Download - True BASIC

Constants, Variables, and Expressions
11
of the string in place of b. If a is less than 1, then True BASIC substitutes a value of 1. If a is larger than the length
of the string, or a is greater than b, the result is an empty string. Thus, any values of a and b are legal. For example,
the program:
PRINT “House”[3:100]
PRINT “House”[-5:20]
PRINT “House”[4:4]
PRINT “House”[5:3]
PRINT “Done”
END
produces the following output:
use
House
s
Done
Note that the substring expression in the fourth line returns the null string because a is larger than b, and thus a
blank line is printed.
You may use parentheses to control the order of evaluation when combining substring expressions with other
string expressions. If you don’t use parentheses, substring extraction occurs before concatenation. Thus,
“Abcde” & “fghijklm”[3:7]
equals “Abcdehijkl”, while
(“Abcde” & “fghijklm”)[3:7]
equals “cdefg”.
Here’s a simple program that uses string constants, variables, and expressions:
LET first_name$ = “Abraham”
LET last_name$ = “Lincoln”
PRINT first_name$ & “ “ & last_name$ & “, 16th President of the US”
PRINT “Subscription Code: “ & last_name$[1:4] & first_name$[1:1]
END
It produces the following output:
Abraham Lincoln, 16th President of the US
Subscription Code: LincA
Assignment Statements
The LET statement is the primary means of assigning values to variables. In assigning values you must assign
numeric values to numeric variables and string values to string variables. (True BASIC does not perform automatic
type conversions.) Examples of numeric assignments are:
LET e = 2.718282828
LET answer = Sin(pi/2) * Exp(-x^2)
LET length = last - first + 1
LET k = k + 1
LET i, j = 2
In an assignment statement, the expression to the right of the equal sign (=) is evaluated first, and the resulting
value is assigned to the variable to the left of the equal sign. This means that you can use the variable appearing
on the left in the expression on the right, and its old value will be used in the expression. In the next to the last
example above, the present value of k increases by one and becomes the new value of k.