:
            if letter == "c":
                li.append(word.index(letter))
print(li)

# 列表生成式做法
a = [word.index(letter) for word in words if word == "machine" for letter in word if letter == "c"]
print(a)

# 结果如下
[2]
[2]

• 在列表生成式中传入调用函数

def func(x):
    return x * 2

s = [func(a) for a in range(5)]
print(s)

# 结果如下：
[0, 2, 4, 6, 8]