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Defining new functions (contd)

Defining new functions (contd). xmemb [x, list] = ; is x a member of list? [ null?[list]  NIL; | eq ?[x, car[list]]  T; | T  xmemb [x, cdr [list]] ] Define a function that will return a count of how many times x appears in list A useful function:

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Defining new functions (contd)

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  1. Defining new functions (contd) xmemb[x, list] = ; is x a member of list? [ null?[list]  NIL; | eq?[x, car[list]]  T; | Txmemb[x, cdr[list]] ] Define a function that will return a count of how many times x appears in list A useful function: not[x] = ; returns negation; i.e., T if x is NIL and NIL if x is T [ eq[x, T]  NIL; | T  T; ] CSE 3341/655; Part 4

  2. Function Definitions (contd.) equal[ x, y ] = ; x and y may not be atoms [ atom[x]  [atom[y]  eq[x,y]; | T  NIL; ] | atom[y]  NIL; | equal[car[x], car[y]]  equal[cdr[x], cdr[y]]; | T  NIL; ] CSE 3341/655; Part 4

  3. Defining new functions (contd) xunion[s1, s2] = ; union of atomic lists, less duplicates [ null?[s1]  s2 | null?[s2]  s1 | T  [ xmemb[car[s1],s2]  xunion[cdr[s1], s2] | T cons[ car[s1], xunion[cdr[s1], s2] ] ] ] ; better: xunion[s1, s2] = [ null?[s1]  s2 ; | null?[s2]  s1 ; | xmemb[car[s1],s2]  xunion[cdr[s1], s2] | T cons[ car[s1], xunion[cdr[s1], s2] ] ] CSE 3341/655; Part 4

  4. Function Definitions (contd.) addUpList[ L ] =; returns sum of all numbers in L [ null?[ L ]  0 | T  +[ car[L], addUpList[ cdr[L] ] ] ] CSE 3341/655; Part 4

  5. Function Definitions (contd.) nNil[ n ] = ; returns a list with as many NILs as value of n [ =[ n, 0]  NIL | T  cons[ NIL, nNIL[ -[n, 1] ] ] ] number[x] : returns T if x is a numerical atom, NIL otherwise genEq[x,y] = ; works for both numbers and non-numeric atoms [ number[x]  [number[y]  =[x,y]; | T  NIL; ]; | number[y]  NIL; | T  eq[x,y]; ] CSE 3341/655; Part 4

  6. Different styles in Lisp maxList[L] = ; returns max of number in non-empty list L ; Functional: [ null?[cdr[L]]  car[L] | >[car[L], maxList[cdr[L]]  car[L] | TmaxList[cdr[L]] ] ; Functional but better: [ null?[cdr[L]]  car[L]; |T bigger[ car[L], maxList[cdr[L]]] ] ; define bigger ; imperative: maxList[L] = max2[car[L], cdr[L]] max2[x, L] :: [ null?[L] x | >[x, car[L]]  max2[x, cdr[L]] | T max2[ car[L], cdr[L] ] ] CSE 3341/655; Part 4

  7. More functions • How do you obtain firstelement of a list? • How do you obtain the last element? [watch out!] • How do you append two lists? [No, not just cons!] • How do you reverse a list? [best: use imperative trick] CSE 3341/655; Part 4

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