[英]Iterating through array
I have an array of bools and now I want to swap those entries for numbers. 我有一系列bools,现在我想将这些条目换成数字。
False => 0
True => 1
I have written two different pieces of code and I would like to know, which one is better and why. 我写了两段不同的代码,我想知道哪一个更好,为什么。 This is not so much about actually solving the problem, as about learning.
这不是关于实际解决问题,而是关于学习。
arr = [[True,False],[False,True],[True,True]]
for i,row in enumerate(arr):
for j,entry in enumerate(row):
if entry:
arr[i][j] = 1
else:
arr[i][j] = 0
print(arr)
And the second approach: 第二种方法:
arr = [[True,False],[False,True],[True,True]]
for i in range(len(arr)):
for j in range(len(arr[i])):
if arr[i][j]:
arr[i][j] = 1
else:
arr[i][j] = 0
print(arr)
I read that there are ways to do this with importing itertools
or similar. 我读到有很多方法可以通过导入
itertools
或类似方法来实现。 I am really not a fan of importing things if it can be done with “on-board tools”, but should I rather be using them for this problem? 如果可以使用“板载工具”,我真的不喜欢导入东西,但我是否应该使用它们来解决这个问题?
Let's define your array: 让我们定义你的数组:
>>> arr = [[True,False],[False,True],[True,True]]
Now, let's convert the booleans to integer: 现在,让我们将布尔值转换为整数:
>>> [[int(i) for i in row] for row in arr]
[[1, 0], [0, 1], [1, 1]]
Alternatively, if we want to be more flexible about what gets substituted in, we can use a ternary statement: 或者,如果我们想要更加灵活地替换什么,我们可以使用三元语句:
>>> [[1 if i else 0 for i in row] for row in arr]
[[1, 0], [0, 1], [1, 1]]
If you want to stay with a for-loop (eg because you want to mutate the existing array instead of creating a new one), you should simplify the code. 如果你想继续使用for循环(例如,因为你想改变现有的数组而不是创建一个新数组),你应该简化代码。
I would first simplify the outer loop by removing the indexing (there is no need for it since it's even easier to modify a row than a nested array): 我首先通过删除索引来简化外部循环(不需要它,因为它比嵌套数组更容易修改行):
for row in arr:
for j, entry in enumerate(row):
if entry:
row[j] = 1
else:
row[j] = 0
these kinds of simple if statement can often be simplified by using an if expression: 通过使用if表达式,通常可以简化这些简单的if语句:
row[j] = 1 if entry else 0
but in this case we can do even better. 但在这种情况下,我们可以做得更好。
bool
is a subclass of int
(ie. all bool
's are int
's), and True
and False
are defined to be exactly 1
and 0
respectively -- if you scroll down to the specification section of PEP 285 ( https://www.python.org/dev/peps/pep-0285/ ) you'll see that that equivalence is not accidental but very much by design. bool
是int
的子类(即所有bool
都是int
的),而True
和False
分别定义为1
和0
- 如果向下滚动到PEP 285的规范部分( https:// www.python.org/dev/peps/pep-0285/ )你会发现这种等价不是偶然的,而是非常多的设计。
We can therefore use the int
constructor to grab the underlying integer values[*], since int(True) == 1
and int(False) == 0
, the if-expression can be simplified to: 因此我们可以使用
int
构造函数来获取底层整数值[*],因为int(True) == 1
和int(False) == 0
,if-expression可以简化为:
row[j] = int(entry)
[*] technically this is an explicit upcast to a base class, and not a conversion constructor.. [*]从技术上讲,这是对基类的显式向上转换,而不是转换构造函数。
The simplified code: 简化代码:
for row in arr:
for j, entry in enumerate(row):
row[j] = int(entry)
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