[22] | 1 | /*************************************************************************/ |
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| 2 | /* */ |
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| 3 | /* Calculate information, information gain, and print dists */ |
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| 4 | /* -------------------------------------------------------- */ |
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| 5 | /* */ |
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| 6 | /*************************************************************************/ |
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| 7 | |
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| 8 | |
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| 9 | #include "buildex.i" |
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| 10 | |
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| 11 | |
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| 12 | /*************************************************************************/ |
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| 13 | /* */ |
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| 14 | /* Determine the worth of a particular split according to the */ |
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| 15 | /* operative criterion */ |
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| 16 | /* */ |
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| 17 | /* Parameters: */ |
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| 18 | /* SplitInfo: potential info of the split */ |
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| 19 | /* SplitGain: gain in info of the split */ |
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| 20 | /* MinGain: gain above which the Gain Ratio */ |
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| 21 | /* may be used */ |
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| 22 | /* */ |
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| 23 | /* If the Gain criterion is being used, the information gain of */ |
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| 24 | /* the split is returned, but if the Gain Ratio criterion is */ |
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| 25 | /* being used, the ratio of the information gain of the split to */ |
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| 26 | /* its potential information is returned. */ |
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| 27 | /* */ |
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| 28 | /*************************************************************************/ |
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| 29 | |
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| 30 | |
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| 31 | float Worth(ThisInfo, ThisGain, MinGain) |
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| 32 | /* ----- */ |
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| 33 | float ThisInfo, ThisGain, MinGain; |
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| 34 | { |
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| 35 | if ( GAINRATIO ) |
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| 36 | { |
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| 37 | if ( ThisGain >= MinGain - Epsilon && ThisInfo > Epsilon ) |
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| 38 | { |
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| 39 | return ThisGain / ThisInfo; |
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| 40 | } |
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| 41 | else |
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| 42 | { |
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| 43 | return -Epsilon; |
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| 44 | } |
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| 45 | } |
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| 46 | else |
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| 47 | { |
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| 48 | return ( ThisInfo > 0 && ThisGain > -Epsilon ? ThisGain : -Epsilon ); |
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| 49 | } |
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| 50 | } |
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| 51 | |
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| 52 | |
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| 53 | |
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| 54 | /*************************************************************************/ |
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| 55 | /* */ |
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| 56 | /* Zero the frequency tables Freq[][] and ValFreq[] up to MaxVal */ |
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| 57 | /* */ |
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| 58 | /*************************************************************************/ |
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| 59 | |
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| 60 | |
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| 61 | ResetFreq(MaxVal) |
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| 62 | /* --------- */ |
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| 63 | DiscrValue MaxVal; |
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| 64 | { |
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| 65 | DiscrValue v; |
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| 66 | ClassNo c; |
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| 67 | |
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| 68 | ForEach(v, 0, MaxVal) |
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| 69 | { |
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| 70 | ForEach(c, 0, MaxClass) |
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| 71 | { |
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| 72 | Freq[v][c] = 0; |
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| 73 | } |
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| 74 | ValFreq[v] = 0; |
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| 75 | } |
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| 76 | } |
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| 77 | |
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| 78 | |
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| 79 | |
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| 80 | /*************************************************************************/ |
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| 81 | /* */ |
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| 82 | /* Given tables Freq[][] and ValFreq[], compute the information gain. */ |
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| 83 | /* */ |
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| 84 | /* Parameters: */ |
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| 85 | /* BaseInfo: average information for all items with */ |
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| 86 | /* known values of the test attribute */ |
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| 87 | /* UnknownRate: fraction of items with unknown ditto */ |
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| 88 | /* MaxVal: number of forks */ |
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| 89 | /* TotalItems: number of items with known values of */ |
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| 90 | /* test att */ |
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| 91 | /* */ |
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| 92 | /* where Freq[x][y] contains the no. of cases with value x for a */ |
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| 93 | /* particular attribute that are members of class y, */ |
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| 94 | /* and ValFreq[x] contains the no. of cases with value x for a */ |
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| 95 | /* particular attribute */ |
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| 96 | /* */ |
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| 97 | /*************************************************************************/ |
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| 98 | |
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| 99 | |
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| 100 | float ComputeGain(BaseInfo, UnknFrac, MaxVal, TotalItems) |
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| 101 | /* ----------- */ |
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| 102 | float BaseInfo, UnknFrac; |
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| 103 | DiscrValue MaxVal; |
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| 104 | ItemCount TotalItems; |
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| 105 | { |
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| 106 | DiscrValue v; |
