Character string retrieval system using index and unit for making the index

A character string retrieval system for retrieving a character string corresponding to a retrieval key input thereto from a text file including a plurality of characters, said system comprising:

• an index indicating position data identifying a position of each of characters in the text file and indicating next character data identifying either a character or a character string formed of a plurality of characters arranged next to each of the characters in the text file;
• selecting means for selecting a first character from characters forming the character string corresponding to the retrieval key;
• first determining means for determining, with reference to said index, whether or not a character or a character string next to the first character, selected by said selecting means, in the retrieval key is equal to a character or a character string identified by the next character data corresponding to the first character;
• second determining means for determining whether or not a character string formed of the characters in the retrieval key is in a predetermined region in the text file, said region including a position identified by the position data for the first character;
• and
• output means for outputting a retrieval result when said second determining means determines that the character string is in the region in the text file, the retrieval result being obtained based on the position data corresponding to the first character in said index.

A unit for making an index used in a character string retrieval system for retrieving a character string corresponding to a retrieval key input thereto from a text file including a plurality of characters, said index indicating frequency data for each of the characters in the text file, the frequency data indicating a frequency with which each of the characters appears in the text file, said unit comprising:

• searching means for successively searching characters in the text file one by one;
• counting means for counting a number of appearances of a character in the text file, said character being searched by said searching means;
• and
• listing means for listing the frequency data corresponding to a count value obtained by said counting means, so that the index indicating the frequency data is made.