Homework 1 (20pts)
due electronically by the end of Jan 28, 2009
The homework requires you to solve the following problems via scheme programming. You need
to develop your scheme programs using the DrScheme environment and submit your solution file
electronically online at
www.cs.utsa.edu/~cs3723.
1. (10pts) Define a recursive function count-symbols which takes an arbitrary value yand returns
how many symbols are contained within y. Use the following test cases for your function.
> (count-symbols 7)
0
> (count-symbols ’y)
1
> (count-symbols ’(x 3 y))
2
> (count-symbols ’((x 3) (y 2) (3 z)))
3
2. (10pts) Define a recursive function count2symbols which takes an arbitrary value yand returns
how many times two symbols are next to each other within y. Use the following test cases for
your function.
> (count2symbols 7)
0
> (count2symbols ’y)
0
> (count2symbols ’(x 3 y))
0
> (count2symbols ’(3 x y))
1
> (count2symbols ’((x y z) (3 z 4)))
2
3. Extra-credit(10pts) Define a recursive function collect-symbols which takes an arbitrary value
yand returns a new list that contains only the symbols in y. Use the following test cases for
your function.
> (collect-symbols 7)
empty
> (collect-symbols ’y)
’y
> (collect-symbols ’(x 3 y))
(list ’x ’y)
> (collect-symbols ’((x 3) (y 2) (3 z)))
(list (list ’x) (list ’y) (list ’z))
1
