NumPy系统是Python的一种开源的数值计算扩展。这种工具可用来存储和处理大型矩阵，比Python自身的嵌套列表（nested list structure)结构要高效的多（该结构也可以用来表示矩阵（matrix））。
说明
使用 arr 代表 一个numpy数组对象
导入模块
导入/导出
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| np.loadtxt(‘file.txt’) | 从text文件读取 |
| np.genfromtxt(‘file.csv’,delimiter=',') | 从CSV文件读取 |
| np.savetxt(‘file.txt’,arr,delimiter=' ‘) | 写入到text文本文件 |
| np.savetxt(‘file.csv’,arr,delimiter=’,') | 写入到CSV文件 |
创建数组
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| np.array([1,2,3]) | 一维数组 |
| np.array([(1,2,3),(4,5,6)]) | 二维数组 |
| np.zeros(3) | 长度为3数值都为0的一维数组 |
| np.ones((3,4)) | 3x4数组，所有值为1 |
| np.eye(5) | 对角线为1其余为0的5x5数组 (单位矩阵) |
| np.linspace(0,100,6) | 从0到100的6个值的数组 |
| np.arange(0,10,3) | Array of values from 0 to less than 10 with step 3 (eg [0,3,6,9]) |
| np.full((2,3),8) | 2x3数组，所有值为8 |
| np.random.rand(4,5) | 4x5 array of random floats between 0-1 |
| np.random.rand(6,7)*100 | 6x7 array of random floats between 0-100 |
| np.random.randint(5,size=(2,3)) | 2x3 array with random ints between 0-4 |
检查属性
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| arr.size | 返回数组元素的总数 |
| arr.shape | 返回数组的维度 |
| arr.dtype | 返回数组中元素的类型 |
| arr.astype(dtype) | 将数组元素的类型转换为dtype |
| arr.tolist() | 将数组转换为列表 |
| np.info(np.eye) | 查看np.eye的文档 |
复制/排序/重塑
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| np.copy(arr) | Copies arr to new memory |
| arr.view(dtype) | Creates view of arr elements with type dtype |
| arr.sort() | Sorts arr |
| arr.sort(axis=0) | Sorts specific axis of arr |
| two_d_arr.flatten() | Flattens 2D array two_d_arr to 1D |
| arr.T | Transposes arr (rows become columns and vice versa) |
| arr.reshape(3,4) | Reshapes arr to 3 rows, 4 columns without changing data |
| arr.resize((5,6)) | Changes arr shape to 5x6 and fills new values with 0 |
添加/删除元素
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| np.append(arr,values) | Appends values to end of arr |
| np.insert(arr,2,values) | Inserts values into arr before index 2 |
| np.delete(arr,3,axis=0) | Deletes row on index 3 of arr |
| np.delete(arr,4,axis=1) | Deletes column on index 4 of arr |
结合/分裂
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| np.concatenate((arr1,arr2),axis=0) | Adds arr2 as rows to the end of arr1 |
| np.concatenate((arr1,arr2),axis=1) | Adds arr2 as columns to end of arr1 |
| np.split(arr,3) | Splits arr into 3 sub-arrays |
| np.hsplit(arr,5) | Splits arr horizontally on the5th index |
索引/切割/构造子集
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| arr[5] | Returns the element at index 5 |
| arr[2,5] | Returns the 2D array element on index [2][5] |
| arr[1]=4 | Assigns array element on index 1 the value 4 |
| arr[1,3]=10 | Assigns array element on index [1][3] the value 10 |
| arr[0:3] | Returns the elements at indices 0,1,2 (On a 2D array: returns rows 0,1,2) |
| arr[0:3,4] | Returns the elements on rows 0,1,2 at column 4 |
| arr[:2] | Returns the elements at indices 0,1 (On a 2D array: returns rows 0,1) |
| arr[:,1] | Returns the elements at index 1 on all rows |
| arr<5 | Returns an array with boolean values |
| (arr1<3) & (arr2>5) | Returns an array with boolean values |
| ~arr | Inverts a boolean array |
| arr[arr<5] | Returns array elements smaller than 5 |
标量的数学
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| np.add(arr,1) | Add 1 to each array element |
| np.subtract(arr,2) | Subtract 2 from each array element |
| np.multiply(arr,3) | Multiply each array element by 3 |
| np.divide(arr,4) | Divide each array element by 4 (returns np.nan for division by zero) |
| np.power(arr,5) | Raise each array element to the 5th power |
向量的数学
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| np.add(arr1,arr2) | Elementwise add arr2 to arr1 |
| np.subtract(arr1,arr2) | Elementwise subtract arr2 from arr1 |
| np.multiply(arr1,arr2) | Elementwise multiply arr1 by arr2 |
| np.divide(arr1,arr2) | Elementwise divide arr1 by arr2 |
| np.power(arr1,arr2) | Elementwise raise arr1 raised to the power of arr2 |
| np.array_equal(arr1,arr2) | Returns True if the arrays have the same elements and shape |
| np.sqrt(arr) | Square root of each element in the array |
| np.sin(arr) | Sine of each element in the array |
| np.log(arr) | Natural log of each element in the array |
| np.abs(arr) | Absolute value of each element in the array |
| np.ceil(arr) | Rounds up to the nearest int |
| np.floor(arr) | Rounds down to the nearest int |
| np.round(arr) | Rounds to the nearest int |
统计数据
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| np.mean(arr,axis=0) | Returns mean along specific axis |
| arr.sum() | Returns sum of arr |
| arr.min() | Returns minimum value of arr |
| arr.max(axis=0) | Returns maximum value of specific axis |
| np.var(arr) | Returns the variance of array |
| np.std(arr,axis=1) | Returns the standard deviation of specific axis |
| arr.corrcoef() | Returns correlation coefficient of array |