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4.1 Some Programming Constructs
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The algorithms in this chapter will use programming constructs that allow for repeating a section of code many times. We will discuss the various constructs to achieve this. Also, RobotBASIC has commands to obtain input from the user and to display output back. The two devices for accepting input are the Keyboard and the Mouse. The device for displaying output is the Screen. 4.1.1 VARIABLES A variable is a storage space for holding a number or a string (text). A variable name is assigned to the storage space for use in a program. A variable name must start with a letter followed by any combination of letters and numbers. A variable can be used anywhere a number (or string) is needed. Of course you must assign the variable a value before using it.
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NOTE: Variable names are case sensitive, so Distance, DISTANCE, and distance are not the same variable. RobotBASIC is generally not case sensitive. Variable names, array names and labels are the exceptions to this rule.
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In most computer languages variables have to be assigned a type and cannot be used to store values of different types. So, for example, in the standard BASIC language you have to name a variable with a $ at the end of the name to indicate that it is to hold a string value. If you try to store an integer in it you will get an error. The variables in RobotBASIC are more versatile than this. When you name a variable you are not restricted as to what to name it and you can store values of any type in it. Furthermore, you can change the type and value of the data stored in a variable at any time. This is a very powerful feature. You can read all about variables in Secs. B.7.3 and C.4. Also you can read further about data types in Secs. B.7.1 and B.7.2.
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4.1.2 THE KEYBOARD There are various commands to obtain input from the user using the keyboard. We will only use two of them here. See Sec. C.7 for details. 4.1.2.1 GetKey Var This command is useful in loops like for-loops, or whileloops (see later) where you want something done repeatedly but want the user to be able to affect the action by pressing a key. You do not want the repeated action to pause until the user presses a key, but you want it to change if she/he does press a key. This command checks if any key is pressed on the keyboard. If there is a key pressed its code value is placed in the variable Var. If no key is pressed the value 0 is placed in the variable. The program ow is not paused until the user presses a key. If the user presses a key when the command is executed the key will be reported, but if the user does not press any key by that time, the program will report a 0 and go on to the next command and proceed with the rest of the program. (See also the command GetKeyE Var.) 4.1.2.2 WaitKey {ExprS }, Var This command pauses the program ow until the user presses a key. This command is useful for allowing the user to press buttons on the keyboard to achieve various actions, where each action is assigned a key value. Once the user presses a key, the key code is placed in the variable Var and then the program continues with the next command. Read Sec. C.7 for more information on this command. The two commands above obtain input from the user but only one key press at a time. The information obtained is a number that represents the key the user pressed. This number is a standard code for computers called the ASCII code. You can convert this code back to a letter by using the Char() function. You can also convert a letter to its code by using the Ascii() function. So Ascii( A ) gives the numeric code 65 and Char (66) gives B . See Sec. C.8 for details of these functions. There is another way to obtain input from the user. This way allows the user to input any combination of keystrokes to form a sentence or a number. The user then presses Enter to indicate completing the entry. The command is Input ExprS,Var. Read more about this command in Sec. C.7. 4.1.3 THE MOUSE The Mouse is a very useful input device. In RobotBASIC there is a function that enables you to obtain the information a mouse provides. The command looks like this:
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