1.使用列表推导计算笛卡儿积

```>>> colors = ['black', 'white']
>>> sizes = ['S', 'M', 'L']
>>> tshirts = [(color, size) for color in colors for size in sizes] ➊
>>> tshirts
[('black', 'S'), ('black', 'M'), ('black', 'L'), ('white', 'S'),
('white', 'M'), ('white', 'L')]

#julia除了表示字符串要用双引号，其余照抄
julia>  colors = ['black', 'white']
ERROR: syntax: invalid character literal

julia>  colors = ["black", "white"]
2-element Array{String,1}:
"black"
"white"

julia> sizes = ["S", "M", "L"]
3-element Array{String,1}:
"S"
"M"
"L"

julia> tshirts = [(color, size) for color in colors for size in sizes]
6-element Array{Tuple{String,String},1}:
("black", "S")
("black", "M")
("black", "L")
("white", "S")
("white", "M")
("white", "L")
```

2.生成器表达式

```>>> colors = ['black', 'white']
>>> sizes = ['S', 'M', 'L']
>>> for tshirt in ('%s %s' % (c, s) for c in colors for s in sizes):  ➊
...     print(tshirt)
...
black S
black M
black L
white S
white M
white L
#julia除了输出格式字符串不同，其余照抄
julia> colors = ["black", "white"]
2-element Array{String,1}:
"black"
"white"

julia> sizes = ["S", "M", "L"]
3-element Array{String,1}:
"S"
"M"
"L"

julia> for tshirt in ("\$c, \$s" for c in colors for s in sizes)
println(tshirt)
end
black, S
black, M
black, L
white, S
white, M
white, L
#生成器表达式可以赋值给一个变量
julia> ge=("\$c, \$s" for c in colors for s in sizes)
Base.Iterators.Flatten{Base.Generator{Array{String,1},##17#19}}(Base.Generator{A
rray{String,1},##17#19}(#17, String["black", "white"]))

julia> for tshirt in ge
println(tshirt)
end
black, S
black, M
black, L
white, S
white, M
white, L
```