解决Euler项目问题我得到的是我需要使用长数字的数字作为字符串进行操作。我使用sbcl在linux，emacs，slime中工作。
例如，为了得到这个数字2的数字之和，我就这样工作，
1）获得权力
CL-USER> (defparameter *answer-as-integer* (expt 2 1000))
*ANSWER-AS-INTEGER*
CL-USER> *ANSWER-AS-INTEGER*
10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376
感谢Common Lisp，这非常简单。现在我认为应该将reduce应用于这个数字序列。
2）获取字符串
CL-USER> (defparameter *answer-as-string* (write-to-string *ANSWER-AS-INTEGER*))
*ANSWER-AS-STRING*
CL-USER> *ANSWER-AS-STRING*
"10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376"
现在我有了序列，所以应用reduce，但是我得错了：这是一个char所以我将转换字符应用于整数：
CL-USER> (reduce #'(lambda (x y) (+ (digit-char-p x) (digit-char-p y))) *ANSWER-AS-string*)
但是我收到了错误：
The value 1 is not of type CHARACTER.
[Condition of type TYPE-ERROR]
Restarts:
0: [RETRY] Retry SLIME REPL evaluation request.
1: [*ABORT] Return to SLIME's top level.
2: [ABORT] Abort thread (#<THREAD "repl-thread" RUNNING {1005DE80B3}>)
Backtrace:
0: (DIGIT-CHAR-P 1) [optional]
1: ((LAMBDA (X Y)) 1 #\7)
2: (REDUCE #<FUNCTION (LAMBDA (X Y)) {100523C79B}> "1071508607186267320948425049060001810561404811705533607443750388370351051124936122493198378815695858127594672917553146825187145285692314043598457757469..
3: (SB-INT:SIMPLE-EVAL-IN-LEXENV (REDUCE (FUNCTION (LAMBDA # #)) *ANSWER-AS-STRING*) #<NULL-LEXENV>)
4: (EVAL (REDUCE (FUNCTION (LAMBDA # #)) *ANSWER-AS-STRING*))
5: (SWANK::EVAL-REGION "(reduce #'(lambda (x y) (+ (digit-char-p x) (digit-char-p y))) *ANSWER-AS-string*) ..)
Locals:
SB-DEBUG::ARG-0 = "(reduce #'(lambda (x y) (+ (digit-char-p x) (digit-char-p y))) *ANSWER-AS-string*)\n"
6: ((LAMBDA NIL :IN SWANK-REPL::REPL-EVAL))
7: (SWANK-REPL::TRACK-PACKAGE #<CLOSURE (LAMBDA NIL :IN SWANK-REPL::REPL-EVAL) {10051F065B}>)
8: (SWANK::CALL-WITH-RETRY-RESTART "Retry SLIME REPL evaluation request." #<CLOSURE (LAMBDA NIL :IN SWANK-REPL::REPL-EVAL) {10051F059B}>)
9: (SWANK::CALL-WITH-BUFFER-SYNTAX NIL #<CLOSURE (LAMBDA NIL :IN SWANK-REPL::REPL-EVAL) {10051F057B}>)
10: (SWANK-REPL::REPL-EVAL "(reduce #'(lambda (x y) (+ (digit-char-p x) (digit-char-p y))) *ANSWER-AS-string*) ..)
11: (SB-INT:SIMPLE-EVAL-IN-LEXENV (SWANK-REPL:LISTENER-EVAL "(reduce #'(lambda (x y) (+ (digit-char-p x) (digit-char-p y))) *ANSWER-AS-string*) ..)
12: (EVAL (SWANK-REPL:LISTENER-EVAL "(reduce #'(lambda (x y) (+ (digit-char-p x) (digit-char-p y))) *ANSWER-AS-string*) ..)
13: (SWANK:EVAL-FOR-EMACS (SWANK-REPL:LISTENER-EVAL "(reduce #'(lambda (x y) (+ (digit-char-p x) (digit-char-p y))) *ANSWER-AS-string*) ..)
