Mercurial > hg > medcouple
annotate pymedcouple @ 23:29a178b23219
Whitespace style fixes
author | Jordi Gutiérrez Hermoso <jordigh@octave.org> |
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date | Sat, 17 Jan 2015 20:04:46 -0500 |
parents | f5b4a2ab6204 |
children | cd3094a08e03 |
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1 #!/usr/bin/python |
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2 |
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3 import random |
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4 import numpy as np |
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7 | 6 from itertools import tee, izip |
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7 |
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8 |
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9 def partsort(L, n): |
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10 """This is a partial sort of the weighted list L = [(a,w) for a in A, |
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11 w in W], so that the element at position n is in the correct spot. |
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12 Also known as quickselect. The elements of W are ignored for the |
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13 purposes of this partial sort. |
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14 """ |
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15 |
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16 beg = 0 |
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17 end = len(L) |
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18 |
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19 while True: |
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20 pivot = random.randint(beg, end - 1) |
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21 pivotval = L[pivot][0] |
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22 L[pivot], L[end - 1] = L[end - 1], L[pivot] |
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23 |
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24 idx = beg |
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25 for i in xrange(beg, end): |
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26 if L[i][0] < pivotval: |
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27 L[idx], L[i] = L[i], L[idx] |
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28 idx += 1 |
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29 |
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30 L[idx], L[end-1] = L[end-1], L[idx] |
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31 |
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32 if idx == n: |
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33 return L[idx] |
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34 elif idx < n: |
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35 beg = idx + 1 |
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36 else: |
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37 end = idx |
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38 |
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39 |
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40 def wmedian(A, W): |
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41 """This computes the weighted median of array A with corresponding |
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42 weights W. |
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43 """ |
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44 |
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45 AW = zip(A, W) |
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46 n = len(AW) |
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47 |
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48 wtot = sum(W) |
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49 |
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50 beg = 0 |
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51 end = n - 1 |
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52 |
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53 while True: |
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54 mid = (beg + end)//2 |
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55 |
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56 partsort(AW, mid) |
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57 trial = AW[mid][0] |
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58 |
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59 wleft = wright = 0 |
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60 for (a, w) in AW: |
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61 if a < trial: |
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62 wleft += w |
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63 else: |
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64 # This also includes a == trial, i.e. the "middle" |
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65 # weight. |
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66 wright += w |
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67 |
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68 if 2*wleft > wtot: |
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69 end = mid |
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70 elif 2*wright < wtot: |
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71 beg = mid |
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72 else: |
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73 return trial |
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74 |
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75 |
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76 def medcouple_1d(X, eps1 = 2**-52, eps2 = 2**-1022): |
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77 """Calculates the medcouple robust measure of skewness. |
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78 |
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79 Parameters |
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80 ---------- |
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81 y : array-like, 1-d |
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82 |
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83 Returns |
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84 ------- |
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85 mc : float |
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86 The medcouple statistic |
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87 |
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88 .. [1] G. Brys, M. Hubert, and A. Struyf "A Robust Measure of |
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89 Skewness." Journal of Computational and Graphical Statistics, Vol. |
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90 13, No. 4 (Dec., 2004), pp. 996- 1017 |
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91 |
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92 .. [2] D. B. Johnson and T. Mizoguchi "Selecting the Kth Element |
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93 in $X + Y$ and $X_1 + X_2 + \cdots X_m$". SIAM Journal of |
9 | 94 Computing, Vol. 7, No. 2 (May 1978), pp. 147-53. |
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95 |
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96 """ |
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97 # FIXME: Figure out what to do about NaNs. |
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98 |
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99 n = len(X) |
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100 n2 = (n - 1)//2 |
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101 |
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102 if n < 3: |
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103 return 0 |
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104 |
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105 Z = sorted(X, reverse=True) |
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106 |
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107 if n % 2 == 1: |
7 | 108 Zmed = Z[n2] |
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109 else: |
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110 Zmed = (Z[n2] + Z[n2 + 1])/2 |
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111 |
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112 #Check if the median is at the edges up to relative epsilon |
