Reading Files

Basic steps: Reading from files

While reading from a file, you can format the data from each field individually, and you can use Express functions to process the information before assigning it to an OLAP DML object. Reading a file generally involves the following steps.

1. Open the file you want to read.

2. Read data from the file one record or line at a time.

3. Process the data and assign it to one or more Express objects.

4. Close the file.

Methods for reading a file

The FILEREAD and FILEVIEW commands have the same attributes and can do the same processing on your data. However, they differ in important ways:

• The FILEREAD command loops automatically over all records in the file.

• The FILEVIEW command processes one record at a time.

Consequently, there are two basic methods for reading a file.

Method	Command	Description	Use
1	FILEREAD	Processes all records in a file automatically.	Use when all records in the file are the same.
2	FILENEXT and FILEVIEW	Reads and processes one record at a time from the file.	Use when there is more than one type of record in a file.

Creating a program to read data

The following table shows, for each method, the commands you need to open and close the input file, to read the file, and to handle errors that might occur.

Program Section	Method 1(FILEREAD)	Method 2(FILENEXT and FILEVIEW)
Initialization	variable funit integer trap on error	variable funit integer trap on error
Body	funit = fileopen(- 'datafile' read) fileread funit . . . fileclose funit	funit = fileopen(- 'datafile' read) while filenext(funit) do fileview funit . . . doend fileclose funit
Normal Exit	return	return
Abnormal Exit	error: if funit ne na then fileclose funit	error: if funit ne na then fileclose funit

Note: The error handling in the abnormal exit section of the programs closes the file only when the file is open. The FILEOPEN function signals an error when for any reason the system can not open the file. The program tries to close the file after the ERROR label only when FUNIT holds a valid file unit number. You can add additional commands to the error handling section as well. These sections of the program are the same for both methods.