数组比较与numpy中的元素比较不匹配

Question

我有一个 numpy 数组 arr。这是一个numpy.ndarray，大小是(5553110,)，dtype=float32。

当我这样做时：

(arr > np.pi )[3154950]
False
(arr[3154950] > np.pi )
True

为什么第一次比较就出错了？我该如何解决？

值：

arr[3154950]= 3.1415927
np.pi= 3.141592653589793

精度有问题吗？

Answer 1

问题是由于 np.float32 与 np.float64 的准确性有关。

使用np.float64，你不会发现问题：

import numpy as np

arr = np.array([3.1415927], dtype=np.float64)

print((arr > np.pi)[0])  # True

print(arr[0] > np.pi)    # True

---

正如@WarrenWeckesser 评论的那样：

It involves how numpy decides to cast the arguments of its operations. Apparently, with arr > scalar, the scalar is converted to the same type as the array arr, which in this case is np.float32. On the other hand, with something like arr > arr2, with both arguments nonscalar arrays, they will use a common data type. That's why (arr > np.array([np.pi]))[3154950] returns True.

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