70.考虑向量[1,2,3,4,5]，如何建立一个新的向量，在每个值之间交错有3个连续的零？ (★★★)
(提示: array[::4])
# Author: Warren Weckesser Z = np.array([1,2,3,4,5]) nz = 3 Z0 = np.zeros(len(Z) + (len(Z)-1)*(nz)) Z0[::nz+1] = Z print(Z0)
71. 考虑一个维度(5,5,3)的数组，如何将其与一个(5,5)的数组相乘？ (★★★)
(提示: array[:, :, None])
A = np.ones((5,5,3)) B = 2*np.ones((5,5)) print(A * B[:,:,None])
72. 如何对一个数组中任意两行做交换? (★★★)
(提示: array[[]] = array[[]])
# Author: Eelco Hoogendoorn A = np.arange(25).reshape(5,5) A[[0,1]] = A[[1,0]] print(A)
73. 思考描述10个三角形（共享顶点）的一组10个三元组，找到组成所有三角形的唯一线段集 (★★★)
(提示: repeat, np.roll, np.sort, view, np.unique)
# Author: Nicolas P. Rougier faces = np.random.randint(0,100,(10,3)) F = np.roll(faces.repeat(2,axis=1),-1,axis=1) F = F.reshape(len(F)*3,2) F = np.sort(F,axis=1) G = F.view( dtype=[('p0',F.dtype),('p1',F.dtype)] ) G = np.unique(G) print(G)
74. 给定一个二进制的数组C，如何生成一个数组A满足np.bincount(A)==C? (★★★)
(提示: np.repeat)
# Author: Jaime Fernández del Río C = np.bincount([1,1,2,3,4,4,6]) A = np.repeat(np.arange(len(C)), C) print(A)
75. 如何通过滑动窗口计算一个数组的平均数? (★★★)
(提示: np.cumsum)
# Author: Jaime Fernández del Río def moving_average(a, n=3) : ret = np.cumsum(a, dtype=float) ret[n:] = ret[n:] - ret[:-n] return ret[n - 1:] / n Z = np.arange(20) print(moving_average(Z, n=3))
76. 思考以为数组Z，构建一个二维数组，其第一行是(Z[0],Z[1],Z[2])， 然后每一行移动一位，最后一行为 (Z[-3],Z[-2],Z[-1]) (★★★)
(提示: from numpy.lib import stride_tricks)
# Author: Joe Kington / Erik Rigtorp from numpy.lib import stride_tricks def rolling(a, window): shape = (a.size - window + 1, window) strides = (a.itemsize, a.itemsize) return stride_tricks.as_strided(a, shape=shape, strides=strides) Z = rolling(np.arange(10), 3) print(Z)
77. 如何对布尔值取反，或改变浮点数的符号(sign)? (★★★)
(提示: np.logical_not, np.negative)
# Author: Nathaniel J. Smith Z = np.random.randint(0,2,100) np.logical_not(Z, out=Z) Z = np.random.uniform(-1.0,1.0,100) np.negative(Z, out=Z)
78. 思考两组点集P0和P1去描述一组线(二维)和一个点p,如何计算点p到每一条线 i (P0[i],P1[i])的距离？ (★★★)
def distance(P0, P1, p): T = P1 - P0 L = (T**2).sum(axis=1) U = -((P0[:,0]-p[...,0])*T[:,0] + (P0[:,1]-p[...,1])*T[:,1]) / L U = U.reshape(len(U),1) D = P0 + U*T - p return np.sqrt((D**2).sum(axis=1)) P0 = np.random.uniform(-10,10,(10,2)) P1 = np.random.uniform(-10,10,(10,2)) p = np.random.uniform(-10,10,( 1,2)) print(distance(P0, P1, p))
79. 考虑两组点集P0和P1去描述一组线(二维)和一组点集P，如何计算每一个点 j(P[j]) 到每一条线 i (P0[i],P1[i])的距离? (★★★)
# Author: Italmassov Kuanysh
# based on distance function from previous question P0 = np.random.uniform(-10, 10, (10,2)) P1 = np.random.uniform(-10,10,(10,2)) p = np.random.uniform(-10, 10, (10,2)) print(np.array([distance(P0,P1,p_i) for p_i in p]))
80. 思考一个任意的数组，编写一个函数，该函数提取一个具有固定形状的子部分，并以一个给定的元素为中心(在该部分填充值) (★★★)
(提示: minimum, maximum)
# Author: Nicolas Rougier Z = np.random.randint(0,10,(10,10)) shape = (5,5) fill = 0 position = (1,1) R = np.ones(shape, dtype=Z.dtype)*fill P = np.array(list(position)).astype(int) Rs = np.array(list(R.shape)).astype(int) Zs = np.array(list(Z.shape)).astype(int) R_start = np.zeros((len(shape),)).astype(int) R_stop = np.array(list(shape)).astype(int) Z_start = (P-Rs//2) Z_stop = (P+Rs//2)+Rs%2 R_start = (R_start - np.minimum(Z_start,0)).tolist() Z_start = (np.maximum(Z_start,0)).tolist() R_stop = np.maximum(R_start, (R_stop - np.maximum(Z_stop-Zs,0))).tolist() Z_stop = (np.minimum(Z_stop,Zs)).tolist() r = [slice(start,stop) for start,stop in zip(R_start,R_stop)] z = [slice(start,stop) for start,stop in zip(Z_start,Z_stop)] R[r] = Z[z] print(Z) print(R)
