在Python中的循环中创建和使用多个变量 [英] Create and use multiple variables in a loop in Python
问题描述
我正在尝试用Python编写一个循环,该循环创建变量,并且还使用创建的尚未创建的变量来生成线性方程组.这是到目前为止我得到的:
I'm trying to write a loop in Python that creates variables and also uses created an not-yet-created variables to produce a system of linear equations. This is what I've got so far:
P=[[0, 0, 0, 0.5, 0, 0.5],
[0.1, 0.1, 0, 0.4, 0, 0.4],
[0, 0.2, 0.2, 0.3, 0, 0.3],
[0, 0, 0.3, 0.5, 0, 0.2],
[0, 0, 0, 0.4, 0.6, 0],
[0, 0, 0, 0, 0.4, 0.6]]
for j in range(6):
globals()['x%s' % j] = 0
for i in range(6):
globals()['x%s' % j] += P[i][j]*globals()['x%s' % i]
但是由于最后一行,我得到了KeyError
输出.我猜是因为还没有定义第二个变量(globals()['x%s' % i]
),但是我不知道如何在稍后要定义它们时使用等号右边的变量.
but I get a KeyError
output because of the last line. I guess it's because the second variable ( globals()['x%s' % i]
) is not defined yet but I don't know how to use the variables to the right of the equals sign when they are going to be defined later.
推荐答案
首先,不要不要为此使用globals()
(或者实际上几乎不使用).创建自己的字典以存储您动态创建的值:
First of all, do not use globals()
for this (or in fact almost never); create your own dictionary to store your dynamically created values:
vars = {}
vars['blabla'] = 123
尽管得到KeyError
的原因是,您试图从globals()
读取不存在的变量(但这也将与自定义字典一起发生).这行:
The reason you are getting the KeyError
though is that you're trying to read a variable from globals()
that doesn't exist (but this would also happen with a custom dict). This line:
globals()['x%s' % j] += P[i][j]*globals()['x%s' % i]
实际上是以下简称:
globals()['x%s' % j] = globals()['x%s' % j] + P[i][j]*globals()['x%s' % i]
但尚未定义globals()['x%s' % j]
.因此,您正在尝试向不存在的内容中添加内容.
but globals()['x%s' % j]
is not defined yet. So you're trying to add something to something that doesn't exist.
相反,在执行+=
操作之前,您需要执行以下操作:
Instead, you need to do something like the following before you do the +=
operation:
if 'x%s' % j not in globals():
globals()['x%s' % j] = 0
但是,如果正确执行并使用dict
和其他一些增强功能,它将看起来像这样:
BUT, if you do it properly and use a dict
, and some other enhancements, it would look like this:
# it's nicer to use floats uniformly, not a mix of ints and floats;
# it's also safer because in Python 2.x, int divided by float will yield a floored down int
P = [[0.0, 0.0, 0.0, 0.5, 0.0, 0.5],
[0.1, 0.1, 0.0, 0.4, 0.0, 0.4],
[0.0, 0.2, 0.2, 0.3, 0.0, 0.3],
[0.0, 0.0, 0.3, 0.5, 0.0, 0.2],
[0.0, 0.0, 0.0, 0.4, 0.6, 0.0],
[0.0, 0.0, 0.0, 0.0, 0.4, 0.6]]
vars = {}
for j in range(len(P)):
vars['x%s' % j] = 0
for i in range(len(P[0])):
vars.setdefault('x%s' % j, 0.0) # shorthand for the `if` I described
vars['x%s' % j] += P[i][j] * vars['x%s' % i]
此外,由于vars
似乎最终只包含键x0
至x5
,所以我宁愿使用list
来代替
Also, since vars
seems to end up just containing the keys x0
to x5
, I would rather use a list
instead;
X = [0] * len(P) # creates [0, 0, 0, 0, 0, 0]
for j in range(len(P)):
for i in range(len(P[0])):
X[j] += P[i][j] * X[i]
这样,您甚至不需要setdefault
调用.
This way, you don't even need the setdefault
call.
但是请记住,我对您要实现的数学算法/计算并不熟悉,因此我也无法发现您的代码中存在任何非技术性的错误或缺点.
Keep in mind, though, that I'm not familiar with the mathematical algorithm/computation you're trying to implement, so I'm also not able to spot any non-technical bugs or shortcomings in your code.
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