The primary logic in a program, which is input-process-output. In an online, interactive program, everything happens when an input occurs, such as a mouse click, mouse movement, keyboard depression or arriving network packet. The main loop takes care of this by continually testing for these inputs and calling the appropriate routines to process them. Outputs are the data on screen and updated files.
In a batch processing program where data are processed without human interaction, the main loop reads the next record and processes it. Output is often continuously updated files with summaries displayed at the end. See loop
and event loop
A Batch Processing Print Job
This example reads order records and prints invoices. After printing date and bill-to/ship-to data, the program prints a variable number of line items. The line item routine is a subloop within the main loop.