# STREAMS 10. Basics of Streams. Practice with Streams COMPUTER SCIENCE 61AS. 1. What is a stream? 2. How does memoization work?

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1 STREAMS 10 COMPUTER SCIENCE 61AS Basics of Streams 1. What is a stream? 2. How does memoization work? 3. Is a cons-stream a special form? Practice with Streams 1. Define a procedure (ones) that, when run with no arguments, returns a cons pair whose car is 1, and whose cdr is a procedure that, when run, does the same thing. Do NOT use cons-stream. 2. Define a procedure (integers-starting n) that takes in a number n and, when run, returns a cons pair whose car is n, and whose cdr is a procedure that, when run with no arguments, does the same thing for n+1. Again, do NOT use cons-stream for this part. 1

2 DISCUSSION 10: STREAMS Page 2 3. Describe what the following expressions define: a. (define s1 (add-stream (stream-map (lambda(x) (* x 2)) s1) s1)) b. (define s2 (cons-stream 1 (add-stream (stream-map (lambda(x) (* x 2)) s2) s2))) c. (define s3 (cons-stream 1 (stream-filter (lambda(x) (not (= x 1))) s3)))

3 DISCUSSION 10: STREAMS Page 3 d. (define s4 (cons-stream 1 (cons-stream 2 (stream-filter (lambda(x) (not (= x 1))) s4)))) e. (define s5 (cons-stream 1 (add-streams s5 integers))) 4. Define facts without defining any procedures; the stream should be a stream of 1!, 2!, 3!, 4!, etc. More specifically, it returns a stream with elements ( ). Hint: use the integers stream. 5. (HARD!) Define powers; the stream should be 1 1, 2 2, 3 3 or, ( ). You cannot use the exponents procedure.

4 DISCUSSION 10: STREAMS Page 4 Practice with Streams 1. Define a procedure (lists-starting n) that takes in n and returns a stream containing (n), (n n+1), (n n+1 n+2)... For example, (lists-starting 1) returns a stream containing like ((1) (1 2) (1 2 3) ( )) 2. Define a procedure, (list->stream ls) that takes in a list and converts it into a stream. Remember, streams don t have to be infinite, and finite streams end with the-empty-stream 3. Define a procedure (chocolate name) that takes in a name and returns a stream like so: STk>(chocolate chung) (chung really likes chocolate chung really really likes chocolate...) Youll want to use helper procedures.

5 DISCUSSION 10: STREAMS Page 5 Stream-processing 1. Define a procedure, (stream-censor s replacements) that takes in a stream s and a table replacements and returns a stream with all instances of all the car of entries in replacements replaced with the cadr of the entries. STk> (stream-censor (hello you weirdo...) ((you I-am) (weirdo an-idiot))) (hello I-am an-idiot...) 2. Define a procedure (make-alternating s) that takes in a stream of positive numbers and alternate their signs. So STk>(make-alternating ones) ( ) and

6 DISCUSSION 10: STREAMS Page 6 STk>(make-alternating integers) ( ). Assume s is an infinite stream. My Bodys Floating Down The Muddy Stream 1. Given streams ones, twos, threes and fours, write down the first ten elements of: (interleave ones (interleave twos (interleave threes fours))) 2. Construct a stream all-integers that includes 0 and both the negative and positive integers. You may use procedures that you have defined above. 3. Suppose we were foolish enough to try to implement stream-accumulate: (define (stream-accumulate combiner null-value s)

7 DISCUSSION 10: STREAMS Page 7 (cond ((stream-null? s) null-value) (else (combiner (stream-car s) (stream-accumulate combiner null-value (stream-cdr s)))))) 4. What happens when we do: a. (define foo (stream-accumulate + 0 integers)) b. (define bar (cons-stream 1 (stream-accumulate + 0 integers))) c. (define baz (stream-accumulate (lambda (x y) (cons-stream x y)) the-empty-stream integers)) 5. Louis Reasoner thinks that building a stream of pairs from three parts is unnecessarily complicated. Instead of separating the pair (S 0, T 0 ) from the rest of the pairs in the first row, he proposes to work with the whole first row, as follows:

8 DISCUSSION 10: STREAMS Page 8 (define (pairs s t) (interleave (stream-map (lambda (x) (list (stream-car s) x)) t) (pairs (stream-cdr s) (stream-cdr t)))) Does this work? Consider what happens if we evaluate (pairs integers integers) using Louis s definition of pairs.

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