Thread: a few questions on array of ptrs

Hi,

I'll be obliged if someone answers my following queries regarding array of pointers:

1. Say I've a some character delimited file and I want to read each string into each element of an array of ptr. How do I dynamically change the size of the array?

2. Say the array of ptr is names: char *names [MAX]; Now say, names[1] contains "Angkar\0". How do I access each element of names[1]? I mean 'A', 'n' ...

3. Say I dynamically read the contents of a file (delimited by \n) into an array of pointers. How do I know the number of elements in it (i mean the # of strings read)? One thing I can think of doing is count the # of times I read \n. But are there any better methods for it?

Thanks,
AK

1. malloc(), realloc() and free()
2. (names[1])[charindex]
3. Best way would be to have some int that keeps track of the total amount of entries. Another way would be to let the last element of the array be NULL. That way you won't know the size of the array without traversing through it though and is probably most useful for linked lists.

>How do I dynamically change the size of the array?
You can't change the size of an array, it's a compile-time constant. You can simulate an array using malloc, calloc, or realloc, and free.

>How do I access each element of names[1]?
In this case, the same way you would a two dimensional array:

Code:

for ( i = 0; names[i] != '\0'; i++ )
  putchar ( names[1][i] );

>How do I know the number of elements in it (i mean the # of strings read)?
You have to have some kind of counter to make sure that while reading you don't overflow the array. Just use that counter, because when you finish readin the file, it should have the same value as the number of lines you read.

If you've stored the \0 properly, then
printf( "%s", names[1] );
will do the whole string for you, no loop required.

Thanks to you all.

Now as you instructed I'm trying to read a # delimited file into an array of pointers as:

Code:

FILE *f;
	  int i=0;
	  int j=0;
	  char ch;
	  char *data[6];
	  clrscr();
	  f=fopen("test.txt","r");
	  ch = fgetc(f);
	  printf("%c\n",ch);
	  if ( feof(f) || ferror(f) ) {
	   printf("Error opening file");
	  }
	  else {

		  while (!(feof(f))){
			for (i=0;i<6;i++){
			 printf("Hi\n");
			 do{
			  ch=fgetc(f) ;
			  (data[i])[j++] = ch;
			 }while(ch!='#');

			 (data[i])[j]='\0';
			}
		      }
	  }
	  fclose(f);

My file looks like (since i know there are 3 entries I've fixed array size to 6 - first I want to learn to getdata rather than playing with the size):

#S1
AAJHKSHA--AHSA
#S2
AIWJOHWHDHSA
#S3
AILKJDHKLHLKJ--

Unfortunately the o/p its throwing is:
#
Hi
Hi
Hi
Hi
and then going into an infinite loop.

Could anyone shed some light upon the reason?
Also, does this depend upon the i/p format? I mean shouldn't I expect (data[0])=S1 AAJHKSHA--AHSA ?

Thanks,
AK

Can you please show an example how to allocate something for data?

I'm sorry quzah but I'm new to this domain and trying to learn things under the guidance of expericed programmers like you.

I'm using a book by Herbert Schildt (Turbo C/C++). But it has lots of info instead of analysis.
And about the malloc I read that it returns a pointer to the 1st byte of the allocated space. Is this a good practice to allocate like this?

Code:

  if ((data[i] = malloc(sizeof(char) * 100)) == NULL) {
    printf("Error : Null pointer returned)
  }

and loop i 6 times?
I do this since I fee that the left hand side must also contain a pointer and a * and a [] are both pointers. am i thinking right?

AK

Trying hard to find why even after allocating data my program gives o/p as:

#
Hi
Hi
Hi
Hi
and then infinite loop...

Code:

FILE *f;
	  int i=0;
	  int j=0;
	  char ch;
	  char *data[6];
	  for(i=0;i<6;i++){
	   if ((data[i] = (char *) malloc(sizeof(char) * 600)) == NULL)
	      printf("Error: Null pointer returned\n");
	  }
	  clrscr();
	  if((f=fopen("test.mul","r"))==NULL)
	  {
	  printf("Error\n");
	  }
	  ch = fgetc(f);
	  printf("%c\n",ch);
	   while (!(feof(f))){
		for (i=0;i<6;i++){
		     printf("Hi\n");
			 do{
			   ch=fgetc(f) ;
			   (data[i])[j++] = ch;
			 } while(ch!='#');

			 (data[i])[j]='\0';
		   }
		   printf("%s",data[0]);
	       }
	  fclose(f);
	  for(i=0;i<6;i++)
	  free(data[i]);

but a good point is that when i use a simple while !EOF with fgetc, the entire file is being printed as it is.



AK

> I'm using a book by Herbert Schildt (Turbo C/C++)
Well that explains a lot - your book is broken.