https://www.machinelearningplus.com/python/101-numpy-exercises-python/ How to create a boolean array np.ones((3,3), dtype=bool)

复制数组
np.r_[np.repeat(a, 3), np.tile(a, 3)]

■ How to get the common items between two python numpy arrays:
取相同数据·  
np.intersect1d(a,b)

■去除在B中的数据：
np.setdiff1d(a,b)  

去除a中重复数据：
np.unique(a)  

How to make a python function that handles scalars to work on numpy arrays:
np.vectorize(funobj)  

How to swap two columns in a 2d numpy array:
arr[:,[2,1,0]]  

How to extract all numbers between a given range from a numpy array?:  
a = np.arange(15)

# Method 1
index = np.where((a >= 5) & (a <= 10))
a[index]

# Method 2:
index = np.where(np.logical_and(a>=5, a<=10))
a[index]
#> (array([6, 9, 10]),)

# Method 3: (thanks loganzk!)
a[(a >= 5) & (a <= 10)]

How to print only 3 decimal places in python numpy array?
np.set_printoptions(precision=3)
How to pretty print a numpy array by suppressing the scientific notation (like 1e10)?
np.set_printoptions(suppress=True)
如何向 Python NumPy 导入包含数字和文本的数据集，同时保持文本不变？
How to import a dataset with numbers and texts keeping the text intact in python numpy?
np.genfromtxt(url, delimiter=',', dtype='object')
How to find the percentile scores of a numpy array?
np.percentile(arr,[50])
How to find the correlation between two columns of a numpy array?
np.corrcoef(iris[:, 0], iris[:, 2])
■ How to convert a numeric to a categorical (text) array?
如何将一个数值转换为一个类别（文本）数组？
petal_length_bin = np.digitize(iris[:, 2].astype('float'), [0, 3, 5, 10])
Map it to respective category

label_map = {1: 'small', 2: 'medium', 3: 'large', 4: np.nan}
petal_length_cat = [label_map[x] for x in petal_length_bin]
■维度增加（1维变为2维，行数不变，增加一列）
volume = volume[:, np.newaxis]
How to get the positions of top n values from a numpy array?
np.argsort()[-n:]
np.argpartition(-a, n)[:n]
np.sort(a)[-n:]
np.partition(a, kth=-n)[-n:]
如何在 2 维 NumPy 数组中找到每一行的最大值
np.amax(arr, axis=1)
np.apply_along_axis(np.max, arr,axis=1)
np.argmax(arr,axis=1)
如何在一个 1 维数组中找到所有的局部极大值（peak）？
doublediff=np.diff(np.sign(np.diff(arr)))
np.where(doublediff==-2)[0]+1
从字符数组中随机选取N个
np.take(list('abcdefgh'), np.random.randint(N, size=30))
np.random.random　と　np.random.randの違いは何？
▲42. How to do probabilistic sampling in numpy?

如何创建一个 Python 函数以对 NumPy 数组执行元素级的操作？
pair_max = np.vectorize(maxx, otypes=[float])
a = np.array([5, 7, 9, 8, 6, 4, 5])
b = np.array([6, 3, 4, 8, 9, 7, 1])
pair_max(a, b)

能力大评估
https://www.machinelearningplus.com/python/101-pandas-exercises-python/