Debugging strings program help plz

This is a discussion on Debugging strings program help plz within the C Programming forums, part of the General Programming Boards category; Hi, I'm having some problems with this program. The program is suppose to ask for an input sentence and print ...

  1. #1
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    Nov 2009
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    7

    Debugging strings program help plz

    Hi, I'm having some problems with this program. The program is suppose to ask for an input sentence and print the order of the words in reverse. The program complies, but crashes...not sure why.
    Thanks,

    Example run:
    Enter a sentence: you can cage a swallow can't you
    Reversal of sentence: you can't swallow a cage can you


    Code:
    #include <stdio.h>
    #include <ctype.h>
    #include <string.h>
    #define MAX_LEN 80
    #define MAX_WORDS 10
    
    int get_words (char *line, char tokens[MAX_WORDS][MAX_LEN+1]);
    void replace_whitespace (char *str);
    
    int main(void)
    { 
      int i;
      char *str, tokens[MAX_WORDS][MAX_LEN+1];
    
      printf("Enter a sentence: ");
      gets(str);
    
      get_words(str, tokens);
    
      printf("Reversal  of sentence: ");
    
      for (i = strlen(str); i > 0; i--)
        printf("%s ", str[i]);
    
    return 0;
    }
    
    /* store the words from string line in the array 
             tokens, one word per entry. */
    
    int get_words (char *line, char tokens[MAX_WORDS][MAX_LEN+1])
    {
      int i, char_count = 0;
      char *p = line;
      int len;
    
      replace_whitespace(p);
    
      len = strlen(p);
      
      while (*p == '\0' && *p < len) 
      {
         p++;
         char_count++;
      }
      
      while (*p != '\0')
      {  
         strcpy(tokens[i], p);
         p += strlen(p);
         while(*p == '\0')
            p++;  
         i++;
      }
    
    
    }
    
    /* replaces every whitespace character in str with the null 
       character; uses the function isspace(char c) from ctype.h */
    
    void replace_whitespace (char *str)  
    {  
       int i;
    
       for (i = 0; i < strlen(str); i++)
         if (isspace(str[i]))
           str[i] = '\0';
            
    }

  2. #2
    Woof, woof! zacs7's Avatar
    Join Date
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    Australia
    Posts
    3,459
    Code:
    #include <stdio.h>
    #include <ctype.h>
    #include <string.h>
    #define MAX_LEN 80
    #define MAX_WORDS 10
    
    int get_words (char *line, char tokens[MAX_WORDS][MAX_LEN+1]);
    void replace_whitespace (char *str);
    
    int main(void)
    { 
      int i;
      char *str, "str" doesn't point anywhere useful.
              tokens[MAX_WORDS][MAX_LEN+1];
    
      printf("Enter a sentence: ");
      gets(str);  gets() is dangerous, due to buffer overruns -- see the FAQ, use fgets() instead. See above, "str" does not point anywhere. 
    
      get_words(str, tokens);
    
      printf("Reversal  of sentence: ");
    
      for (i = strlen(str); i > 0; i--)
        printf("%s ", str[i]);
    
    return 0;
    }
    
    /* store the words from string line in the array 
             tokens, one word per entry. */
    
    int get_words (char *line, char tokens[MAX_WORDS][MAX_LEN+1])
    {
      int i, char_count = 0;
      char *p = line;
      int len;
    
      replace_whitespace(p);  This is incorrect logic, how do you know where the last whitespace character was replaced?
    
      len = strlen(p);  This will give you the length of the first part (upto the first whitespace character). 
      
      while (*p == '\0' && *p < len) 
      {
         p++;
         char_count++;
      }
      
      while (*p != '\0')
      {  
         strcpy(tokens[i], p);
         p += strlen(p);
         while(*p == '\0')
            p++;  
         i++;
      }
    
    
    }
    
    /* replaces every whitespace character in str with the null 
       character; uses the function isspace(char c) from ctype.h */
    
    void replace_whitespace (char *str)  
    {  
       int i;
    
       By replacing whitespace with null terminators, you're changing the length of the string, thus this loop is useless. It will halt on the first whitespace character.
       for (i = 0; i < strlen(str); i++)
         if (isspace(str[i]))
           str[i] = '\0';
            
    }
    Do you have to reverse the string in-place?
    Even so, why not just replace the whitespace with null terminators and have an array of pointers to each of the tokens (in your array).

    What I mean is:
    Code:
    zac@neux:code (1) $ gcc rev.c -o rev
    zac@neux:code (1) $ ./rev 
    Token 1: This
    Token 2: is
    Token 3: an
    Token 4: example
    zac@neux:code (1) $ cat rev.c 
    #include <stdio.h>
    
    #define MAX_WORDS 4
    
    int main(void)
    {
       char str[] = "This is an example";
       char * tokens[MAX_WORDS] = {NULL};
       size_t i = 0;
    
       size_t numberOfWords = 4;        /* obviously you'd have to calculate this, and below at runtime */
    
       tokens[0] = str;           /* This */
       str[4] = '\0';
    
       tokens[1] = (str + 5);     /* is */
       str[7] = '\0';
    
       tokens[2] = (str + 8);     /* an */
       str[10] = '\0';
    
       tokens[3] = (str + 11);    /* example */
    
       for(i = 0; i < numberOfWords; ++i)
       {
          printf("Token %d: %s\n", i + 1, tokens[i]);
       }
    
       return 0;
    }
    Last edited by zacs7; 11-08-2009 at 07:31 PM.

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