[编程交流] lambda , function , mapcar , a

(+ 1 2 3) ; Returns 6(apply '+ (list 1 2 3)) ; Returns 6

The reason you'd use apply instead of simply writing the normal call is, sometimes you don't know what contents are in the list or you obtain the list from somewhere else. Note also the single-quote before the +, this is a shorthand way of writing

( + (+ 2 3) (*2 5)))

 
The interpreter does the following:
 
evaluate 5 as 5 and 2 as 2
applies those numbers to the * function and evaluate it as 10
evaluate 3 as 3 and 2 as 2
applies those numbers to the + function and evaluate it as 5
applies the evaluated 10 and 5 to the + function and evaluate it as 15
Autolisp evaluation
 
Evaluation is actually the result of an expression or any data within a parenthesis. Note that arguments evaluated first and then applied to the function.
 
Evaluation prevention
Autolisp evaluate everything by its nature, so there is a need to control when we don't want things to be evaluated or skipped by the interpreter.
 
To prevent evaluation we use the Quote function
 
(Quote (a b)) or '(a b)
 
Variables
 
Variables are container to hold data. A name of a variable is always the same but the value can be change by user manipulation only.
 
Data types
Integer
 
An integer is a whole number without a decimal point and its range between 32000 to -32000. If an equation that emulate to a number with a decimal point its being dropped out, for example
 
(/ 15 2) evaluate to 7
 
Integers are evaluated to themselves.
 
Real
 
Is a number with decimal point .
 
(/15 2) 7.5
 
Note that values between 0.0 and 1.0 must start with 0 or 1,
 
Not .425 but 0.425.
 
String
 
A string is a sequence of data types so called characters.
 
Lists
 
Can be treated as a container that holds all kind of data types and even other lists.
 
Assigning values to variables
Assigning values to variables is issued by two functions combination, set and quote which form one function setq .
 
set means set the value of the first argument to be equal to the evaluation of the second argument.
 
Quote means don't evaluate the first argument
 
Setq means set the value of the first argument to be equal to the evaluation of the second argument but don't evaluate the first argument.
 

+ + 2 3 * 2 5--> + 5 * 2 5--> + 5 10--> 15

This is because pure polish means each "operator" is a binary operator and can only be applied to 2 values. Lisp allows multiple values to be passed into the "operator" (zero or more), that's why it needs to group them together and why it need parentheses in all cases.
 
So in the Lisp idea the following happens in your formula:

(/ 1 2) ; Returns 0 integer(/ 1 2.0) ; Returns 0.5 real

This is a bit different from other Lisps. E.g. using some implementations of Common Lisp you would get a result which uses a fraction data type:

(setq test 123 test2 "A string")(type test) ;Returns 'INT, but ...(princ INT) ;Prints nil(princ 'INT) ;Prints INT(type test2) ;Returns 'STR, but ...(princ STR) ;Prints nil(princ 'STR) ;Prints STR, BUT ...(= 'STR 'INT) ;Returns nil(= STR INT) ;Returns T

This can be used to make much more efficient comparisons. You can use a quoted symbol as a piece of data, this means it uses less RAM but also when compared to another uses less CPU. A normal value would need to be checked for its symbol, then from that the RAM position needs to be extracted and finally the value stored in that position in RAM needs to be extracted - only then can it be compared to something else. Comparing 2 quoted symbols means 2 of those steps are eliminated.