event loop

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event loop

A programming structure that continually tests for external events and calls the appropriate routines to handle them. An event loop is often the main loop in a program that typically waits for the user to trigger something. The following example is the main loop in the first software engine for this encyclopedia. Written in Turbo C, the main loop is constantly testing for menu selections, keystrokes and mouse clicks.

The statements with double parentheses are calls to functions (subroutines). For example, the bookmark() function creates a bookmark, and control is returned to the break statement. Break creates a go-to instruction to the beginning of the loop.

The while (1) statement creates a continuous loop until some condition is satisfied. In this example, the (1) means always true and keeps looping continuously. At the bottom (END OF EVENT LOOP), the curly brace (}) tells the compiler to insert a go-to instruction at that point that jumps to the beginning of the loop.

When testing for several conditions, the switch statement saves the programmer from having to write compare statements multiple times (if something is equal to, greater than or less than something else). In this example, the numbers are keyboard key codes (what key was just pressed?). See C, curly brace, event, event handler and event driven.

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