while loop

This is a discussion on while loop within the C Programming forums, part of the General Programming Boards category; im Hi im trying to understand this sorting program and im not sure why in this example in the while ...

  1. #1
    Musicman - Canora
    Join Date
    Aug 2005
    Location
    Melbourne
    Posts
    252

    while loop

    im Hi im trying to understand this sorting program and im not sure why in this example
    in the while loop there checking for NULL twice in the while condition


    Code:
    #define SIZE 81 /*string length including \0*/
    #define LIM 20 /*maximun number of lines to be read*/
    #define HALT "" /*null string to stop input*/
    void stsrt(char *strings[], int num); /*string sort function*/
    
    int main(void)
    {
    	char input[LIM][SIZE];/*array to store input*/
    	char *ptstr[LIM]; /*array of pointer variables*/
    	int ct = 0; /*input count*/
    	int k;/*output count*/
    	
    	printf("Input up to %d lines and i will sort them\n", LIM);
    	printf("To stop, press the enter key at a lines start\n");
    	
    	while(ct <LIM && gets(input[ct])!=NULL
    	&& input[ct][0] != '\0')
    	{
    	   ptstr[ct] = input[ct]; /*set pointers to strings*/
    	   ct++;	
    		
    	}
    	stsrt(ptstr, ct); /*string sorter*/
    	
    	puts("Here's the sorted list\n");
    	
    	for(k=0; k<ct; k++)
    	{
    	  puts(ptstr[k]); /*sorted pointers*/	
    	}
    	
    	
      return EXIT_SUCCESS;	
    }
    
    void stsrt(char *strings[], int num)
    {
      char *temp;
      int top, seek;
      
      for(top = 0; top < num -1; top++)
      {
      	  for(seek = top + 1; seek < num; seek++)
      	  {
      	  	 if(strcmp(strings[top], strings[seek])>0)
      	  	 {
      	  	   temp = strings[top];
      	  	   strings[top] = strings[seek];
      	  	   strings[seek] = temp;	
      	  	 }
      	  }
      }	
    	
    }

  2. #2
    Registered User
    Join Date
    Jun 2005
    Posts
    6,210
    Code:
    while(ct <LIM && gets(input[ct])!=NULL && input[ct][0] != '\0')
    is not "checking for NULL twice".

    After checking ct < LIM, it then checks if gets(input[ct]) has returned a NULL pointer (which indicates that gets() has encountered an error reading from stdin). If a NULL pointer has not been returned, it checks if the first character read is zero (which indicates that input was received by gets() before a '\n' was encountered).

    Essentially, this means the loop read one line at a time until an error occurs, or until a line is entered with no text (eg "Enter" key is hit twice).

    Unrelated to your question, use of gets() is not a good idea because it will happily yield undefined behaviour (typically tromping random memory locations) if the user enters a long line.
    Last edited by grumpy; 06-02-2007 at 11:13 PM.

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. nested loop, simple but i'm missing it
    By big_brother in forum C Programming
    Replies: 19
    Last Post: 10-23-2006, 10:21 PM
  2. While loop misbehaving (or misunderstanding)
    By mattAU in forum C Programming
    Replies: 2
    Last Post: 08-28-2006, 02:14 AM
  3. loop in a linked linked lists
    By kris.c in forum C Programming
    Replies: 6
    Last Post: 08-26-2006, 12:38 PM
  4. while loop help
    By bliznags in forum C Programming
    Replies: 5
    Last Post: 03-19-2005, 11:30 PM
  5. loop issues
    By kristy in forum C Programming
    Replies: 3
    Last Post: 03-05-2005, 08:14 AM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21