QBasic/Advanced Input< QBasic
Getting real time information from the user is a little more difficult. To do so, we will use the INKEY$ command, which checks to see if a user typed a key and provides the keypress to the program.
Look at this code and then we will look at it in depth:
DO LET k$ = INKEY$ LOOP UNTIL k$ <> "" SELECT CASE k$ CASE "q" QuitProgram CASE "c" MakeCircle CASE "s" MakeSquare END SELECT
The first part is the DO-LOOP which constantly polls INKEY$ for a return value. In the basic use, INKEY$ returns an empty string if no keys are being pressed and continues with the program. Once a key is pressed, INKEY$ will return that key immediately.
The keyboard bufferEdit
What is INKEY$ doing and how does it work?
While the INKEY$ command looks like it returns the key currently being pressed, this is not the case. It is used by the program to answer the question, "What is IN the KEYboard buffer?" To understand this you will need to understand what a basic buffer is and why it is used.
In older systems (not necessarily the IBM PC) a single chip processed keyboard input, and controlled the LED lights for caps lock and number lock. Because a computer does many things at once (e.g., take input from the mouse, crunch numbers, call subroutines, display new information on the screen), it needs to be able to remember what was pressed on the keyboard while it is busy. This chip contained some memory (called a buffer) that allow keeping track of a limited number of keypresses.
Within the Dos platform under IBM PCs, the hardware has changed slightly. Instead of a hardware buffer, pressing or releasing a key will interrupt the running program to add a keystroke to a software buffer located in the BIOS. This procedure is usually unnoticed by the user and has minimal impact on system performance. However, this buffer allows for 15 characters - attempting to overflow it when the computer is busy will cause a short beep and drop any further characters.
The INKEY$ command uses this buffer as a FIFO (First In First Out) buffer. As an example let's say you have a game that has a bouncing ball on the screen and a paddle at the bottom. The computer program constantly has to update the screen to show the movement of the ball. While it does this the program passes by an INKEY$ command to see what value is returned. If the user has pressed a key since the last time the command was invoked it will return that key. Let's say the ball is moving over to the right and the user needs to press the "R" key to tell the program to move the paddle right. Since the program is busy moving the ball and updating the screen, it does not instantaneously notice that the user has pressed the key. Instead, the key press is stored in the keyboard buffer, and retrieved a few milliseconds (or microseconds) later when the INKEY$ command is used.
In many programs (as above), INKEY$ will appear nested in a loop. It is requested over and over again. This allows the program to get user input one character at a time. Using our example above, the user may need to press R over and over again until the paddle is under the ball. On the other hand, the user may press R too many times and need to press L to move it left. Because the INKEY$ command is using a FIFO buffer it will always retrieve the keys pressed in the same order as they where typed.
In summary, the INKEY$ command will always return and remove the first character in the keyboard buffer. Generally speaking, it is used over and over to retrieve every key that has been pressed, and to allow a user to interact with a program in a close approximation to "real time." If there is no key in the keyboard buffer, INKEY$ it will return an empty string (no character).
Some keypresses are not associated with an ASCII character. When one of these keys is pressed, INKEY$ returns a string with two characters = the first character is a null (ASCII code 0), and the second is the raw scan code for the keyboard. A full listing of the scancodes can be found within the QBASIC help file - you can also determine the scan codes by examining the results of INKEY$ as you press those keys in question.