LISP新手请求逐行解析count-lower-case-vowels函数：do循环与赋值语句疑问

First, let's restate your code with proper formatting for readability:

(defun count-lower-case-vowels (str)
  (do ((i 0 (1+ i))
       (acc 0)
       (len (length str)))
      ((= i len) acc)
    (when (or (equal (aref str i) #\a)
              (equal (aref str i) #\e)
              (equal (aref str i) #\i)
              (equal (aref str i) #\o)
              (equal (aref str i) #\u))
      (:= acc (1+ acc)))))

1. Function Definition #

(defun count-lower-case-vowels (str))
This defines a new function named count-lower-case-vowels that accepts one parameter: str (the input string we want to scan for lowercase vowels).

2. The do Loop: Core Iteration Logic #

The do loop in Common Lisp is a flexible iteration tool with this structure:
(do (variable-initializations) (end-condition return-value) loop-body)

Let’s break down each part of your do loop:

Variable Initialization Block #

((i 0 (1+ i))
 (acc 0)
 (len (length str)))

Each line defines a loop variable with up to three components: name, starting value, and update step (run at the end of each iteration):

• i 0 (1+ i):
  • i starts at 0 (our starting index for traversing the string).
  • After every loop iteration, i increments by 1 ((1+ i) is shorthand for (+ i 1)).
• acc 0:
  • acc (short for "accumulator") starts at 0—this tracks how many lowercase vowels we’ve found.
  • No update step here means acc won’t change automatically; we have to modify it manually when we find a match.
• len (length str):
  • len is set to the length of str once, at the start of the loop.
  • No update step keeps len constant—it’s our stopping point (when we’ve checked every character in the string).

End Condition & Return Value #

((= i len) acc)

This is the loop’s exit rule:

• When i equals len (we’ve reached the end of the string), the loop stops.
• The function returns acc—the total count of lowercase vowels found.

3. The when Block: Vowel Check & Count Update #

Inside the do loop, the when statement runs only if its condition is true:

(when (or (equal (aref str i) #\a)
          (equal (aref str i) #\e)
          (equal (aref str i) #\i)
          (equal (aref str i) #\o)
          (equal (aref str i) #\u))
  (:= acc (1+ acc)))

Let’s unpack this:

• (aref str i): aref (array reference) grabs the character at index i from the string str (strings in Common Lisp are arrays of characters).
• #\a: This is the character literal for lowercase 'a'—same logic applies to #\e, #\i, etc.
• (or ...): Combines all equality checks—if any character matches a lowercase vowel, the condition is true.
• (:= acc (1+ acc)):
  • := is likely a custom macro (not standard Common Lisp) that acts like setq or setf—it assigns a new value to acc.
  • (1+ acc) increments acc by 1. We need this line to update our count every time we find a vowel—without it, acc would stay at 0 forever, and the function would always return 0!

Quick Summary of the Flow #

1. Initialize i=0 (starting index), acc=0 (vowel counter), and len (string length).
2. For each iteration:
   • Check if the character at i is a lowercase vowel.
   • If yes, increment acc by 1.
   • Increment i by 1.
3. When i reaches the string length, return acc as the final count.

内容的提问来源于stack exchange，提问作者snk