Download - True BASIC

27
Input Statements
CHAPTER
4
Most useful programs have some flexibility built into them; that is, they can produce different results when provided
with different data. Commonly, the user of the program provides this data and thus has some control over
the program’s behavior.
In True BASIC, the simplest way to obtain data from the user is the INPUT statement. The data provided by the
user may consist of numeric or string values, and it may come from the keyboard or a file.
This chapter introduces the fundamentals of user input. It discusses the INPUT statement, the LINE INPUT
statement, and the GET KEY statement for getting single keystrokes. For information on graphical input from
the GET MOUSE and GET POINT statements, see Chapter 13 “Graphics.” For input from menus, buttons, check
boxes, special edit fields, and dialog boxes, see Chapter 14 “Interface Elements.”
Basic Input
Chapter 2 on “Constants, Variables, and Expressions” illustrates the use of the LET statement to assign data values
to variables. Although it is extremely useful, the LET statement is limited in that you must know the value you wish
to assign when you are writing the program. To change the value, you must change the program.
Since it is impractical and undesirable for the user to change the source code before each run, True BASIC provides
the INPUT statement to let the user assign values to variables during the run. The INPUT statement complements
the PRINT statement. While the PRINT statement lets your program give information (the “answer”) to the user,
the INPUT statement lets the user give information (the “question”) to your program.
The INPUT statement pauses the program, prints a question mark (?) at the current text cursor position, and then
displays the text cursor (if it is turned on). A visible text cursor indicates that the user must enter data before the
program will continue.
The number and type of variables specified as input items determine the number and type of values the user must
supply. The program may include as many input items as necessary in an INPUT statement, separating them
with commas. Consider the following example lines:
INPUT a
INPUT x, y
INPUT name$, age, phone$
The first line expects the user to enter a single number, the second line expects two numbers, and the third expects
three pieces of information — a string, a number, and another string (in that order).
The user must enter the proper number of items, of the proper types, in the proper order, and then press the Enter
(or Return) key to tell the program to continue. Before continuing, the program matches the values entered with
the list of input items from left to right. If the number and types of items do not match exactly, then True BASIC
prints a message and asks the user to re-enter the data. If they do match, True BASIC assigns each entered value
to the corresponding input item. Thus, if a user responds to the third example line above by typing:
? Chris Jones, 32, 555-4321
then the program will assign the string value “Chris Jones” to name$, the numeric value 32 to age, and the string
value “555-4321” to phone$. Notice that the user must separate multiple items by commas.