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| 107 | float ThisInfo=0.0, ThisGain, TotalInfo(); |
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| 108 | short ReasonableSubsets=0; |
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| 109 | |
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| 110 | /* Check whether all values are unknown or the same */ |
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| 111 | |
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| 112 | if ( ! TotalItems ) return -Epsilon; |
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| 113 | |
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| 114 | /* There must be at least two subsets with MINOBJS items */ |
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| 115 | |
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| 116 | ForEach(v, 1, MaxVal) |
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| 117 | { |
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| 118 | if ( ValFreq[v] >= MINOBJS ) ReasonableSubsets++; |
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| 119 | } |
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| 120 | if ( ReasonableSubsets < 2 ) return -Epsilon; |
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| 121 | |
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| 122 | /* Compute total info after split, by summing the |
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| 123 | info of each of the subsets formed by the test */ |
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| 124 | |
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| 125 | ForEach(v, 1, MaxVal) |
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| 126 | { |
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| 127 | ThisInfo += TotalInfo(Freq[v], 0, MaxClass); |
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| 128 | } |
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| 129 | |
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| 130 | /* Set the gain in information for all items, adjusted for unknowns */ |
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| 131 | |
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| 132 | ThisGain = (1 - UnknFrac) * (BaseInfo - ThisInfo / TotalItems); |
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| 133 | |
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| 134 | Verbosity(5) |
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| 135 | printf("ComputeThisGain: items %.1f info %.3f base %.3f unkn %.3f result %.3f\n", |
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| 136 | TotalItems + ValFreq[0], ThisInfo, BaseInfo, UnknFrac, ThisGain); |
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| 137 | |
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| 138 | return ThisGain; |
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| 139 | } |
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| 140 | |
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| 141 | |
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| 142 | |
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| 143 | /*************************************************************************/ |
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| 144 | /* */ |
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| 145 | /* Compute the total information in V[ MinVal..MaxVal ] */ |
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| 146 | /* */ |
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| 147 | /*************************************************************************/ |
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| 148 | |
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| 149 | |
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| 150 | float TotalInfo(V, MinVal, MaxVal) |
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| 151 | /* --------- */ |
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| 152 | ItemCount V[]; |
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| 153 | DiscrValue MinVal, MaxVal; |
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| 154 | { |
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| 155 | DiscrValue v; |
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| 156 | float Sum=0.0; |
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| 157 | ItemCount N, TotalItems=0; |
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| 158 | |
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| 159 | ForEach(v, MinVal, MaxVal) |
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| 160 | { |
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| 161 | N = V[v]; |
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| 162 | |
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| 163 | Sum += N * Log(N); |
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| 164 | TotalItems += N; |
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| 165 | } |
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| 166 | |
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| 167 | return TotalItems * Log(TotalItems) - Sum; |
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| 168 | } |
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| 169 | |
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| 170 | |
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| 171 | |
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| 172 | /*************************************************************************/ |
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| 173 | /* */ |
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| 174 | /* Print distribution table for given attribute */ |
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| 175 | /* */ |
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| 176 | /*************************************************************************/ |
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| 177 | |
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| 178 | |
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| 179 | PrintDistribution(Att, MaxVal, ShowNames) |
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| 180 | /* ----------------- */ |
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| 181 | Attribute Att; |
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| 182 | DiscrValue MaxVal; |
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| 183 | Boolean ShowNames; |
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| 184 | { |
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| 185 | DiscrValue v; |
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| 186 | ClassNo c; |
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| 187 | String Val; |
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| 188 | |
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| 189 | printf("\n\t\t\t "); |
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| 190 | ForEach(c, 0, MaxClass) |
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| 191 | { |
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| 192 | printf("%7.6s", ClassName[c]); |
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| 193 | } |
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| 194 | printf("\n"); |
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| 195 | |
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| 196 | ForEach(v, 0, MaxVal) |
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| 197 | { |
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| 198 | if ( ShowNames ) |
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| 199 | { |
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| 200 | Val = ( !v ? "unknown" : |
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| 201 | MaxAttVal[Att] ? AttValName[Att][v] : |
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| 202 | v == 1 ? "below" : "above" ); |
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| 203 | printf("\t\t[%-7.7s:", Val); |
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| 204 | } |
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| 205 | else |
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| 206 | { |
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| 207 | printf("\t\t[%-7d:", v); |
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| 208 | } |
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| 209 | |
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| 210 | ForEach(c, 0, MaxClass) |
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| 211 | { |
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| 212 | printf(" %6.1f", Freq[v][c]); |
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| 213 | } |
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| 214 | |
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| 215 | printf("]\n"); |
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| 216 | } |
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| 217 | } |
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