There are three types of file organization used in practice. Let us consider the example of a dictionary. The dictionary contains listing of English words sorted alphabetically. The top left and right hand corners provide the starting and the ending of each page. Let us assume that the information of the dictionary is stored on a disk. Now suppose, we want to find a word, say, “Computer”. This can be done in the following ways:
Sequential Access: We start from the first word and check whether or not it is the desired word. We continue checking till the required word is found. This method implies searching the whole file from beginning to end and is known as sequential processing. Time required to locate a specific word will obviously depend on its location from the starting point.
Indexed Sequential Access: To locate “Computer”, we can also search only the top corner words to first identify which particular page the word belongs to. After knowing the specific page, we can then search sequential till the required word is found. In this method, firstly an index has been searched followed by a complete sequential search. This method is known as indexed sequential method and is much faster compared to sequential method and is very commonly used in practice.
Random Access: Suppose each word is assigned a specific sequence number and “Computer” was 25th from the beginning. Then we could go directly to the 25th location on the disk without any searching. This type of processing is known as Random Access and the response in this method will be very fast. However, the concept of allocating a unique number, to each data item, becomes difficult as the complexity increases.