notes37 - Tree recursion Compare the car/cdr version of...

Info icon This preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
Tree recursion Compare the car/cdr version of deep-map with ordinary map : (define (map fn seq) (if (null? seq) ’() (cons (fn (car seq)) (map fn (cdr seq))))) Each non-base-case invocation of map gives rise to one recursive call, to handle the cdr of the sequence. The car, an element of the list, is not handled recursively. By contrast, in deep-map there are two recursive calls, one for the car and one for the cdr. This is what makes the difference between a sequential, one-dimensional process and the two-dimensional process used for deep lists and for the Tree abstraction. A procedure in which each invocation makes more than one recursive call is given the name tree recursion because of the relationship between this pattern and tree structures. It’s tree recursion only if each call (other than a base case) gives rise to two or more recursive calls; it’s not good enough to have two recursive calls of which only one is chosen each time, as in the following non-tree-recursive procedure: (define (filter pred seq) (cond ((null? seq) ’())
Image of page 1
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern