Logical Expressions

Logical expressions (also called Boolean expressions) can have only two values; true or false. They generally result from comparisons you make in your DataBasic code. For example, the code:

A EQ 27

tests the variable A to see if it is equal to 27. If it is, the result is true; otherwise the result is false.

Logical expressions are typically used in branching and looping control structures. For example, the IF statement performs one action if a logical expression is true and another if it is false:

IF A EQ 27 THEN
    B = B + 1
END ELSE
    A = A + 1
END

This code adds 1 to B if A is equal to 27; otherwise it adds 1 to A.

However, relational, arithmetic and string expressions can all be used as logical expressions:

Relational expressions are false if they evaluate to 0 and true if they evaluate to 1.
Arithmetic expressions and numeric strings are false if they evaluate to 0 and true if they evaluate to anything other than 0.
Non-numeric strings evaluate to false and generate the run-time error [B16] Non-numeric data when numeric required; zero used.

Note: This behaviour can be changed with the TRUE.NONNULL compatibility option.
Null strings are false.

You can use the NOT function to negate (invert) a logical expression.

Logical Operators

The following operators can be used to combine logical expressions:

Symbol	Operation	Example
AND or &	logical AND	The expression `A OR B` is true if A and/or B is true. It is false only if both A and B are false.
OR or !	logical OR	The expression `A AND B` is true only if both A and B are true. It is false if A and/or B is false.

For example,

IF A EQ 27 AND B > 0 THEN
    B = B + 1
END ELSE
    A = A + 1
END

adds 1 to B if both A is equal to 27 and B is greater than 0; otherwise it adds 1 to A. On the other hand,

IF A EQ 27 OR B > 0 THEN
    B = B + 1
END ELSE
    A = A + 1
END

adds 1 to B if either A is equal to 27 or B is greater than 0 (or both); otherwise it adds 1 to A.

Notes:

The value returned by a compound AND expression is that of the leftmost expression if the complete expression evaluates to true, or 0 if it evaluates to false.
The value returned by a compound OR expression depends on whether any of the second or subsequent expressions evaluate to true. If this is the case, 1 is returned; otherwise, if the leftmost expression evaluates to true, the value of the leftmost expression is returned.
In a compound logical expression, every sub-expression is evaluated.

The behaviour of all these can be changed with the TRUE.BOOL compatibility option.

Precedence

Logical operators have the lowest precedence and are only evaluated after the other operations have been performed. If two or more operators appear in an expression, the leftmost is performed first.

Parentheses may be included into an expression to alter the precedence. Any logical operation included in parentheses is carried out first. Parentheses can be used to combine two or more logical operations in the same expression and re-define the order in which they are carried out.

Examples

A = 16
X = 1 AND A

X evaluates to to 1, because the current value of A is true (non-zero).

A = 4
B = 1
J = 13
Y = B < (A * 2 - 5) AND J > 20

Y evaluates to 0 (false) because, although B < (A * 2 - 5) is true, J > 20 is false. Both expressions would have to be true in order for X to be true.

A = 12
B = 0
C = 400
X = 5 * A AND (B OR C)

Evaluates to 60, because 5 * A equals 60 and the expression within parentheses (evaluated first) is true (C is non-zero); see notes.

Y = X1 AND X2 AND X3

Sets Y to the value of X1 if variables X1, X2 and X3 are all non-zero; otherwise, sets Y to 0.

Y = "XYZ1" MATCHES "4X" AND X

Sets Y to 1 if X is non-zero, because the first expression "XYZ1" MATCHES "4X" is true.