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# Boolean Logic using VB functions

Posted on 2004-04-21
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I need to get visual basic to do boolean algebra so it can act like a combinational logic circuit.
I programmed it to give an output with a boolean equation but it has been returning decimal values. Why is this? This is the code I used :

Gout = ((NOT B) OR ((NOT A) AND (NOT D)) OR ((NOT A) AND (NOT C) AND (NOT F)) OR ((NOT A) AND (NOT C) AND (NOT E) AND (NOT H))

Please note also that the variables A,B,C,D,E,F,H all represent smaller formula's. These formula's use binary inputs from text boxes. The smaller formula's are all written before the Gout output in the program.
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Expert Comment

you must define variables' types

Dim A As Boolean, B As Boolean, C As Boolean, D As Boolean, E As Boolean, F As Boolean, H As Boolean, Gout As Boolean
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Or use the CBool function to convert each variable to Boolean
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>> Please note also that the variables A,B,C,D,E,F,H all represent smaller formula's

Your formulaes must return Boolean values. For testing purpose do this.

MsgBox A
MsgBox B
MsgBox C
MsgBox D
MsgBox E
MsgBox F
MsgBox G
MsgBox H

If any of them display any values other than a "True" or a "False", then you know where to check!
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I don't think you HAVE to define the variables types, although it certainly makes sense to do so.  Consider the online help text for the Boolean data type:

----

Boolean variables are stored as 16-bit (2-byte) numbers, but they can only be True or False. Boolean variables display as either True or False (when Print is used) or #TRUE# or #FALSE# (when Write # is used). Use thekeywords True and False to assign one of the two states to Boolean variables.

When other numeric types are converted to Boolean values, 0 becomes False and all other values become True. When Boolean values are converted to other data types, False becomes 0 and True becomes -1.

----

If the values coming back are 0 and/or -1, then you are getting the numeric representation of a True/False result.  If the values are something other than 0 and -1, then as the help text states "all other values become true".  Declaring the variables as type Boolean will make interpreting the results easier and will preclude a varibale from having a value other than true or false, but it's not absolutely essential so long as you understand the rules the system uses to determine true and false values in non-Boolean data.
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I think that the author meant something different.

Suggestion:
These variables A,B,C,D,E,F,G and H are 8 bit numbers from 0 to 255.
So he want to make some bitwise operations like AND:
01110100
AND
00110010
________
00110000

This is "boolean algebra" for me.

junebrady: How many states (different values) your variables from A to H can have ? Is your A-H variables can contain only 0 and 1 (1 bit) or they can contain 8 bit values ?

"I programmed it to give an output with a boolean equation but it has been returning decimal values. Why is this" - Because these AND , NOT , OR operates over bits in bytes. So as above AND example, this is equivalent to
116  (Decimal)
AND
50    (Decimal)
______________

48   (Decimal)

For example try this:

MsgBox 116 And 50 ' Returns 48 as above
MsgBox 255 And 47 ' Returns 47
MsgBox 1 Or 2 ' Returns 3
MsgBox 1 Or 3 ' Returns 3
MsgBox 2 Or 3 ' Returns 3
MsgBox Not 255 ' Returns 0

You can "feel" boolean algebra logic in last 4 MsgBox examples.
Is that you need ?
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