CSE240 Lecture 22

Introduction to Programming Languages
Control Structures in LISP
(202011)

Transcript

1. jgs
   CSE 240
   Introduction to Programming Languages
   Lecture 22: Control Structures in LISP
   Dr. Javier Gonzalez-Sanchez
   javiergs@asu.edu
   javiergs.engineering.asu.edu | javiergs.com
   PERALTA 230U
   Office Hours: By appointment
   

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2. jgs
   Previously …
   

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3. Javier Gonzalez-Sanchez | CSE240 | Spring 2018 | 3
   jgs
   Control Structures
   § (if (<= 3 2) (* 3 9) (+ 4 2 3))
   ; if (3<=2) {return 3*9;} else {return 4+2+3;}
   9
   § (if (> 2 3) 9)
   ; if (2>3) {return 9;} else {return nil}
   NIL
   § (if (= 2 2) (if (> 3 2) 4 6) 9)
   4
   § (+ 4 (if (= 2 2) (* 9 2) 7) )
   22
   

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4. Javier Gonzalez-Sanchez | CSE240 | Spring 2018 | 4
   jgs
   Control Structures
   § if only allows one test-expression, one then-expression, and one optional-
   else-expression.
   § To do more things in the then-expression we need to make a block. Blocks
   are made with the special form progn, which takes the form:
   (progn expr1 expr2 expr3 ...)
   • (if (> 3 2)
   (progn (print "hello")
   (print "yo")
   (print "whassup?")
   (+ 2 2))
   (+ 1 2 3))
   "hello"
   "yo"
   "whatsup?"
   4
   

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5. jgs
   Loops
   

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6. Javier Gonzalez-Sanchez | CSE240 | Spring 2018 | 6
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   Loop (dotimes)
   dotimes is an iterator with this format
   (dotimes (var high-val optional-return-val)
   expr1
   expr2
   …)
   • (dotimes (x 4) (print "hello"))
   "hello"
   "hello"
   "hello"
   "hello"
   NIL
   • (dotimes (x 4 100) (print "hello"))
   "hello"
   "hello"
   "hello"
   "hello"
   100
   

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7. Javier Gonzalez-Sanchez | CSE240 | Spring 2018 | 7
   jgs
   Loop (dotimes)
   dotimes is an iterator with this format
   (dotimes (var high-val optional-return-val)
   expr1
   expr2
   …) • (setf bag 2)
   2
   • (dotimes (x 3) (setf bag (* bag bag)))
   NIL
   • (print bag)
   256
   

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8. jgs
   Variables
   

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9. Javier Gonzalez-Sanchez | CSE240 | Spring 2018 | 9
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   Global Variables
   § Variables are set with the macro setf.
   • (setf x (* 3 2))
   6
   • x
   6
   • (setf y (+ x 3))
   9
   

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10. Javier Gonzalez-Sanchez | CSE240 | Spring 2018 | 10
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    Local Variables
    § let declares local variables with each declaration. Then it evaluates the
    expressions in order (as a block).
    § let returns the value of the last expression.
    • (setf x 4)
    (let ((x 3)) ; x declared local
    (print x)
    (setf x 9) ; change the value of x
    (print x)
    (+ x x) ;this is the returned value
    )
    3
    9
    18
    • x ;outside the let, we're back to global again
    4
    

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11. Javier Gonzalez-Sanchez | CSE240 | Spring 2018 | 11
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    Nested Variable Scope
    § (let ((x 3) (y (+ 4 9))) (* x y))
    39
    § (let ((x 3))
    (print x)
    (let (x)
    (print x)
    (let ((x "hello"))
    (print x))
    (print x))
    (print x)
    (print ”end"))
    3
    NIL
    "hello"
    NIL
    3
    "end"
    "end"
    

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12. Javier Gonzalez-Sanchez | CSE240 | Spring 2018 | 12
    jgs
    Questions
    

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13. jgs
    CSE 240 Introduction to Programming Languages
    Javier Gonzalez-Sanchez, Ph.D.
    javiergs@asu.edu
    Fall 2021
    Copyright. These slides can only be used as study material for the class CSE240 at Arizona State University.
    They cannot be distributed or used for another purpose.
    

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