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The base case should describe all lists of length one, the shortest possible non-empty lists:
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; A non-empty-list-of-numbers (nelon) is one of |
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; - a cons, which has a first, a number, and rest, empty |
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; (cons num empty) |
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; - a cons, which has a first, a number, and rest, a nelon |
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; (cons num nelon) |
In this case, the template looks as follows:
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(define (func-for-nelon a-nelon) |
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(cond |
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\[(empty? (rest a-nelon)) |
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…(first a-nelon)… |
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\] \[(cons? (rest a-nelon)) |
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…(first a-nelon)…(func-for-nelon (rest a-nelon))…\])) |
Solution 2:
We could use the definition of (perhaps-empty) list-of-numbers, that we've already seen. ; A non-empty-list-of-numbers is
; a cons, which has a first, a number, and rest, a list-of-numbers
; (cons num lon)
Here, the template is different, since it must refer to the template for a regular (non-empty) list-of-numbers. In particular, you don't make a recursive call; you think to yourself "Ah, a-nelon contains a (perhaps-empty) list; I'd better call a function for such regular ol' lists": (define (f a-nelon)
…(first a-nelon)…(func-for-lon (rest a-nelon))…)
Here, to compute the average, what function(s) of general (perhaps-empty) lists do you want to call, as your helper?
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