Logout

--- Simple logic gates ---

2.1.11

Define the Boolean operators: AND, OR, NOT, NAND, NOR and XOR.

 

Teaching Note:

LINK Introduction to programming, approved notation sheet.

 

 

Sample Question (in this case, sort of...) - FORMER CURRICULUM

The method logic1(), shown below, returns the output from a particular logic gate.
The logic gate has two inputs, a and b.

//parameters a and b can only have the values of 0 or 1
public boolean logic1(int a, int b){ 
    
    if(!(a==b)) return true;
else return false; }

(a) Identify the logic gate represented by the above method. [1 mark]
(b) Construct the method logic2(), which would similarly return the output from
a NAND gate. [2 marks]


 

Possible "Spiraling" Through Logic Gates:

With this material basic taken at face value, there's no real need to go into details. But why not? It's very interesting stuff. So to do so with these notes, here's a suggested approach:

Step 1: Look at all three 2.1.11-13 in a "mercenary" way; just, how can you do these things /answer these assessment statement "at face value" And definitely mainly relate them to what you already know: compound boolean conditions in Java. So look superficially at just the parts of each notes page that relate directly to the assessment statement.

Step 2: Then look at the advanced material, including the video on transistors, the videos showing logic gates as dominoes, and even the crazy intense Crash Course videos, as well as the in depth parts of the notes about how all this ultimately has to do with the logic gates of circuits.

Step 3: Then, spiral through once again the more IB CS assessment statement related part, keeping in mind what you are doing with truth tables and so on is mimicking what is actually happening at the very low level of transistors.

____________________________


Preamble

The only confusing part of this assessment statement is that you can interpret/apply it four different ways. But the fundamental concept is the same.

You can "define" boolean operators by:

1. A truth table.
2. In a dictionary, Boolean Logic, or Boolean Math way.
3. How they are applied in a specific programming languages like Java.
4. How they operate as actual cricuit logic gates.

The details will follow in this assessment statement and the next, but as an overview, and also as a very useful "second spiral" just before a test/exam, take a look at this question.

Question:

(a.) Define the AND boolean operator by truth table.
(b.) Define the AND boolean operator according to Boolean logic.
(c.) Define the AND boolean operator as it is used in compound conditions in the Java proramming language.
(d.) Define the AND logic gate used in microprocessors.

Answer:

(a.) Truth table for AND:

A

B

table 1

0

0

0

0

1

0

1

0

0

1

1

1

(b.) Boolean Logic AND: The overall value of an AND expression is true only when both operands are true.

(c.) Java AND operator: The AND operator in Java is &&. A compound condition using the && operator will only evalulate to true if both conditions/variables are true. For example, if boolean a = true; boolean b = false; and boolean c = true; then (a && b) evaluates to false, but (a && c) evaluates to true.

(d.) AND Logic Gate: The AND logic gate is a circuit which has two inputs. It is designed such that only when both of the circuits have current applied to them will the gate allow electricity to be passed through it.

 

______________________________________

Boolean Operators in Programming

Of these four applications of boolean operators, it makes sense to start with Boolean operators in the context you have (likely) already seen them: programming, in particular compound conditional statements in Java.

--------> Do make a point of going to a programming IDE (like Netbeans or IntelliJ IDEA) and playing around with boolean logic again to remind yourself how it works.

Java Boolean Operators:

AND - &&
OR - ||
NOT - !
XOR - ^
NAND - !( && )
NOR - !( || )


 5 package mrrayworthoctober;
 6 
 7 /**
 8  *
 9  * @author John Rayworth
10  */
11 public class ForBooleanOperators {
12     public static void main(String[] args) {
13         boolean a = false;
14         boolean b = false;
15         boolean c = false;
16         
17         // AND
18         if(a && b){
19             System.out.println("a and b are true");
20             
21         }
22             
23         // OR 
24         if(a || b){
25             System.out.println("Either a or b are true (and possibly both are.)");
26         }
27         
28         // NOT
29         if(!a){
30             System.out.println("a is the opposite of what it was first declared.");
31         }
32         
33         // NAND
34         if(!(a && b){
{
35             System.out.println("It is not the case that a and be are true.");
36         }
37         
38         // NOR
39         if(!(a || b)){
40             System.out.println("It is not the case that one or the other (or both) are true");
41         }
42         
43         // XOR
44         if(a ^ b){
45             System.out.println("Either a or b are true, but not both.");
46         }
47     }
48     
49 }
50 

Here is an second example which is a code fragment you may recognize from being used in Topic 4, where the various boolean operators are used.

