Function Definitions

 6  -  Arithmetic Tests (Checks)

NOTE  REGARDING  ACCURACY

Different data types and different sources of calculations yield different levels of accuracy.

Integers are completely accurate within their allowed range (-2,147,483,648 through +2,147,483,647).

Floating point real numbers are accurate to between 14 and 17 significant digits.

Trigonometric functions are somewhat less accurate than reals.  This can cause some misleading results from the tests described below.  Numbers which theoretically should be equal, may not be stored accurately enough for the tests to give the result you expect.  For example, the sine of 30° is 0.5.  But if we ask AutoLISP to compare the sine of 30° with 0.5, it says they are not equal!  Try it out (since the trig functions work with radians, you need to substitute one-sixth p for 30°):   (=  0.5  (sin (/  pi  6)))  returns nil !

This highlights the importance of (1) knowing which data type your program is working with, and (2) when needed, using the equal function, explained below, which allows for a margin of error. 


= ..... Tests for equality, can have more than 2 arguments

(= 7 7.0)  returns T
After (setq a 7 b 6.9)
(= a b)  returns nil
(= 4.8 4.80 4.8000)  returns T
(= "hello" "hello")  returns T
(= "hello" "Hello")  returns nil
(= 10 "ten")  returns nil


eq ..... Tests whether the same value (stored in the same memory location) is assigned to two different variables.  Variables that are not eq may still be equal.  Variables that are eq are always equal.  (See equal below).

After (setq L1  '(j k)  L2  '(j k)  L3  L1)
(eq L1 L2)  returns nil
(eq L1 L3)  returns T


equal ..... Tests whether two expressions evaluate to the same thing.  Expressions that are equal may not be eq.  Expressions that are not equal cannot be eq.  (See eq above).

After (setq L1  '(j k)  L2  '(j k)  L3  '(j q))
(equal L1 L2)  returns T
(equal L1 L3)  returns nil

Many times there will be two real numbers which you would consider equal for all practical purposes, but which actually differ very slightly.  (See the "NOTE REGARDING ACCURACY" at the top of this page.) This creates a problem because the equal function considers these two numbers unequal. However, the equal function will accept a third argument which indicates the allowable margin of error.

(equal 3.007 3.008)  returns nil
(equal 3.007 3.008 0.001)  returns T because the margin of error is 0.001
(equal 95 188 100)  returns T because the margin of error is 100
After (setq a 5.00006 b 5.00008)
(equal a b)  returns nil
(equal a b 0.00002)  returns T

This margin of error can also be applied to the distance between points.
After (setq p1 (list 1.00000000 1.00000000) p2 (list 1.00000001 0.99999999)
(equal p1 p2)  returns nil
(equal p1 p2 0.00000001)  returns nil
(equal p1 p2 0.00000002)  returns T


/= ..... Tests for inequality (accepts only two arguments)

(/= 7 6.9)  returns T
(/= 7 7.0000)  returns nil
(/= "high" "low")  returns T
(/= "High" "high")  returns T


< ..... Tests for less-than

(< 5 10)  returns T
(< 10 9.999999999)  returns nil
(< -5 1)  returns T
After (setq a 2 b 4 c 6 d 8)
(< a b c d)  returns T
(< a b c c)  returns nil


<= ..... Tests for less-than or equal-to

(<=  5  10)  returns T
(<=  2  4  6  6)  returns T
(<=  11  -12  13)  returns nil


> ..... Tests for greater-than

(> 5 10)  returns nil
(> 10.0 10.0)  returns nil
After (setq a 18 b 17 c 16 d 15)
(> a b c d)  returns T
(> a b b c)  returns nil


>= ..... Tests for greater-than or equal-to

(>=  10  5)  returns T
(>=  18  17  17  16)  returns T
(>=  -99  2)  returns nil


minusp ..... Tests for a negative number

(minusp 0)  returns nil
(minusp -3.3)  returns T
(minusp 4)  returns nil


numberp ..... Tests whether an item is a number (real or integer)

(numberp 2)  returns T
(numberp 2.193)  returns T
(numberp "ten")  returns nil


zerop ..... Tests for equal-to-zero

(zerop 0)  returns T
(zerop 0.000)  returns T
(zerop 0.00000000000000001)  returns nil


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Copyright © 1988, 1998 Ronald W. Leigh