Welcome to the lesson on comparisons in AutoLISP! In any programming language, the ability to compare values is fundamental. This capability enables us to make decisions, filter data, and control program flow. In AutoLISP, the primary comparison operators we will be covering are <
(less than), <=
(less than or equal to), >
(greater than), and /=
(not equal to). By the end of this lesson, you’ll have a comprehensive understanding of how to use these operators effectively.
Learning OutcomesBy the end of this lesson on comparisons in AutoLISP, students will be able to:

Understanding Comparison Operators
Comparison operators, as the name suggests, allow us to compare two values. In AutoLISP, these operators return a T (true) or nil (false), depending on the result of the comparison.
1. The <
(Less Than) Operator
The <
operator checks if the value on the left is less than the value on the right.
Syntax:
(< value1 value2) 
Example 1:
(< 3 5) ; This will return T, because 3 is less than 5 
In this example, we’re comparing the numbers 3 and 5. Since 3 is indeed less than 5, the function returns T, indicating a true comparison.
Example 2:
(< 10 5) ; This will return nil, because 10 is not less than 5 
Here, we’re comparing 10 to 5. Since 10 is not less than 5, the result is nil, indicating the statement is false.
2. The <=
(Less Than or Equal To) Operator
The <=
operator checks if the value on the left is either less than or equal to the value on the right.
Syntax:
(<= value1 value2) 
Example 1:
(<= 4 5) ; This will return T, because 4 is less than 5 
In this scenario, we’re comparing 4 to 5. Since 4 is less than 5, the function returns T.
Example 2:
(<= 5 5) ; This will return T, because 5 is equal to 5 
Here, we compare the number 5 to itself. The <=
operator checks for both “less than” and “equal to” conditions. Since 5 equals 5, the result is T.
The >
(Greater Than) Operator
The >
operator checks if the value on the left is greater than the value on the right.
Syntax:
(> value1 value2) 
Example 1:
(> 5 3) ; This will return T, because 5 is greater than 3 
In this example, we’re comparing the numbers 5 and 3. Since 5 is indeed greater than 3, the function returns T, indicating a true comparison.
Example 2:
(> 4 10) ; This will return nil, because 4 is not greater than 10 
Here, we’re comparing 4 to 10. Since 4 is not greater than 10, the result is nil, indicating the statement is false.
The /=
(Not Equal To) Operator
The /=
operator checks if the two values are not equal.
Syntax:
(/= value1 value2) 
Example 1:
(/= 5 4) ; This will return T, because 5 is not equal to 4 
In this example, the number 5 is being compared to 4. Since they are not equal, the function returns T, indicating the values are indeed different.
Example 2:
(/= 7 7) ; This will return nil, because both values are equal 
Here, we’re comparing the number 7 to itself. Since both values are equal, the result is nil, indicating no difference between the two values.
Practical Applications of Comparison Operators
Comparison operators are not just for mathematical operations. They play a pivotal role in many aspects of programming:
 Conditional Expressions: Using
if
,cond
, and other conditional functions, comparisons guide program decisions.  Loop Control: In iterative loops, comparisons can determine whether a loop should continue or terminate.
 Data Filtering: When processing lists or arrays, comparisons help in filtering and sorting data.
Practical Examples
To deepen your understanding, let’s explore some realworld applications of these operators:
1. Determining the Smallest Number:
(defun smallest (num1 num2)(if (< num1 num2) num1 num2 ) )(smallest 7 10) ; This will return 7 
In this function, we use the <
operator to compare two numbers. If num1
is smaller, it returns num1
; otherwise, it returns num2
.
2. Determining the Largest Number:
(defun largest (num1 num2) (if (> num1 num2) num1 num2 ) ) (largest 7 10) ; This will return 10 
Here, we employ the >
operator to determine the larger of two numbers. If num1
is greater, it returns num1
; otherwise, it returns num2
.
3. Checking Eligibility for a Senior Discount:
(defun seniorDiscount (age) (if (<= age 65) “Eligible for senior discount” “Not eligible for senior discount” ) ) (seniorDiscount 64) ; This will return “Eligible for senior discount” 
Here, we use the <=
operator to check if someone’s age qualifies them for a senior discount. If their age is 65 or older, they are eligible.
4. Youth Discount Function:
(defun youthDiscount (age) (if (/= age 18) “Eligible for youth discount” “Not eligible for youth discount” ) ) (youthDiscount 17) ; This will return “Eligible for youth discount” 
For this function, the /=
operator checks if someone’s age is different from 18. If their age is not 18, they qualify for a youth discount.
These practical examples showcase the versatility and importance of understanding and correctly applying the comparison operators in AutoLISP. Whether you’re designing tools for CAD operations or automating tasks, these operators will frequently be at the heart of your logic and decisionmaking processes.
Summary
In this lesson, we delved into the key comparison operators in AutoLISP:
<
(Less Than): Compares if the left value is strictly smaller than the right.<=
(Less Than or Equal To): Checks if the left value is either smaller or equal to the right.>
(Greater Than): Assesses if the left value is strictly larger than the right./=
(Not Equal To): Determines if the two values are different.
These operators are quintessential tools in your AutoLISP toolkit. Gaining proficiency in these comparisons establishes a robust foundation for delving into more intricate logical and conditional operations as you advance in your programming journey.
It’s pivotal to practice using these operators consistently and to explore varied scenarios. The more handson experience you gain, the more secondnature these concepts will become. As you continue on your AutoLISP educational journey, you’ll unearth the extensive applications of these seemingly straightforward, yet immensely potent tools.