Figure 9.20  Text Editor Program
/*
 *  Performs text editing operations on a source string
 */

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

#define MAX_LEN   100
#define NOT_FOUND -1

char *delete(char *source, int index, int n);
char *do_edit(char *source, char command);
char  get_command(void);
char *insert(char *source, const char *to_insert, int index);
int   pos(const char *source, const char *to_find);

int
main(void)
{
      char source[MAX_LEN], command;

      printf("Enter the source string:\n> ");
      gets(source);

      for (command = get_command();  
           command != 'Q';
           command = get_command()) {
          do_edit(source, command);
          printf("New source: %s\n\n", source);
      }

      printf("String after editing: %s\n", source);
      return (0);
}

/*
 *  Returns source after deleting n characters beginning with source[index].
 *  If source is too short for full deletion, as many characters are
 *  deleted as possible.
 *  Pre:  All parameters are defined and 
 *        strlen(source) - index - n < MAX_LEN
 *  Post:  source is modified and returned
 */
char *
delete(char *source,  /* input/output - string from which to delete part */
       int   index,   /* input - index of first char to delete           */
       int   n)       /* input - number of chars to delete               */
{
      char rest_str[MAX_LEN];  /* copy of source substring following 
                                  characters to delete */

      /*  If there are no characters in source following portion to 
          delete, delete rest of string */
      if (strlen(source) <= index + n) {
            source[index] = '\0';

      /*  Otherwise, copy the portion following the portion to delete
          and place it in source beginning at the index position        */
      } else {
            strcpy(rest_str, &source[index + n]);
            strcpy(&source[index], rest_str);
      }

      return (source);
}

/*
 *  Performs the edit operation specified by command
 *  Pre:  command and source are defined.
 *  Post: After scanning additional information needed, performs a
 *        deletion (command = 'D') or insertion (command = 'I') or
 *        finds a substring ('F') and displays result; returns
 *        (possibly modified) source.
 */
char *
do_edit(char *source,  /* input/output - string to modify or search */
        char  command) /* input - character indicating operation    */
{
      char str[MAX_LEN];  /* work string */
      int  index;

      switch (command) {
      case 'D':
            printf("String to delete> ");
            gets(str);
            index = pos(source, str);
            if (index == NOT_FOUND)
                  printf("'%s' not found\n", str);
            else
                  delete(source, index, strlen(str));
            break;

      case 'I':
            printf("String to insert> ");
            gets(str);
            printf("Position of insertion> ");
            scanf("%d", &index);
            insert(source, str, index);
            break;

      case 'F':
            printf("String to find> ");
            gets(str);  
            index = pos(source, str);
            if (index == NOT_FOUND)
                  printf("'%s' not found\n", str);
            else
                  printf("'%s' found at position %d\n", str, index);
            break;

      default:
            printf("Invalid edit command '%c'\n", command);
      }
      
      return (source);
}

/*  
 *  Prompt for and get a character representing an edit command and
 *  convert it to uppercase. Return the uppercase character and ignore
 *  rest of input line.
 */
char
get_command(void)
{
      char command, ignore;

      printf("Enter D(Delete), I(Insert), F(Find), or Q(Quit)> ");
      scanf(" %c", &command);
   
      do
          ignore = getchar();
      while (ignore != '\n');

      return (toupper(command));
}

/*
 *  Returns source after inserting to_insert at position index of 
 *  source. If source[index] doesn't exist, adds to_insert at end of
 *  source.
 *  Pre:  all parameters are defined, space available for source is
 *        enough to accommodate insertion, and 
 *        strlen(source) - index - n < MAX_LEN
 *  Post: source is modified and returned
 */
char *
insert(char       *source,    /* input/output - target of insertion */
       const char *to_insert, /* input - string to insert           */
       int         index)     /* input - position where to_insert 
                                         is to be inserted          */
{
      char rest_str[MAX_LEN]; /* copy of rest of source beginning
                                 with source[index] */

      if (strlen(source) <= index) {
            strcat(source, to_insert);
      } else {
            strcpy(rest_str, &source[index]);
            strcpy(&source[index], to_insert);
            strcat(source, rest_str);
      }

      return (source);
}

/*
 *  Returns index of first occurrence of to_find in source or
 *  value of NOT_FOUND if to_find is not in source.
 *  Pre:  both parameters are defined
 */
int
pos(const char *source,   /* input - string in which to look for to_find */
    const char *to_find)  /* input - string to find                      */

{
      int  i = 0, find_len, found = 0, position;
      char substring[MAX_LEN];

      find_len = strlen(to_find);

      while (!found  &&  i <= strlen(source) - find_len) {
          strncpy(substring, &source[i], find_len);
          substring[find_len] = '\0';

          if (strcmp(substring, to_find) == 0)
                found = 1;
          else
                ++i;
      }

      if (found)
            position = i;
      else
            position = NOT_FOUND;

      return (position);
}