Boolean Expressions

A Boolean expression is an expression that answers a true or false question.

BASIC-256 has no separate Boolean type. True is the whole number 1 and false is the whole number 0 (see Boolean Constants), so a Boolean expression is an integer expression whose value happens to be one of those two. The answer can therefore be stored in a variable, printed, or even added up.

print 3 > 2
print 3 = 2
print (3 > 2) + (4 > 3)

displays

1
0
2

Boolean expressions are built with Comparison Operators, Logical Operators, the constants true and false, and any function that answers a true or false question, such as Exists or IsNumeric.

When a Value Counts as True

Anywhere a true or false value is expected — in an If, a While or an Until, or on either side of NOT, AND, OR and XOR — any value may be used. It is judged like this:


Value	Counts as
A whole number	True when it is not zero.
A decimal number	True when it differs from zero by more than 0.00000001. A smaller number, such as 0.000000001, counts as false.
A string	True when it is not empty. Every non-empty string is true, including “0” and “false”.
A variable that has never been assigned	False.
An array or a map	Not allowed. The program stops with “Unable to convert to a Boolean”.

How Two Values are Compared

What a comparison actually compares depends on the types of the two values:


Both values	Are compared
Two whole numbers	Exactly.
Two strings	Character by character, using the character codes. The comparison is case sensitive and “Z” comes before “a”.
Anything else	As decimal numbers, and they count as equal when they differ by no more than 0.00000001.

The last row also covers a string compared against a number: the string is converted to a number first, and a string that is not a number counts as 0 and reports “Unable to convert to a number”.

print "10" = 10
print "abc" < "abd"
print "Z" < "a"
print 0.1 + 0.2 = 0.3

displays

The last line is not what most programming languages give. 0.1 + 0.2 is not exactly 0.3 in binary arithmetic, but the difference is far smaller than 0.00000001, so BASIC-256 calls the two equal.

Both Sides are Always Evaluated

AND and OR do not stop early. Both sides are worked out before the operator is applied, even when the left-hand side has already settled the answer. A function call on the right of an AND therefore always happens.

Not the Same as the Bit Operators

NOT, AND, OR and XOR work on the truth of a whole value. The Bitwise Operators work on the individual bits of a number. They are not interchangeable:

print 6 and 3
print 6 & 3

displays

1
2

Six and three are both non-zero, so both are true and “6 and 3” is true. The bits of six (110) and three (011) have only one bit in common, so “6 & 3” is two.

Availability

Present in BASIC-256 but never described — this page was empty. Written from the BASIC-256 v2.1 continuation project source.