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POProblem with Precision floating point operation in C
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  1. POProblem with Precision floating point operation in C
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    copied!<p>For one of my course project I started implementing "Naive Bayesian classifier" in C. My project is to implement a document classifier application (especially Spam) using huge training data.</p> <p>Now I have problem implementing the algorithm because of the limitations in the C's datatype. </p> <p>( Algorithm I am using is given here, <a href="http://en.wikipedia.org/wiki/Bayesian_spam_filtering" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Bayesian_spam_filtering</a> )</p> <p>PROBLEM STATEMENT: The algorithm involves taking each word in a document and calculating probability of it being spam word. If p1, p2 p3 .... pn are probabilities of word-1, 2, 3 ... n. The probability of doc being spam or not is calculated using </p> <p><img src="https://upload.wikimedia.org/math/f/1/d/f1d1c65ee72c294f1fc9b4eb156f5768.png" alt="alt text"></p> <p>Here, probability value can be very easily around 0.01. So even if I use datatype "double" my calculation will go for a toss. To confirm this I wrote a sample code given below.</p> <pre><code>#define PROBABILITY_OF_UNLIKELY_SPAM_WORD (0.01) #define PROBABILITY_OF_MOSTLY_SPAM_WORD (0.99) int main() { int index; long double numerator = 1.0; long double denom1 = 1.0, denom2 = 1.0; long double doc_spam_prob; /* Simulating FEW unlikely spam words */ for(index = 0; index &lt; 162; index++) { numerator = numerator*(long double)PROBABILITY_OF_UNLIKELY_SPAM_WORD; denom2 = denom2*(long double)PROBABILITY_OF_UNLIKELY_SPAM_WORD; denom1 = denom1*(long double)(1 - PROBABILITY_OF_UNLIKELY_SPAM_WORD); } /* Simulating lot of mostly definite spam words */ for (index = 0; index &lt; 1000; index++) { numerator = numerator*(long double)PROBABILITY_OF_MOSTLY_SPAM_WORD; denom2 = denom2*(long double)PROBABILITY_OF_MOSTLY_SPAM_WORD; denom1 = denom1*(long double)(1- PROBABILITY_OF_MOSTLY_SPAM_WORD); } doc_spam_prob= (numerator/(denom1+denom2)); return 0; } </code></pre> <p>I tried Float, double and even long double datatypes but still same problem. </p> <p>Hence, say in a 100K words document I am analyzing, if just 162 words are having 1% spam probability and remaining 99838 are conspicuously spam words, then still my app will say it as Not Spam doc because of Precision error (as numerator easily goes to ZERO)!!!.</p> <p>This is the first time I am hitting such issue. So how exactly should this problem be tackled?</p>
 

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