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Reference.org
Count-distinct problem
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<p>In computer science, the count-distinct problem (also known in applied mathematics as the cardinality estimation problem) is the problem of finding the number of distinct elements in a data stream with repeated elements. This is a well-known problem with numerous applications. The elements might represent <a href="/facts/IP_addresses/z9V22Sw9">IP addresses</a> of packets passing through a <a href="/facts/Router_(computing)/Nh5cIyVf">router</a>, <a href="/facts/Unique_visitor/tEsFaN23">unique visitors</a> to a web site, elements in a large database, motifs in a <a href="/facts/DNA/nB89Iauz">DNA</a> sequence, or elements of <a href="/facts/RFID/4CnmIV2b">RFID</a>/<a href="/facts/Sensor_networks/r5LIAVsU">sensor networks</a>. </p>