Question

RACKET

a) Write a recursive function (gen-list start end). This function will generate a list of consecutive integers, from start to end. If start > end then an empty list is generated. For example: (gen-list 1 5) ---> (1 2 3 4 5)

b) write a recursive function pair-sum? that takes an integer sequence as generated by the gen-list function in exercise 4 above. This function tests whether any two adjacent values in the given list sum to the given val. For example,

   (pair-sum? '(1 2 3) 3) ---> #t since 1+2=3. Similarly,

   (pair-sum? (gen-list 1 100) 1000) ---> #f since no two adjacent integers in the range 1 to 100 can sum to 1000.

You must use recursion, and not iteration. Please include explanation thanks.

Answer 1

Solution :-

a)

Previous Version:

//suppose start=n and end=N

void gen_list(int n, int N) // This gen_list will generate  list of consecutive integers, from start (n) to end (N)
{

   if(n>N) //Start >End

{

printf("The list is empty"); //It will print empty list

return; // return to main()

}
if (n <= N) //Start <End
{
printf("%d,", n); //print the number
n++; // increment n
gen_list(n,N); //recursion call
}
else
return; // return to main()
}

In RACKET:

(define (gen-list Start End)
(if (> Start End) // check if Start grearted than End or not
#f   
(cond ((<= Start End) // check if Start less than End or not
(gen-list Start (- End 1)) //Call to recursion
(display End) //show the output
(display " ")))))

b)

Previous Version:

int i=1, int j=1; //intilialize to variables

void pairsum?(int a[], int i, int j, int sum, int n) //list of integers are stored in the array a, sum represents sum of two numbers and n is the numbe of elements in the array
{

int k;

if(i>n || j>n) //required sum not exists

{

printf("#f");   

return; // return to main()

}

k=a[i]+a[j];

if(k==sum)

{

printf("#t");

return; // return to main()

}

else{

pairsum?(a,i+1,j+1,sum,n); //call to recursion after incrementing i and j
}
}

In RACKET:

(define (pair-sum? 1st s) // 1st is the input list of numbers and s is the required sum  
(if (null? (cdr lst)) // checking is list is null or not
#f
(if (= (+ (car lst) (car (cdr lst))) s) // checking the sum of paires is equal to s or not
#t
(pair-sum? (cdr lst) s)))) // Call to recursion

Thank you.... if any Quire's Feel free to ask me Sir