尝试绘制迭代，但plt.plot为空 [英] try plotting an iteration but plt.plot is empty

问题描述

a = frame_query("select ....",db)

使用我的代码，变量'c'是我要绘制的numpy.ndarray.但是，当我执行以下代码时，我得到了一个空图！

With my code, the var 'c' is a numpy.ndarray which I would like to plot. However when I execute the following code, i get an empty plot!

for i in a.values:
    c = (i[:1]-a.values[-1:])/a.values[-1:]*100
    plt.plot(c)
plt.show()

print c

给予:

[[ 28.57142857]]
[[ 27.27272727]]
[[ 27.92207792]]
[[ 28.57142857]]
[[ 22.07792208]]
[[ 22.07792208]]
[[ 22.07792208]]

我到底哪里出问题了?

谢谢.

推荐答案

适应代码的最简单方法可能是在循环之前启动一个空列表，将c中的值附加到循环中，然后绘图它在循环之后.例如:

The simplest way to adapt your code is probably to start an empty list before the loop, append the values from c onto it within the loop and then plot it after the loop. For example:

c_series = []

for i in a.values:
    c = (i[:1]-a.values[-1:])/a.values[-1:]*100
    c_series.append(c[0])

plt.plot(c)
plt.show()

在这里使用c[0]的

注意 是安全的，因为上面一行的逻辑保证ndarray仅具有一个成员.

Note using c[0] is safe here, because the logic of the line above guarantees that the ndarray will only have one member.

但是 ，这是处理数据结构的一种奇怪方法.似乎a.values是ndarray，您也可以通过使用numpy提供的对数组执行算术运算的功能来完成此操作(由于我没有您的逐字记录，因此我无法对其进行测试a.values):

However, that's a bit of an odd way to deal with your data structure. As it seems that a.values is an ndarray, you could also simply do this by using the facilities that numpy provides to perform arithmetical operations on arrays (I can't test it as I don't have a verbatim copy of your a.values):

const = a.values[-1:]
c_series = (a.values - const) / const*100
plt.plot(c_series)
plt.show()

通常-使用numpy数组时，通常最好将它们保留为数组(称为向量化代码)，而不是逐个循环地逐个处理它们.

In general - when using numpy arrays, it's often better to keep them as arrays (called vectorising the code), rather than dealing with them element by element in a loop.

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