In fact, search engines and directories on the principle that we are the same books, the books we can find our content through the appropriate directory, such as navigation station is actually an index structure:
Forward index:
Keyword Site 001 = A + B and so on keywords. . . . . .
Keyword Site 002 = A + B and so on keywords. . . . . .
Like this page is divided into a number of keywords is the forward index
Inverted index:
1, the document
Like a website, which is divided into a file (such as TXT, RAR, JS, CSS, EXE, JPG), which called for the document
2, document collection
There are a lot of documents consisting of a collection, so he called for the document collection.
3, document number
Internet Each file has its own a number (can be understood as a document of their md5 value only)
4, the word number
Each word has its own unique number, unique, with a number to represent words or phrases or sentences again
5, inverted index
Inverted index is among the search engine search engine library, the storage form of the word correspondence page, that can quickly get to the relevant documents based on the words.
More intuitive point illustrated in Figure 17:
Aware of inverted index, we need to know about keyword density referred TF, words such as we have just ID1, the word is: I can appear in multiple documents, indicating the frequency of occurrence pricey, so we need to follow the page find TF, i.e. keyword density, there are three methods of calculating keyword density.
Method 1: Image page number ÷ total number of words X 100%
Method 2: Image page number ÷ total number of words Word Count X 100% ÷ Keywords
Method 3: ÷ words the number of sub-page number of words (number of words to be understood that the figure right) X 100%
TF when sorting the results in the search engines, analyze the query words and refer to factors more relevant document libraries that document
In fact, on a similar frequency calculation method to calculate this figure can, of course, the inverted blue font list if more than one appears, the frequency will be + 1, and so on, this is the legendary keyword density play slightly ~ ~!
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