Tutorial I wrote a while back re True/False. Hope it helps.

by Moneo (no login)

 
TRUE/FALSE TESTING
==================

Testing conditions or switches for TRUE or FALSE can be confusing, so first let's look at some QBasic/QuickBasic (QB) rules:

RULE #1: A boolean expression: any expression that evaluates as follows:
* to TRUE (nonzero)
* to FALSE (zero)

RULE #2: Conditional statements like IF, DO...LOOP, WHILE...WEND test a boolean expression and will execute, branch or loop based on whether the boolean expression is TRUE (nonzero) or FALSE (zero).

RULE #3: However, when a boolean expression is used as a value, assigned to a variable or output in any way, QB will convert the result, after evaluation, to TRUE (-1) or FALSE (0).

EXAMPLES:
x=2
if x then print "is true" .....Will print because x is TRUE (nonzero)
if x-2 then print "is true" ...Will not print because x-2 is FALSE (zero)
if x-5 then print "is true" ...Will print because x-5 is -3 is TRUE (nonzero)
result = (x=2) ...............result will be -1 (TRUE) because x=2 is true
result = (x>0) ...............result will be -1 (TRUE) because x>0 is true
result = (x<0) ...............result will be 0 (FALSE)

EXAMPLE OF TESTING A SWITCH WITH AN IF STATEMENT:

if switch then goto switchon

The boolean expression containing the variable "switch" is evaluated for zero or non-zero in accordance with Rule #2.
* If the result is non-zero, the condition is considered TRUE, and the goto takes place.
* If the result is zero, the condition is considered FALSE, and the goto does not take place.

Let me stress a point regarding the value of "switch". The above IF would do the goto under many nonzero circumstances, including:
* A value of -1
* A value of 1

The fact that the IF works for -1 or 1 tricks some people into thinking that either of these can always be considered as TRUE. It depends, see Rules #2 and #3 above.

EXAMPLE OF STRAIGHTLINE VERSUS TRICKY BOOLEAN CODE:

Here's a straightforward line of code. If the code is 1 then we want to increment count1 by 1.
if code=1 then count1=count1+1

A clever, but confusing alternative is:
count1 = count1 - (code1=1)

Here's how it works. The boolean expression (code1=1) is evaluated and the result is either TRUE (-1) or FALSE (0), in accordance with Rule #3.
* If it's TRUE (-1), count1 will be incremented because -(-1) is +1.
* If it's FALSE (0), count1 will not be incremented because -(0) is 0.

This kind of cleverness dates back to assembly language where IF-type conditional logic was avoided. With today's languages, you don't need to be so cute. You'll only be confusing yourself or the next programmer doing maintenance.

EXAMPLE OF OVER-SIMPLIFYING CODE WITH BOOLEAN EXPRESSIONS:

The following is the code I use to determine if a year is leap year. The resultant variable Isleap is set to TRUE (-1) or FALSE (0).

IsLeap = (Y MOD 4 = 0 AND Y MOD 100 <> 0) OR (Y MOD 400 = 0)

The logic is fairly straightforward following the standard definition of leap year which says:
If the year is evenly divisible by 4 and not divisible by 100,
or if the year is evenly divisible by 400,
then it is a leap year.

The code has two boolean expressions enclosed in quotes to comply with the definition. So, if either expression is true, it is a leap year.

Here's another implementation of the same logic which at first glance looks a little simpler.

Isleap = (Y MOD 4 = 0) - (Y MOD 100 = 0) + (Y MOD 400 = 0)

This code, which also works by the way, has three boolean expressions enclosed in quotes. Taking into account that TRUE is -1 and FALSE is 0, arithmetic is performed on the results of these expressions to produce a final TRUE/FALSE condition.

It took me a while to understand this code, and I tested it with a little program. I'll let you figure it out. Again, the second version of the code looks simpler, but I consider it more complicated. Like Albert Einstein said: "We have to make things simple, but not simpler."

RECOMMENDATIONS:

1) Be aware of Rule #2 when writing conditional statements, that is, TRUE is nonzero and FALSE is zero.

2) If you're going to use or assign the results of boolean expessions, remember that TRUE is -1 and FALSE is 0.

3) If you're going to use switches, avoid using 1 for on, and 0 for off, even if that seems to make more sense to you. Use -1 for on, and 0 for off.

The recommended and more explicit way of handling switches is as follows:

CONST FALSE = 0
CONST TRUE = NOT(FALSE) ..... this is equal to -1

if somecondition then switch is TRUE

if switch = TRUE then goto swon
if switch = FALSE then goto swoff

To flip a switch, turn it off if on, or turn it on if off:

switch = not(switch)

*****

Posted on Oct 14, 2010, 12:33 PM

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Response TitleAuthor and Date
*"enclosed in quotes"? I don't see any quotes.A ninny mouse on Oct 16
 Re: *"enclosed in quotes"? I don't see any quotes.Moneo on Oct 24
  * Moneo, he is just ACTING like a NINNY, get it? ninny...:-) on Oct 25
rule #3 on Oct 20
 Re: rule #3Moneo on Oct 21
  Re: rule #3 on Oct 27
   Re: rule #3 moreMoneo on Oct 29
    Constants add meaning to numbers. on Oct 29

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