Relational Operations - QB64Official/qb64 GitHub Wiki
QB64 supports several relational operations, which are binary operations that test numeric or string values and return an INTEGER value representing a boolean true (-1) or false (0) result. These operations are primarily used in expressions where a condition is required, such as the IF...THEN statement.
List of relational operations
The following table describes the relational operations, where A is the left-hand side operand, and B is the right-hand side operand:
MISSING: Relational Operations Table
True statements return -1 in Boolean evaluations. False returns zero.
Comparing numerical values and variables
For numeric operands, such as INTEGER or DOUBLE values, the relational operations behave as one would expect. For example, the expression (-3.4 Less_Than 1.2) evaluates to true, and the expression (50 Equal 100) evaluates to false.
Example(s)
When a user enters a value greater than or equal to 5, the boolean statement returns -1. Zero is printed otherwise.
INPUT "Enter a value from 1 to 10: ", number
PRINT number >= 5
Comparing string values and variables
For string operands, the Equal and Not_Equal operators test, respectively, if the operands are of the same or differing length, and that each of the corresponding characters match or don't match. For example, the three expressions ("abc" Equal "abc"), ("abc" Not_Equal "abd") and ("abc" Not-Equal "abcdef") all evaluate to true.
When two strings are compared, the left string sets the number of characters to evaluate when not checking for equality. The Less_Than and Greater_Than operators find the first set of non-matching characters and test if the ASCII code values of those characters are less than or greater than each other, respectively. Equal strings MUST be of the same length with identical UCASE$ letters or ASCII characters!
If one string is identical to part of the other, Less_Than returns false if the left-hand side string is shorter than the right-hand side string, while Greater_Than returns true if the left-hand side string is longer than the right-hand side string. For example, the expressions ("abc" Less_Than "abd") and ("abcdef" Greater_Than "abc") both evaluate to true. Even SPACE$ ASCII character values are evaluated!
Example(s)
Shows that the left hand string sets the number of characters to evaluate when using > or < and are not equal when longer.
PRINT "abc" < "abcd"
PRINT "abc" = "abc"
PRINT "abc" = "abcd"
PRINT "abcd" > "abc"
0
-1
0
-1
Comparing user-defined type variables
Variables of a TYPE (UDT) cannot be used as operands to the relational operators, but numeric or string type fields can be used as described above. For example:
TYPE T
a AS INTEGER
END TYPE
TYPE U
b AS INTEGER
END TYPE
DIM x AS T : x.a = 10
DIM y AS U : y.b = 20
PRINT x < y ' <- error: type mismatch
PRINT x.a < y.b ' <- outputs "-1"
Boolean values
The INTEGER values for true and false are such that the bitwise Logical Operators, such as NOT, AND and OR can be used to invert and combine test results. For example, the expression (NOT (10 Greater-Than-Or-Equal 20)) evaluates to true, while the expression ((2 Less-Than 1) AND ("three" Equal "four")) evaluates to false.
MISSING: Logical Truth Table