- In this teaching example:
- I will illustrate the
storage technique using
2 different data types:
- int data type
- String data type
- I will illustrate the
storage technique using
2 different data types:
- In project 1, you
will be asked to write a
program that can
process
3 different data types:
- int data type
- String data type
- double dat atype
- How to store the
data
when we do not know
the structure of the
data fields in advance:
- We store the
fields of
a data record
scattered using
arrays
of all possible
data types:
String[ ] c_buf; // Stores the String typed fields int[ ] i_buf; // Stores the int typed fields
Since we will store int type and String type data, I will use 2 different types arrays:
- c_buf is used to store String type data fields in the data record
- i_buf is used to store int type data fields in the data record
Graphically:
- The data record
multiple
data fields.
The data type of a data field can be int or String (because we only handle these 2 different data types)
Example:
- We store a
data as follows:
- Use c_buf[0] to
store the first field of the
data if its
data type is
String
Use i_buf[0] to store the first field of the data if its data type is int - Use c_buf[1] to
store the second field of the
data if its
data type is
String
Use i_buf[1] to store the second field of the data if its data type is int - Use c_buf[2] to
store the 3rd field of the
data if its
data type is
String
Use i_buf[0] to store the 3rd field of the data if its data type is int - And so on...
- Use c_buf[0] to
store the first field of the
data if its
data type is
String
- We store the
fields of
a data record
scattered using
arrays
of all possible
data types:
- Example 1:
- Suppose the
record format of a file
is:
class DataFile1 { String name; int age; int id; String address; }Graphically:
we store the fields of a data record as follows:
- Suppose the
record format of a file
is:
- Example 2:
- Suppose the
record format of a file
is:
class DataFile1 { String name; String address; int age; int id; }Graphically:
we store the fields of a data record as follows:
- Suppose the
record format of a file
is:
- We now apply all that
we have learned and
write
program code to
store the data from a file into
program variable
- Summary of
what we have learned so far:
- Values of a
given data type
must be stored in
a variable of the
same data type:
/* =========================================================== How to read ONE record USING the meta data ONE record has n_fields fields according to the Meta Data =========================================================== */ for ( int i = 0; i < n_fields; i++ ) { /* -------------------------------------------------------------- HOW you access the i-th field depends on the TYPE of the field ! -------------------------------------------------------------- */ if ( dataDes[i].fieldType.equals("I") ) { /* -------------------------------------------------------- Field i is an integer, use i_buf[i] to store the value -------------------------------------------------------- */ intVar = file.ReadInt( ); // Store in an int type var } else { /* -------------------------------------------------------- Field i is an String, use c_buf[i] to store the value -------------------------------------------------------- */ stringVar = in.nextLine(); // Store in a String type variable } }
- To store
user data of the supported types
(character string and integer)
we will use two arrays
--- one array for
each supported data type:
/* ------------------------------------------------------- Variables used to store the data read from file ------------------------------------------------------- */ static String[] c_buf = new String[MAXNFIELDS]; // Used to store String fields static int[] i_buf = new int[MAXNFIELDS]; // Used to store int fields- c_buf[ ] = an array used to store String typed data
- i_buf[ ] = an array used to store Integer typed data
- Values of a
given data type
must be stored in
a variable of the
same data type:
- Combining the
above 2 pieces of knowledge and we
have the following
program code that can
store the
data into
the program variables
c_buf[ ] and i_buf[ ]:
/* =========================================================== How to read ONE record USING the meta data ONE record has n_fields fields according to the Meta Data =========================================================== */ for ( int i = 0; i < n_fields; i++ ) { /* -------------------------------------------------------------- HOW you access the i-th field depends on the TYPE of the field ! -------------------------------------------------------------- */ if ( dataDes[i].fieldType.equals("I") ) { /* -------------------------------------------------------- Field i is an integer, use i_buf[i] to store the value -------------------------------------------------------- */ i_buf[i] = file.ReadInt( ); // Store in an int type var } else { /* -------------------------------------------------------- Field i is an String, use c_buf[i] to store the value -------------------------------------------------------- */ c_buf[i] = in.nextLine(); // Store in a String type variable } }
- The
complete program
to read a
data field "db-data" using the
data description stored in the
meta data file
"db-description":
public static void main(String[] args) throws IOException { /* ----------------------------------------------------------- 1. Find out the STRUCTURE of the data file (and store this info !) Read in the meta data from db-description and store meta data in the DataDes[] array ----------------------------------------------------------- */ DataFile descrFile = new DataFile("db-description"); // Open the meta data file n_fields = 0; // We count the actual number of fields in data while ( true ) { try { /* ======================================================= Read the description of the next field and store it ======================================================= */ dataDes[n_fields].fieldName = descrFile.ReadString(24); dataDes[n_fields].fieldType = descrFile.ReadString(4); dataDes[n_fields].fieldSize = descrFile.ReadInt(); n_fields++; // Remember the number of fields System.out.println("Field: " + dataDes[n_fields].fieldName + ", type: " + dataDes[n_fields].fieldType + ", size: " + dataDes[n_fields].fieldSize); } catch ( IOException e ) { System.out.println("\nFinish reading data description file....\n"); break; // Read error: no more data !!! } }
/* ============================================================ Read the data file USING the meta data ============================================================ */ DataFile dataFile = new DataFile("db-data"); // Open the data file System.out.println( "The data file contains these records:\n"); while ( true ) { /* ============================================================ Read in one record in one for-loop One record contains "n_fields" fields ============================================================ */ try { for ( int i = 0; i < n_fields; i++ ) { /* ------------------------------------------------------ Read in the i-th field and store it How to read and how to store depends on the TYPE of the i-th field ------------------------------------------------------ */ if ( dataDes[i].fieldType.equals("I") ) { /* -------------------------------------------------------- Field i is an integer, use i_buf[i] to store the value -------------------------------------------------------- */ i_buf[i] = dataFile.ReadInt(); // Read an int and store in i_buf[] } else { /* -------------------------------------------------------- In our example, we only handle 2 data types: int and String Field i must be an String, use c_buf[i] to store the value -------------------------------------------------------- */ c_buf[i] = dataFile.ReadString( dataDes[i].fieldSize ); // Read a String and store in c_buf[] } } } catch ( IOException e ) { System.out.print("\nDone.\n"); break; } } }
Demo:
cd /home/cs377001/demo/Phys-Data-Dependance/DB java ReadStuFile
I have 2 sets of data: To use data set 1: cp db-description.1 db-description cp student-file.1 db-data record format: +-------------+--------------------------------+-------+ | ID | Name | level | +-------------+--------------------------------+-------+ 10 chars 30 chars 4 chars To use data set 2: cp db-description.2 db-description cp student-file.2 db-data record format: +-------------+----------------+-------------+-------+ | ID | Name | Address | level | +-------------+----------------+-------------+-------+ 6 chars 20 chars 20 chars 4 chars
- Comment:
- There is a better way to
store the
data if you use
the C programming language
- Since you learned Java in CS170/CS171, we need to use multiple arrays because Java is more strongly typed than C and does not allow us to manipulate data using low level operators
- There is a better way to
store the
data if you use
the C programming language