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Topic 9 Symbols and eq? vs equal?

This article explores the fundamental role of symbols in Lisp/Scheme, highlighting their importance in artificial intelligence and symbolic data manipulation. It delves into programming techniques for comparing symbols using predicates like `eq?`, and demonstrates how to manipulate and analyze English sentences. Additionally, it provides examples of membership testing and counting occurrences of symbols within lists. The guide offers insights into efficiently working with symbolic data structures in Lisp, enhancing programming productivity.

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Topic 9 Symbols and eq? vs equal?

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  1. Topic 9 Symbols and eq? vs equal? Section 2.3.1 October 2008 Programming Development Techniques

  2. Example of Symbol Manipulation • One of the corner stones of Lisp/Scheme is its ability to easily manipulate symbols. • Because of this it has been heavily used in Artificial Intelligence. • As an example, we will write a program to manipulate English Sentences. Programming Development Techniques

  3. Symbolic data (quote a) --> a (quote this-is-a-symbol) --> this-is-a-symbol 'a --> a '(a b d) --> (a b d) '(1 2 (a (3 d) this is the life)--> (1 2 (a (3 d) this is the life) Programming Development Techniques

  4. Some cautions ''c --> (quote c) '(1 'a 2) --> (1 (quote a) 2) Programming Development Techniques

  5. The eq? predicate (eq? x y) returns #t iff x and y are both the same (small) integer, the same symbol or the same pointer to a data structure. It always returns #f if x and y are real numbers. Programming Development Techniques

  6. Some examples (eq? 'a 'a) --> #t (eq? 'a 'b) --> #f (eq? 1234 1234) --> #t (eq? 1234567890 1234567890) --> #f (define x (list 1 2)) (eq? x x) --> #t (eq? (list 1 2) (list 1 2)) --> #f Programming Development Techniques

  7. An efficient membership test (see lisp file) ; takes a symbol named item and a list ; and checks for membership of item in lst (define (memq item lst) (cond ((null? lst) #f) ((eq? item (car lst)) lst) (else (memq item (cdr lst))))) (memq 'a '(b a d a b i n g)) --> (a d a b i n g) Programming Development Techniques

  8. A way to count items in a list ; counts the number of times symbol item ; occurs in top level of lst (define (item-count item lst) (define (item-count-iter sublist count) (let ((next-sublist (memq item sublist))) (if (not next-sublist) count (item-count-iter (cdr next-sublist) (+ count 1))))) (item-count-iter lst 0)) (item-count 'a '(1 a (a a (a)) b 2 a)) --> 2 Programming Development Techniques

  9. What about lists whose elements are not symbols? ; takes two expressions and is #t if they are the ; same symbols, or if they are lists with same elements ; and structure -- i.e., they look the same (define (my-equal? s1 s2) (cond ((symbol? s1) (eq? s1 s2)) ((symbol? s2) #f) ((and (null? s1) (null? s2)) #t) ((or (null? s1) (null? s2)) #f) (else (and (my-equal? (car s1) (car s2)) (my-equal? (cdr s1) (cdr s2)))))) Programming Development Techniques

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