Thread: Having a problem with an infinite loop

Hi everyone,

I'm working on a programming assignment for a cs class and I am having trouble figuring out why I am getting an infinite loop in my function flipstring, a function that reverses the order of words in a string while keeping the characters the same. Flipstring calls the function nextword so I will copy both functions. Here is my code,

Code:

int nextword(char *str);
void flipstring(char *str);

int nextword(char *str)
{
        static char *s;
        static int nextindex;
        int thisindex;
        // reset the static variables
        if (str != NULL)  //string is called once.  NULL is used to find the nextword in str.
        {
                s = str;
                thisindex = 0;


        }
        else
        {
                // set the return value to be the nextindex
                thisindex = nextindex;


        }


        if (nextindex != -1)
          {
          while((s[nextindex] != ' ') && (s[nextindex] != '\n') && (s[nextindex] != '\t')){
            if(s[nextindex]=='\0'){
              thisindex=-1;
              return thisindex; //returns final index
            }
            else
              nextindex++;  //if s is not white space, nextindex is incremented
          }
          s[nextindex]='\0';
          while ((s[nextindex] == ' ') || (s[nextindex] == '\n') || (s[nextindex] == '\t') || (s[nextindex] == '\0'))//nextindex is incremented after \0 is placed
            nextindex++;
        }




        return thisindex;
}

void flipstring(char *str){
  int i = nextword(str);
  char *c[256];
  int j=0,w=0,x=0;
  while(i != -1){


    c[j]=&(str[i]);
    printf("%d: %s\n",j,c[j]);  //Here for testing reasons
    i = nextword(NULL);
    j++;
  }
  j--; //j was incremented too far in the last while loop
  for(j;j>=0;j--){
    w=0;  //reset w for every string in c
    while(c[j][w] != '\0'){
      str[x]=c[j][w];
      printf("str[x] x=%d c[j][w] j=%d w=%d str=%s",x,j,w,str); //Here for testing reasons
      w++;
      x++;
    }
    str[x]=' '; //sets space
    x++;
  }
  str[x]='\n';
  str[x+1]='\0';
}

Here is the code I am using to test flipstring,

Code:

void main(int argc, char **argv) 
{
char str[] = "Hello Everyone.  I'm doing great!";
flipstring(str);
printf("%s",str);
}

I use nextword to save my strings in an array of strings, c. I then try to copy of the characters of the strings in c back to the string str in the reverse order. For some reason my while loop,

Code:

while(c[j][w] != '\0')

, runs infinitely. The print statement tells me that j is decremented down to 1 and stays at 1, while x and w increment infinitely. My only theory as to why the while statement runs forever is that there are no \0 characters in some/all of my strings, but this theory doesn't explain why j is decremented at all in the first place. Any help with this problem would be much appreciated, I have been trying to figure this out for a while now.

Looks like in flipstring

Code:

while(i != -1)

is always true....

N this declaration is i = nextword(NULL); is keeping it true.....

Code:

int nextword(char *str)
{
...
        if (nextindex != -1)
          {
          while((s[nextindex] != ' ') && (s[nextindex] != '\n') && (s[nextindex] != '\t')){
            if(s[nextindex]=='\0'){
              thisindex=-1;
              return thisindex; //returns final index
            }

If you reach the end of the string ('\0') and the word doesn't end with whitespace you throw the starting index for the last word away.

Bye, Andreas