Character pointer problem

[james123]

Hi all,

I'll warn you all now this is my first attempt at programming in C so this is probably a very basic question. Basically I have an n-ary tree which contains a char pointer called value. I assign this a temporary value of say 'xyz'. I then parse this value to read each character and assign that character to a new node. The code to loop though the value is as follows:

for (j = 0; j < strlen(t->value); j++) {
	add_child_node(t, &(t->value[j]));
}

The problem is if value is 'xyz' the first time round i get 'xyz' then the 2nd i get 'yz' and then finally 'z'. I can see why this happens but obviously it is not what i want. I just want 'x' then 'y' then 'z'. Can anyone give me some help on how I would do this?

Thanks,
James

[Sebastiani]

Have add_child_node take a character as the second parameter, and then just pass t->value[j]. Also, calculating strlen each iteration of the loop is very inefficient. You just need to calculate it once, after all.

[james123]

Thanks for the quick reply! I have tried this and i get a compile error saying 'assignment makes pointer from integer without a cast'. Changing my add_child_node code to:

int add_child_node(struct node *t, char val) {
	if (t->child_count >= MAX_CHILDREN) { return 0; }
	
	struct node *child;
	child = tree_alloc();
	if (child == NULL) { return 0; }
	child->value = (char *)val;
	child->parent = t;
	child->child_count = 0;
	
	t->children[t->child_count] = child;
	t->child_count++;
	
	return 1;
}

gives me a compiler warning 'cast to pointer from integer of different size'

Thanks for pointing out the strlen problem I forgot that that gets checked every iteration!

Cheers,
James

[arpsmack]

You can't just cast a char to a char *, it doesn't work like that.

How you accomplish this task depends on how you defined your struct.

If you allocated a fixed amount of space for the value, like this:

struct node {
   ...
   char value[10];
   ...
}

Then you can store single character into the value by doing this:

child->value[0] = val;
child->value[1] = '\0';

If instead the struct contains a char *, like this:

struct node {
   ...
   char *value;
   ...
}

Then you would have to dynamically allocate some space before you stored the new value there:

child->value = malloc(2 * sizeof(char));
child->value[0] = val;
child->value[1] = '\0';

Notice how I made sure to add the trailing null-terminator onto the end of the value each time. This really is optional. But if you want to make sure that value contains a valid C string capable of being passed to functions like strlen(), then you must always make sure it is properly null terminated.

[james123]

Excellent, thanks to both of you who replied. I've sorted the problem now using the approach suggested. It seems pretty obvious now you mention it!

Cheers,
James