The idea with this is to give a good example of a bunch of boolean operators, but particularly xor; in this case the company will only hire the individual if they have over 10 years experience or over 10 years of university, but not both, because they could not afford to pay such an employee.

21             if(companyDivision.equals("a")){
22                 if(yearsExp > 10 && yearsUni > 10){
23                     System.out.println("You are hired, congratuations.");
24                 }
25                 else{
26                     System.out.println("Sorry.");
27                 }
28             }
29             else if(companyDivision.equals("b"))
30             {
31                 if(yearsExp > 10 ^ yearsUni > 10){
32                     System.out.println(fullName + "You are hired, congratuations.");
33                 }
34                 else{
35                     System.out.println("Sorry.");
36                 } 
37             }
38             else if(companyDivision.equals("c"))
39             {
40                 if(yearsExp > 10 || yearsUni > 10){
41                     System.out.println(fullName + "You are hired, congratuations.");
42                 }
43                 else{
44                     System.out.println("Sorry.");
45                 }
46             }
47             else{
48                 System.out.println("Sorry, you did not type in a or b or c.");
49             }       
50         }

 

Reminder of Evaluating Compound Boolean Expressions

Recall Evaluating Arithmetic Expressions:

For example, c = a + b – (3*4) is known as an expression, and it evaluates to a single numeric value through the order of operations.
If a = 2 and b = 3, this arithmetic expression simplifies to -7:
c = 2 + 3 - (3 * 4)
c = 2 + 3 - (12)
c = 5 - 12
c = -7

Evaluating Boolean Expressions is approached the same way, step by step:

For example, ((x+y)*z == 12 && q.equals(r)) is also an expression, but a Boolean expression, so it ultimately evaluates down to a single Boolean value, in a similar fashion to evaluation of the arithmetic expression above.

If x = 2, y = 1, z = 4, and q and r are both "hello world", this boolean expression simplifies to true in four steps:

((2 + 1) * 4 == 12 && "hello world".equals("hello world"))
((3) * 4 == 12 && true)
(12 == 12 && true)
(true && true)
true

 

____________________________

 

A Program To Lead to and Prove Definitions of Boolean Operators

____________________________

Now to the General Definitions

In terms of the assessment statement, how would we define boolean operators?

Well, there are actually two ways of "defining" boolean operators, one is with truth tables (see the next assessment statement) and one is the dictionary way. This assessment statement deals with the dictionary way. (But even the "dictionary" way can be either in terms of logic gates in the circuitry of computers, or in terms of boolean programming constructs - see the "Details" below.)


("Tier 1") Basic Boolean Math Definitions of the Boolean Operators

AND - both boolean variables must be true for the overall expression to be true.

OR - at least one of the boolean variables must be true for the overall expression to be true.

NAND - the two boolean variables can't both be true for the overall expression to be true.
(Alternatively, "returns the opposite boolean value of an AND expression").

NOR - neither of the boolean variables can be true for the overall expression to be true.
(Alternatively, "returns the opposite boolean value of an OR expression.")

NOT - inverts the value of a boolean expression.

XOR - only one of the boolean variables can be true for the overall expression to be true.

A little bit more on NAND and NOR

____________________________

 

Making the Connection to Logic Gates

The logic discussed above does not just function at a higher level via programming, it is fundamentally the way computers work at the most basic level, right on the CUP and other microprocessing chips.

Transistors on Microprocessors

The first thing you probably need is an idea of what a circuit, or more particularly a transistor is. And you should appreciate that in terms of a computer a transistor is the logic gate which a the single fundamental component of all microprocessing chips like the CPU. Here's an excellent introductory video:

 

(Sorry, but I've lost the reference to credit the source of this.)

