Relational Operations - QB64 Wiki

Relational Operations

From QB64 Wiki

Jump to: navigation, search

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:


Relational Operations
Operation Description
a = b Tests if a is equal to b.
a <> b Tests if a is not equal to b; equivalent to (NOT (a = b)).
a < b Tests if a is less than b.
a > b Tests if a is greater than b.
a <= b Tests if a is less than or equal to b; equivalent to (NOT (a > b)).
a >= b Tests if a is greater than or equal to b; equivalent to (NOT (a < b)).


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 < 1.2) evaluates to true, and the expression (50 = 100) evaluates to false.

Example: 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 = and <> 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" = "abc"), ("abc" <> "abd") and ("abc" <> "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 < and > 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 cased letters or ASCII characters!

If one string is identical to part of the other, < returns false if the left-hand side string is shorter than the right-hand side string, while > returns true if the left-hand side string is longer than the right-hand side string. For example, the expressions ("abc" < "abd") and ("abcdef" > "abc") both evaluate to true. Even space ASCII character values are evaluated!


Example: 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 user-defined 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 >= 20)) evaluates to true, while the expression ((2 < 1) AND ("three" = "four")) evaluates to false.


The results of the bitwise logical operations, where A and B are operands, and T and F indicate that a bit is set or not set:
Operands Operations
A B NOT B A AND B A OR BA XOR BA EQV BA IMP B
T T F T T F T T
T F T F T T F F
F T F F T T F T
F F T F F F T T
Relational Operations return negative one (-1, all bits set) and zero (0, no bits set) for true and false, respectively.
This allows relational tests to be inverted and combined using the bitwise logical operations.


See also



Navigation:
Go to Keyword Reference - Alphabetical
Go to Keyword Reference - By usage
Go to Main WIKI Page
Personal tools