Chapter 3, Modularity, Objects, and State
Exercise 3.52
Using memoization
The sequence if evaluated in full will be:
1 3 6 10 15 21 28 36 45 55 66 78 91 105 120 136 153 171 190 210
Well the 7+1 even number in the sequence is 136.
And next statement prints the output of numbers divisible by 5.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1 ]=> (stream-ref y 7)
;Value: memo-proc
1 ]=>
;Value: 136
1 ]=> (display-stream z)
10
15
45
55
105
120
190
210
;Value: done
Without memoization
Yes, the output will change if memoization is removed because we re-evaluate stream-cdr every time and which causes the sum to change thus running it again even the same statement running again gives different output.
Note that first term is not reevaluated! It is the second term where reevaluation happens because first term does not require stream-cdr. It is present directly just like as if we were using car for the object created using cons but all the later terms are delayed which causes their value to change in every evaluation!
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
1 ]=> (stream-ref y 7)
;Value: memo-proc
1 ]=>
;Value: 162
1 ]=> (display-stream z)
15
180
230
305
;Value: done
1 ]=> (stream-ref y 7)
;Value: 492
1 ]=> (display-stream z)
15
510
560
635
;Value: done
1 ]=>