14: (SWANK::PROCESS-REQUESTS NIL)
15: ((LAMBDA NIL :IN SWANK::HANDLE-REQUESTS))
16: ((LAMBDA NIL :IN SWANK::HANDLE-REQUESTS))
17: (SWANK/SBCL::CALL-WITH-BREAK-HOOK #<FUNCTION SWANK:SWANK-DEBUGGER-HOOK> #<CLOSURE (LAMBDA NIL :IN SWANK::HANDLE-REQUESTS) {1005DF00DB}>)
18: ((FLET SWANK/BACKEND:CALL-WITH-DEBUGGER-HOOK :IN "/home/anquegi/quicklisp/dists/quicklisp/software/slime-2.13/swank/sbcl.lisp") #<FUNCTION SWANK:SWANK-DEBUGGER-HOOK> #<CLOSURE (LAMBDA NIL :IN SWANK::H..
19: (SWANK::CALL-WITH-BINDINGS ((*STANDARD-OUTPUT* . #1=#<SWANK/GRAY::SLIME-OUTPUT-STREAM {1005DCF343}>) (*STANDARD-INPUT* . #2=#<SWANK/GRAY::SLIME-INPUT-STREAM {1006160003}>) (*TRACE-OUTPUT* . #1#) (*ERR..
20: (SWANK::HANDLE-REQUESTS #<SWANK::MULTITHREADED-CONNECTION {1005078BE3}> NIL)
21: ((FLET #:WITHOUT-INTERRUPTS-BODY-1226 :IN SB-THREAD::INITIAL-THREAD-FUNCTION-TRAMPOLINE))
22: ((FLET SB-THREAD::WITH-MUTEX-THUNK :IN SB-THREAD::INITIAL-THREAD-FUNCTION-TRAMPOLINE))
23: ((FLET #:WITHOUT-INTERRUPTS-BODY-647 :IN SB-THREAD::CALL-WITH-MUTEX))
24: (SB-THREAD::CALL-WITH-MUTEX #<CLOSURE (FLET SB-THREAD::WITH-MUTEX-THUNK :IN SB-THREAD::INITIAL-THREAD-FUNCTION-TRAMPOLINE) {7FFFEA81ED1B}> #<SB-THREAD:MUTEX "thread result lock" owner: #<SB-THREAD:THR..
25: (SB-THREAD::INITIAL-THREAD-FUNCTION-TRAMPOLINE #<SB-THREAD:THREAD "repl-thread" RUNNING {1005DE80B3}> #S(SB-THREAD:SEMAPHORE :NAME "Thread setup semaphore" :%COUNT 0 :WAITCOUNT 0 :MUTEX #<SB-THREAD:MU..
26: ("foreign function: call_into_lisp")
27: ("foreign function: new_thread_trampoline")
如果我尝试将此作为数字而不进行转换，则inteerpreter会说这不是整数，所以我变得疯狂，因为这是正确的，但上面的代码不是：
(reduce #'+ *ANSWER-AS-string*)
The value #\1 is not of type NUMBER.
[Condition of type TYPE-ERROR]
Restarts:
0: [RETRY] Retry SLIME REPL evaluation request.
1: [*ABORT] Return to SLIME's top level.
2: [ABORT] Abort thread (#<THREAD "repl-thread" RUNNING {1005DE80B3}>)
Backtrace:
0: (+ #\1 #\0)
1: (REDUCE #<FUNCTION +> "107150860718626732094842504906000181056140481170553360744375038837035105112493612249319837881569585812759467291755314682518714528569231404359845775746985748039345677748242309854..
2: (SB-INT:SIMPLE-EVAL-IN-LEXENV (REDUCE (FUNCTION +) *ANSWER-AS-STRING*) #<NULL-LEXENV>)
3: (EVAL (REDUCE (FUNCTION +) *ANSWER-AS-STRING*))
4: (SWANK::EVAL-REGION "(reduce #'+ *ANSWER-AS-string*) ..)