Logic Gates

Logic gates are the basic building blocks of microprocessing chips, including the CPU and RAM memory. They are simple transistors that have one output, and usually two inputs. Current of two different levels is passed to the inputs. Depending on the kind of logic gate it is (AND, OR, XOR etc.) only certain combinations of low/high current (low-low, or high-low etc.) will cause the circuit to be closed - i.e. to allow electricity to pass through it.

Here is a diagram of an AND gate, in which both inputs have to have high current for the circuit to be closed, and electricity flow through the logic gate.

And here is a diagram of an OR gate, in which one or the other inputs have to have a high current for the circuit to be closed, and electricity flow through the logic gate.

Logic gates can be combined. Here's an example. (More on this in the next two assessment statements.)

******** A key realization for you to understand at this stage is that boolean operators and logic gates operate exactly the same way ********. In fact the way a microprocessor is built from the ground up is entirely based on boolean logic. So everything being said about logic gates can be said about the boolean operators used in a programming language, and vice versa.

So, next, check out this great video explaining logic gates with dominoes. (Thanks Mr Walker!)
And the "logical" follow-up video. (All of this does go a bit beyond what you need for IB CS and this assessment statement, but it's kind of a shame to not go any further, because a few learnings on, and you can understand pretty well completely the basic operations of a computer.)

Plus the two appropriate Crash Course Computer Science videos (Take a deep breath!!):
Crash Course # 3 - Logic Gates
Crash Course # 5 - The ALU

Also note that the former curriculum "Red & Yellow" IB CS textbook has a good summary on pages 230 - 237.


Boolean Expressions & Logic Gates

So back to this assessment statement, and we can see that Boolean operators can actually be defined in terms of both:
--> Boolean Compound Conditions
--> and Logic Gates


("Tier 2") Full Definitions - the Three(Four) Different Applications of Boolean Operators:

 


Boolean Math
: A general definition according to general Boolean theory.

Truth Table: (See next assessment statement.)

Java: The overall value of the compound boolean expression will be...

Circuit: The logic gate is on (has electricity flowing through it) when...

AND


Boolean Math
: only when both boolean values are true is the overall expression true.

Truth Table: (See next assessment statement.)

Java: overall true when both boolean variables/expressions are true
Example: if(3 == 3 && 2 == 2) evaluates to true.

Logic gate level: ...electricity flows when both input switches are closed (i.e. electricity is flowing through both of them).

OR


Boolean Math
: if either of the boolean values is true the overall expression is true.

Truth Table: (See next assessment statement.)

Java: overall true expression if one or the other variable/expression is true:
Example: if(42 == 67 || 99 == 99) evaluates to true.

Logic gate level: ...electricity flows when one input switch or the other is closed.

NAND


Boolean Math
: whenever not both of the boolean values are true the overall expression is true.

Truth Table: (See next assessment statement.)

Java: overall true expression if both variable/expressions are not true
Example: if(!(153 == 153 && 6 == 7)) evaluates to true.

Logic gate level: ...electricity flows when at most, one switch can be closed (so not both).

NOR


Boolean Math
: only when neither boolean value is true is the overall expression true.

Truth Table: (See next assessment statement.)

Java: overall true expression if neither variable/expression is true
Example: if(!(234123 ==9797 || 6879 == 23)) evaluates to true.

Logic gate level: ...electricity flows when woth input switches are open.

NOT


Boolean Math
: the overall boolean expression is the opposite of the boolean value it is applied to.

Truth Table: (See next assessment statement.)

Java: overall true expression if what is being negated is itself false
Example: if(!(123 == 567)) evaluates to true.

Logic gate level: ...electricity flows when the input is not on (or the gate is turned off if its input was on).

XOR


Boolean Math
: the overall expression is true only if one but not both of the boolean values are true.

Truth Table: (See next assessment statement.)

Java: overall true expression if one variable/expression or the other is true, but not both
Example: if(1002 == 1002 || 757 == 434) evaluates to true.

Logic gate level: ...electricity flows when one switch or the other is closed, but not both.


____________________________