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Giá trị trả về bởi các chức năng như một toàn thể sẽ là 6. Bây giờ chúng ta biết những gì sẽ xảy ra khi tam giác-đệ quy được gọi với một đối số 3, nó là rõ ràng những gì sẽ xảy ra nếu nó được gọi với một đối số 4: Trong các cuộc gọi đệ quy, đánh giá | Counting function definitions 201 The function is straightforward except for one subtle feature. The true-or-false test of the inner loop looks like this and car sorted-lengths car sorted-lengths top-of-range instead of like this car sorted-lengths top-of-range The purpose of the test is to determine whether the first item in the sorted-lengths list is less than the value of the top of the range. The simple version of the test works fine unless the sorted-lengths list has a nil value. In that case the car sorted-lengths expression function returns nil. The function cannot compare a number to nil which is an empty list so Emacs signals an error and stops the function from attempting to continue to execute. The sorted-lengths list always becomes nil when the counter reaches the end of the list. This means that any attempt to use the defuns-per-range function with the simple version of the test will fail. We solve the problem by using the car sorted-lengths expression in conjunction with the and expression. The car sorted-lengths expression returns a non-nil value so long as the list has at least one number within it but returns nil if the list is empty. The and expression first evaluates the car sorted-lengths expression and if it is nil returns false without evaluating the expression. But if the car sorted-lengths expression returns a non-nil value the and expression evaluates the expression and returns that value as the value of the and expression. This way we avoid an error. See Section 12.4 forward-paragraph a Goldmine of Functions page 155 for more information about and. Here is a short test of the defuns-per-range function. First evaluate the expression that binds a shortened top-of-ranges list to the list of values then evaluate the expression for binding the sorted-lengths list and then evaluate the defuns-per-range function. Shorter list than we will use later. setq top-of-ranges 110 120 130 140 150 160 170 180 190 200 setq sorted-lengths 85 86 110 116 122 .