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113 if abs(Z[0] - Zmed) < eps1*(eps1 + abs(Zmed)): |
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114 return -1.0 |
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115 if abs(Z[-1] - Zmed) < eps1*(eps1 + abs(Zmed)): |
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116 return 1.0 |
7 | 117 |
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118 # Centre Z wrt median, so that median(Z) = 0. |
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119 Z = [z - Zmed for z in Z] |
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120 |
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121 # Scale inside [-0.5, 0.5], for greater numerical stability. |
7 | 122 Zden = 2*max(Z[0], -Z[-1]) |
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123 Z = [z/Zden for z in Z] |
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124 Zmed /= Zden |
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125 |
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126 Zeps = eps1*(eps1 + abs(Zmed)) |
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127 |
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128 # These overlap on the entries that are tied with the median |
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129 Zplus = [z for z in Z if z >= -Zeps] |
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130 Zminus = [z for z in Z if Zeps >= z] |
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131 |
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132 n_plus = len(Zplus) |
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133 n_minus = len(Zminus) |
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134 |
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135 |
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136 def h_kern(i, j): |
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137 """Kernel function h for the medcouple, closing over the values of |
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138 Zplus and Zminus just defined above. |
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139 |
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140 In case a and be are within epsilon of the median, the kernel |
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141 is the signum of their position. |
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142 |
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143 """ |
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144 a = Zplus[i] |
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145 b = Zminus[j] |
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146 |
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147 if abs(a - b) <= 2*eps2: |
7 | 148 h = signum(n_plus - 1 - i - j) |
149 else: | |
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150 h = (a + b)/(a - b) |
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151 |
7 | 152 return h |
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153 |
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154 # Init left and right borders |
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155 L = [0]*n_plus |
7 | 156 R = [n_minus - 1]*n_plus |
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157 |
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158 Ltot = 0 |
7 | 159 Rtot = n_minus*n_plus |
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160 medc_idx = Rtot//2 |
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161 |
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162 # kth pair algorithm (Johnson & Mizoguchi) |
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163 while Rtot - Ltot > n_plus: |
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164 |
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165 # First, compute the median inside the given bounds |
6 | 166 # (Be stingy, reuse same generator) |
167 [I1, I2] = tee(i for i in xrange(0, n_plus) if L[i] <= R[i]) | |
168 | |
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169 A = [h_kern(i, (L[i] + R[i])//2) for i in I1] |
7 | 170 W = [R[i] - L[i] + 1 for i in I2] |
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171 Am = wmedian(A, W) |
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172 |
7 | 173 Am_eps = eps1*(eps1 + abs(Am)) |
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174 |
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175 # Compute new left and right boundaries, based on the weighted |
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176 # median |
7 | 177 P = [] |
178 Q = [] | |
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179 |
7 | 180 j = 0 |
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181 for i in xrange(n_plus - 1, -1, -1): |
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182 while j < n_minus and h_kern(i, j) - Am > Am_eps: |
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183 j += 1 |
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184 P.append(j - 1) |
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185 |
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186 P.reverse() |
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187 |
7 | 188 j = n_minus - 1 |
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189 for i in xrange(0, n_plus): |
7 | 190 while j >= 0 and h_kern(i, j) - Am < -Am_eps: |
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191 j -= 1 |
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192 Q.append(j + 1) |
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193 |
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194 # Check on which side of those bounds the desired median of |
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195 # the whole matrix may be. |
7 | 196 sumP = sum(P) + len(P) |
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197 sumQ = sum(Q) |
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198 |
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199 if medc_idx <= sumP - 1: |
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200 R = P |
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201 Rtot = sumP |
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202 else: |
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203 if medc_idx > sumQ - 1: |
7 | 204 L = Q |
205 Ltot = sumQ | |
206 else: | |
207 return Am | |
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208 |
7 | 209 # Didn't find the median, but now we have a very small search |
210 # space to find it in, just between the left and right boundaries. | |
211 # This space is of size Rtot - Ltot which is <= n_plus | |
212 A = [] | |
213 for (i, (l, r)) in enumerate(izip(L, R)): | |
214 for j in xrange(l, r + 1): | |
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215 A.append(h_kern(i, j)) |
7 | 216 A.sort() |
217 A.reverse() | |
218 | |
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219 Am = A[medc_idx - Ltot] |
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220 |
7 | 221 return Am |
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222 |
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223 |
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224 def signum(x): |
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225 if x > 0: |
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226 return 1 |
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227 if x < 0: |
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228 return -1 |
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229 else: |
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230 return 0 |
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231 |
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232 |
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233 def main(): |
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234 import sys |
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235 fname = sys.argv[1] |
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236 with open(fname) as f: |
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237 data = [float(x) for x in f.readlines() if x.strip() != ""] |
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238 |
7 | 239 print "%.16g" % medcouple_1d(data) |
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240 |
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241 if __name__ == "__main__": |
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242 main() |