Locals:
SB-DEBUG::ARG-0 = "(reduce #'+ *ANSWER-AS-string*)\n"
5: ((LAMBDA NIL :IN SWANK-REPL::REPL-EVAL))
6: (SWANK-REPL::TRACK-PACKAGE #<CLOSURE (LAMBDA NIL :IN SWANK-REPL::REPL-EVAL) {100566384B}>)
7: (SWANK::CALL-WITH-RETRY-RESTART "Retry SLIME REPL evaluation request." #<CLOSURE (LAMBDA NIL :IN SWANK-REPL::REPL-EVAL) {100566378B}>)
8: (SWANK::CALL-WITH-BUFFER-SYNTAX NIL #<CLOSURE (LAMBDA NIL :IN SWANK-REPL::REPL-EVAL) {100566376B}>)
9: (SWANK-REPL::REPL-EVAL "(reduce #'+ *ANSWER-AS-string*) ..)
10: (SB-INT:SIMPLE-EVAL-IN-LEXENV (SWANK-REPL:LISTENER-EVAL "(reduce #'+ *ANSWER-AS-string*) ..)
11: (EVAL (SWANK-REPL:LISTENER-EVAL "(reduce #'+ *ANSWER-AS-string*) ..)
12: (SWANK:EVAL-FOR-EMACS (SWANK-REPL:LISTENER-EVAL "(reduce #'+ *ANSWER-AS-string*) ..)
13: (SWANK::PROCESS-REQUESTS NIL)
14: ((LAMBDA NIL :IN SWANK::HANDLE-REQUESTS))
15: ((LAMBDA NIL :IN SWANK::HANDLE-REQUESTS))
16: (SWANK/SBCL::CALL-WITH-BREAK-HOOK #<FUNCTION SWANK:SWANK-DEBUGGER-HOOK> #<CLOSURE (LAMBDA NIL :IN SWANK::HANDLE-REQUESTS) {1005DF00DB}>)
17: ((FLET SWANK/BACKEND:CALL-WITH-DEBUGGER-HOOK :IN "/home/anquegi/quicklisp/dists/quicklisp/software/slime-2.13/swank/sbcl.lisp") #<FUNCTION SWANK:SWANK-DEBUGGER-HOOK> #<CLOSURE (LAMBDA NIL :IN SWANK::H..
18: (SWANK::CALL-WITH-BINDINGS ((*STANDARD-OUTPUT* . #1=#<SWANK/GRAY::SLIME-OUTPUT-STREAM {1005DCF343}>) (*STANDARD-INPUT* . #2=#<SWANK/GRAY::SLIME-INPUT-STREAM {1006160003}>) (*TRACE-OUTPUT* . #1#) (*ERR..
19: (SWANK::HANDLE-REQUESTS #<SWANK::MULTITHREADED-CONNECTION {1005078BE3}> NIL)
20: ((FLET #:WITHOUT-INTERRUPTS-BODY-1226 :IN SB-THREAD::INITIAL-THREAD-FUNCTION-TRAMPOLINE))
21: ((FLET SB-THREAD::WITH-MUTEX-THUNK :IN SB-THREAD::INITIAL-THREAD-FUNCTION-TRAMPOLINE))
22: ((FLET #:WITHOUT-INTERRUPTS-BODY-647 :IN SB-THREAD::CALL-WITH-MUTEX))
23: (SB-THREAD::CALL-WITH-MUTEX #<CLOSURE (FLET SB-THREAD::WITH-MUTEX-THUNK :IN SB-THREAD::INITIAL-THREAD-FUNCTION-TRAMPOLINE) {7FFFEA81ED1B}> #<SB-THREAD:MUTEX "thread result lock" owner: #<SB-THREAD:THR..
24: (SB-THREAD::INITIAL-THREAD-FUNCTION-TRAMPOLINE #<SB-THREAD:THREAD "repl-thread" RUNNING {1005DE80B3}> #S(SB-THREAD:SEMAPHORE :NAME "Thread setup semaphore" :%COUNT 0 :WAITCOUNT 0 :MUTEX #<SB-THREAD:MU..
