Files and data storage

Files and Data Storage

Files and Data Storage● Most computers are used for data

processing, as a big growth area in the “information age”

● Data processing from a computer science perspective:

– Storage of data

– Organization of data

– Access to data

– Processing of data

Data Structures vs File Structures

• Both involve:

– Representation of Data

+

– Operations for accessing data

• Difference:

– Data structures: deal with data in the main memory

– File structures: deal with the data in the secondary storage

File Structure in Computer Science

Data Storage in Computer

Goal of the File Structures● Minimize the number of trips to the

secondary storage (SS) in order to get desired information.

● Group related information so that we are likely to get everything we new with fewer trip to the SS.

● Select the right file structures so that performance can be increased.

File and File Operations● A file is a collection of data stored on mass

storage like hard disk, CD etc. ● File data consist of records (student

information) and each record contains number of fields (ID, Name etc.).

● We can perform following operations on a file.– Search for a particular data in a file.– Add a certain data item.– Remove / Update a certain item.

File and File operations– Order the data items according to a

certain criterion, merge of files. – Creation of new files from existing files. – Finally create, open, and close

operations which have implications in the operating system.

Organization of Files● Sequential● Indexed● Hashing

Sequential File Organization● Records are conceptually organized in a

sequential list and can only be accessed sequentially.

● The actual storage might or might not be sequential (on tape or on disk)

Sequential File (Write/Read in C++)

● Create ofstream object (after including file fstream.h at the top)

● Open file for output or for appending at the end of file.

● Test whether the file open operation of step 2 is successful. If not successful then exit else continue.

● Write / Read data to output file.● Close file after writing / reading data.

Sequential File Implementation● #include <iostream.h>

● #include <fstream.h>

● #include <stdlib.h>

●

● Void main() {

● Int i, Roll[N] = { 171,717, 834, 394, 475 };

● float Percentage[N]= {45.3, 84.5, 95.0, 48.2, 39.2 };

● Char* Name[N] = {“wajid”, “Aashir”, “Luqman”, “Tushar”, “Waseem” };

● // Step1: Create ofstream and ifstream objects

● Ofstream outFile; ifstream inFile;

● // Step 2: Open file for output

● outFile.open(“percent.dat”, ios::out);

● // Step 3 Test weather open operation is successful

● If (!outFile) {

– cout<<”File could not open “;

– Exit(1);

● Else

– Cout<<”\n File open successfully\n”;

● //Step 4: Write to file

● For( i=0; i<N; i++)

– OutFile <<Name[i]<<' '<<Roll[i]<< ' '

<<Percentage[i]<<endl;

● cout<<”\n File write successfully. \n”;

● // Step 5: Close file

● outFile.close();

Sequential File Implementation● // Step 6 open file for input

● inFile.open(“percentage.dat”, ios::in);

● // Step 7: Test wether file open successfully.

● if(!inFile) {

– cout<<”File could not open”<<endl;

– Exit(1);

● }

● // Step 8: Read from input File

● While( inFile>> Name >>Roll >> Percentage)

– cout<<setiosflags(ios::left)<<setw(14)<<roll <<setw(16)<<Name <<setw(9)<<Setprecision(4)

– <<setiosflags(ios::showpoint | ios:: right)

– <<percent<<'%'<< endl;

● //Step 9: Close file

● inFile.close(); }

Sequential File Implementation● OUTPUT:

Roll Number Name Percentage

171 Wajid 45.30%

717 Aashir 84.50%

834 Luqman 95.00%

394 Tushar 48.20%

475 Waseem 39.20%

Indexed File Organization● Sequential search is even slower on

disk/tape than in main memory. Try to improve performance using more sophisticated data structures.

● An index for a file is a list of key field values occurring in the file along with the address of the corresponding record in the mass storage.

● Typically the key field is much smaller than the entire record, so the index will fit in main memory.

Indexed File Organization ...● The index can be organized as a list, a

search tree, a hash table, etc. To find a particular record:

● Search the index for the desired key.● When the search returns the index entry,

extract the record’s address on mass storage.

● Access the mass storage at the given address to get the desired record.

Indexed File Organization ...

Hashed File Organization● A hashed file uses a hash function to map

the key to the address.● Eliminates the need for an extra file (index).● There is no need for an index and all of the

overhead associated with it.

● Use an array of M < N linked lists, good choice is M~ N/10

● Hash: map key to integer i between 0 and M-1.

● Insert: put at front of ith chain (if not already there).

● Search: only need to search ith chain.

Collusion Resolution (Separate Chaining)

Collusion Resolution (Separate Chaining)

Separate Chaining Implementation

Separate Chaining Implementation

● Use an array of size M >> N, good choice M~2N

● Hash: map key to integer i between 0 and M-1.

● Insert: put in slot i if free; if not try i+1, i+2, etc.

● Search: search slot i; if occupied but no match, try i+1, i+2, etc.

Collusion Resolution (Open Addressing)

Collusion Resolution (Open addressing)

Open Addressing Implementation