Code Examples and Exercises

11.4.el

; Write a function similar to `triangle' in which each row has a value which is the square of the row number.  Use a `while' loop.

(defun square-triangle (number-of-rows)
"Calculate the total number of pebbles in a triangle where the number of
pebbles in each row is the square of the number of the row."
(let ((total 0)
      (row-number 1))
  (while (<= row-number number-of-rows)
    (setq total (+ total (* row-number row-number)))
    (setq row-number (1+ row-number)))
  total))

; Write a function similar to `triangle' that multiplies instead of adds the values.

(defun factor-triangle (number-of-rows)
"Calculate the product of number of pebbles in each row of a triangle
with NUMBER-OF-ROWS rows."
(let ((total 1)
      (row-number 1))
  (while (<= row-number number-of-rows)
    (setq total (* total row-number))
    (setq row-number (1+ row-number)))
  total))

; Rewrite these two functions recursively.  Rewrite these functions using `cond'.

(defun recursive-square-triangle (number)
"Square-triangle in a recursive manner."
(if (= number 1)
    1
  (+ (* number number) (recursive-square-triangle (1- number)))))

(defun recursive-factor-triangle (number)
"Factor-triangle in a recursive manner."
(if (= number 1)
    1
  (* number
     (recursive-factor-triangle 
      (1- number)))))

(defun cond-square-triangle (number)
(cond ((<= number 0) 0)
      ((= number 1) 1)
      ((> number 1)
       (+ (* number number) (cond-square-triangle (1- number))))))

(defun cond-factor-triangle (number)
(cond ((<= number 0) 0)
      ((= number 1) 1)
      ((> number 1)
       (* number (cond-factor-triangle (1- number))))))

; Write a function for Texinfo mode that creates an index entry at the beginning of a paragraph for every `@dfn' within the paragraph. (In a Texinfo file, `@dfn' marks a definition.  This book is written in Texinfo.)

(defun make-paragraph-index ()
"Create an index entry at the beginning of a paragraph for every `@dfn' within the paragraph."
(interactive)
(save-excursion
  (let ((number-of-dfns 0) (resumepos 0))
    (progn
      (forward-paragraph)
      (setq eop (point))
      (backward-paragraph)
      (setq bop (point))
      (goto-char eop) 
      (while (search-backward "@dfn" nil t)
	(setq number-of-dfns (1+ number-of-dfns))
	(setq resumepos (1- (point))) ; the resume position is 1 char before "@dfn"
	(forward-char 4) ; go to the beginning of the actual word
	(push-mark (point) nil t)
	(copy-region-as-kill (mark) (search-forward "}" nil t 1))
	(goto-char resumepos))
      (goto-char bop)
      (setq pos 1)
      (while (<= pos number-of-dfns)
	(setq kill-ring-yank-pointer kill-ring) ; reset the k-r-y-p so the next
	                                        ; expression doesn't get confused
	(yank pos)
	(message "Inserting item %d" pos)
	(insert "\n")
	(setq pos (1+ pos)))))))  ; kill the text in the dfn bracelets

; TODO: make the last defun work for multiple paragraphs, given as an arg

func_ptr.c

// func_ptr.c -- uses function pointers
// C Primer Plus 5th Edition, Listing 14.16

#include <stdio.h>
#include <string.h>
#include <ctype.h>

char showmenu(void);
void eatline(void);    // read through end of line
void show(void (* fp)(char *), char * str);
void ToUpper(char *);   // convert string to uppercase
void ToLower(char *);   // convert string to lowercase
void Transpose(char *);  // transpose cases
void Dummy(char *);      // leave string unaltered

int main (void)
{
    char line[81];
    char copy[81];
    char choice;
    void (*pfun)(char *);  // points a function having a
                               // char * argument and no return value
    puts("Enter a string (empty line to quit):");
    while (gets(line) != NULL && line[0] != '\0')
    {
        while ((choice = showmenu()) != 'n')
        {
            switch (choice   )  // switch sets pointer
            {
                case 'u' : pfun = ToUpper; break;
                case 'l' : pfun = ToLower; break;
                case 't' : pfun = Transpose; break;
                case 'o' : pfun = Dummy; break;
            }
            strcpy(copy, line); // make copy for show()
            show(pfun, copy);   // use selected function
        }
        puts("Enter a string (empty line to quit):");
    }
    puts("Bye!");
    return 0;
}

char showmenu(void)
{
    char ans;
    puts("Enter menu choice:");
    puts("u) uppercase         l) lowercase");
    puts("t) transposed case   o) original case");
    puts("n) next string");
    ans = getchar();    // get response
    ans = tolower(ans); // convert to lowercase
    eatline();          // dispose of rest of line
    while (strchr("ulton", ans) == NULL)
    {
        puts("Please enter a u, l, t, o or n:");
        ans = tolower(getchar());
        eatline();
    }
    
    return ans;
}

void eatline(void)
{
    while (getchar() != '\n')
        continue;
}

void ToUpper(char * str)
{
    while (*str)
    {
        *str = toupper(*str);
        str++;
    }
}

void ToLower(char * str)
{
    while (*str)
    {
        *str = tolower(*str);
        str++;
    }
}

void Transpose(char * str)
{
    while (*str)
    {
        if (islower(*str))
            *str = toupper(*str);
        else if (isupper(*str))
            *str = tolower(*str);
        str++;
    }
}

void Dummy(char * str)
{
    // leaves string unchanged
}

void show(void (* fp)(char *), char * str)
{
    (*fp)(str);  // apply chosen function to str
    puts(str);   // display result
}

progn.markdown

%Progn

Elisp 中的函数隐式地默认各条语句按顺序执行，在while等形式中也是如此，因此实际上需要显式地使用progn形式的场合并不多，只有and or等形式以及if形式的then部分等等。

progn返回列表中最后一个形式的值。与其副作用相同但返回值不同的还有prog1和prog2，分别返回第一个和第二个形式的值。例如，可以用如下代码返回一个列表的第一个值并获得截短后的列表：

(prog1 (car x) (setq x (cdr x)))