25: ("foreign function: call_into_lisp")
26: ("foreign function: new_thread_trampoline")
我这样解决了这个问题：
CL-USER> (defun sum-digits-in-a-string (str)
(reduce #'+ (map 'list #'digit-char-p str)))
SUM-DIGITS-IN-A-STRING
CL-USER> (sum-digits-in-a-string *ANSWER-AS-STRING*)
1366
所以我的问题是为什么我首先得到一个整数和一个字符后的错误，以及使用长整数的数字的更好的方法是什么。如果我的aproximation是好的：长整数 - ＆gt; string - ＆gt;整数列表 - ＆gt;申请减少。
答案 0 :(得分：6)
这不是Common Lisp特有的，但我认为如果您首先在函数中添加一些调试输出，这可能是一般帮助。例如，在这种情况下，如果在添加并使用它们调用digit-char-p之前打印x和y，则可以看到在处理前两个元素之后，第三个元素将使用前一个添加的结果进行处理，这是一个数字，而不是一个字符：
CL-USER> (reduce (lambda (x y)
(write (list x y))
(+ (digit-char-p x)
(digit-char-p y)))
"1234")
(#\1 #\2)(3 #\3)
; Evaluation aborted on #<TYPE-ERROR expected-type: CHARACTER datum: 3>.
您可以考虑手动展开的样子：
(+ (digit-char-p (+ (digit-char-p (+ (digit-char-p #\1)
(digit-char-p #\2)))
(digit-char-p #\3)))
(digit-char-p #\4))
在那些中间调用中，你可以在数字上调用digit-char-p，而不是字符。
你的最终解决方案得到了一个好处：
(defun sum-digits-in-a-string (str)
(reduce #'+ (map 'list #'digit-char-p str)))
但是这有一个不幸的问题，就是它会创建一个包含数字的全新序列，然后减少它们。另一种常见的反模式是（应用＆＃39; +（地图...））。你的更好一点，因为它使用reduce，但reduce实际上采用了一个关键参数，消除了对map的需求。您可以使用reduce参数reduce来指定如何从序列元素中提取值，并且可以使用初始值（如果序列为空，或者只有一个元素，这很重要）：
CL-USER> (reduce '+ "1234" :key 'digit-char-p :initial-value 0)
;=> 10
答案 1 :(得分：5)
可以避免将数字打印到字符串并直接生成小数位列表。
爆炸和影响
通常此操作称为 explode 。通常， explode 操作可以处理符号，整数和类似操作。它创建了一个组件列表。反向操作称为 implode 。
这会对正整数爆炸：
(defun explode-integer (integer)
"Explode a positve integer."
(labels ((aux-explode-integer (integer)
(nreverse
(loop with i = integer and r = 0
while (> i 0)
do (multiple-value-setq (i r) (floor i 10))
collect r))))
(cond ((plusp integer)
(aux-explode-integer integer))
((zerop integer) (list 0)))))
示例：
CL-USER 31 > (explode-integer 572304975029345020734)
(5 7 2 3 0 4 9 7 5 0 2 9 3 4 5 0 2 0 7 3 4)
答案 2 :(得分：2)
(reduce f "125")执行的计算是
(f (f #\1 #\2) #\5)
在您的情况下，这意味着它将计算
(+ (digit-char-p (+ (digit-char-p #\1) (digit-char-p #\2)))
(digit-char-p #\5))
所以它会评估
(+ (digit-char-p 3) (digit-char-p #\5))
因类型错误而中断。
最简单的解决方案是编写适用于整数的digit-char-p变体：
(defun digit-to-int (x)
(etypecase x
(integer x)
(character (digit-char-p x))))
(reduce #'(lambda (x y) (+ (digit-to-int x) (digit-to-int y))) *ANSWER-AS-string*)
正如约书亚·泰勒所指出的，更清晰的解决方案是使用:key参数reduce，这大致相当于使用map，但避免了生成中间序列的需要：
(reduce #'+ *ANSWER-AS-string* :key #'digit-char-p)