使用指针反转字符串中的单词

Question

I was given a question with two parts. Part A was to reverse the words within a string via string manipulation, for which I used strcpy and strcat. For part B, however, I must reverse the words using pointers. Now I have the code displayed below. Am I on the right track?

The thought behind my function is that I have the original string string1 and I have a pointer at a starting character, then iterate through the string till I hit a white space, giving me the size of the word. Then I place that word at the end of my new string.

Code:

 partb(char * string1, int s)
 {
    int i=0, j=0, k=0, count=0;
    char temp[100]={0}, *sp=string1;

    for(i=0; i<=s; i++)
    {
      if(isalnum(string1[i]))
      {
          k=i;
          break;
      }
      break;
    }

    for(i=0; i<=s; i++)
    {
       if(isalnum(string1[i]))
       {
         count++;
       }
      else if(string1[i] == ' ')
      {
         for(j=0; j<=count; j++)
         {

         }  
      }
    }
 }

Answer 1

A few observations:

• How do you know that temp is going to be big enough to store the reversed string? You should allocate a char * that has the same size as the input string.

• Why do you test string1[i] == ' ' when you know that isalnum(string1[i]) is false? You're already at a word break, so the test is unnecessary.

• You have forgotten to initialize count to 0 inside the loop. You must reset count each time you come upon a new word.

After fixing the errors you can implement the function with your proposed approach, but I'd like to suggest another way. Instead of using count , you can have a pair of indices a and b that traverse a word in opposite directions.

This program demonstrates my approach:

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

int isWordCharacter(char c) {
  return isalnum(c);
}

char * reverseWords(char *s) {
  int pos = 0, seek, a, b, len;
  for (len = 0; s[len] != 0; ++len) {   // Find the string length ourselves to
  }                                     // avoid using anything from string.h.
  char *result = malloc(len * sizeof(char));
  while (1) {
    while (!isWordCharacter(s[pos])) {  // Look for the start of a word.
      result[pos] = s[pos];
      if (pos == len) {                 // Are we at the end of the string?
        return result;
      }
      ++pos;
    }
    for (seek = pos + 1; isWordCharacter(s[seek]); ++seek) {
    }                                   // Look for the end of the word.
    for (a = pos, b = seek - 1; a < seek; ++a, --b) {
      result[b] = s[a];                 // Scan the word in both directions.
    } 
    pos = seek;                         // Jump to the end of the word.
  }
}

int main() {
  char *s = "Hello, world. Today is September 20, 2015.";
  printf("%s\n%s\n", s, reverseWords(s));
}