Python 如何对 numpy 数组进行条件数组算术？
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How do I do conditional array arithmetic on a numpy array?
提问by pr0crastin8r
I'm trying to get a better grip on numpy arrays, so I have a sample question to ask about them:
我试图更好地掌握 numpy 数组，所以我有一个示例问题要问他们：
Say I have a numpy array called a. I want to perform an operation on a that increments all the values inside it that are less than 0 and leaves the rest alone. for example, if I had:
假设我有一个名为 a 的 numpy 数组。我想对 a 执行一个操作，该操作会增加其中所有小于 0 的值，并保留其余的值。例如，如果我有：
a = np.array([1,2,3,-1,-2,-3])
I would want to return:
我想返回：
([1,2,3,0,-1,-2])
What is the most compact syntax for this?
最紧凑的语法是什么？
Thanks!
谢谢！
采纳答案by unutbu
In [45]: a = np.array([1,2,3,-1,-2,-3])
In [46]: a[a<0]+=1
In [47]: a
Out[47]: array([ 1, 2, 3, 0, -1, -2])
回答by John Kugelman
To mutate it:
要改变它：
a[a<0] += 1
To leave the original array alone:
不理会原始数组：
a+[a<0]
回答by Suhail Abdul Rehman Chougule
Just to add to above,In numpy array I wanted to subtract a value based on the ascii value to get a value between 0 to 35 for ascii 0-9 and A-Z and had to write the for loops but the post above showed me how to make it short. So I thought of posting it here as thanks to the post above.
只是为了添加到上面，在 numpy 数组中，我想根据 ascii 值减去一个值，以获得 0 到 35 之间的 ascii 0-9 和 AZ 值，并且不得不编写 for 循环，但上面的帖子向我展示了如何让它简短。所以我想把它贴在这里，感谢上面的帖子。
The code below can be made short
下面的代码可以缩短
i = 0
for y in y_train:
if y < 58:
y_train[i] = y_train[i]-48
else :
y_train[i] = y_train[i] - 55
i += 1
i = 0
for y in y_test:
if y < 58:
y_test[i] = y_test[i]-48
else :
y_test[i] = y_test[i] - 55
i += 1
# The shortened code is below
y_train[y_train < 58] -= 48
y_train[y_train > 64] -= 55
y_test[y_test < 58] -= 48
y_test[y_test > 64] -= 55
