VERY simple problem, can't figure it out for the life of me. :[

**MK27** · 09-27-2009

Originally Posted by rasheemo

so can't i just send '&start' to pass the pointer itself? but the compiler won't allow that for some reason.

Nope. I was confused by this too (but it seems very straightforward to me now).

& the address of operator is something to use when you want to pass the address of a variable.

In C, all function arguments are just VALUES. So

Code:

int x;
myfunc(&x);

now, the prototype for myfunct must be

Code:

myfunc(int *ptr);

So, the VALUE received is a memory address where an int (another value) is stored. If you want to change the value of that int in myfunc, you would use * to dereference the pointer:

Code:

*ptr = 666;

that means to change the value stored at the address x to 666. That is very different from this:

Code:

ptr = 666;

This is probably not a valid memory address, but you can assign it anyway. You did not change the value stored at the address represented by ptr; you changed the address represented by ptr. So now, in myfunc, ptr refers to A DIFFERENT ADDRESS than the one supplied as a parameter. In other words, ptr now has nothing to do with x.

The same thing is true if you use malloc, because ptr = malloc(whatever) will assign a NEW ADDRESS to the value of ptr. Remember, ptr is not x; its value used to be the address of x, but you just changed that.

You can dereference a struct pointer:

Code:

#include <stdio.h>

struct x{
	int i;
};

void test(struct x *ptr) {
	(*ptr).i = 6;
}

int main(void) {
	struct x eg;
	test(&eg);	
	printf("%d",eg.i);
	return 0;
}

However, the normal way to do things with a linked list is to use return values:

Code:

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

struct x{
	int i;
};

struct x *test() {
	struct x *here = malloc(sizeof(struct x));
	return here;
}

int main(void) {
	struct x *eg = test();
	eg->i = 6;
	printf("%d",eg->i);
	return 0;
}

**rasheemo** · 09-27-2009

ok, i figured out how to build the linked list. but now my program freezes when i try to copy a string to string inside of the struct.

here is the latest code:

Code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "template_wordstat.h"

entry * append() ;

int main(int argc, char* argv[])
{
    FILE* fp;
    char line[MAX_LINE];
    int i = 0;
    char *nextWord;
    entry *start = NULL;


    /* Checks for correct usage and handles arguments accordingly */

    if (argc != 2) {
        fprintf(stderr, "Error: wrong number of parameters.\n");
        return -1;
    }

	if (strcmp(argv[1],"-h") == 0) {
        	printf("Usage: wordstat FILE_NAME\n");
        	return 0;
   	}

    if ((fp=fopen(argv[1],"r")) == NULL) {
        fprintf(stderr, "Error: cannot open file %s.\n", argv[1]);
        return -2;
    }

    while (! feof(fp)) {

        /* reads one line at a time */

            if (fgets(line, MAX_LINE, fp) == NULL)
                    break;
            if (line[0] == '\n')    /* empty line */
                    continue;

        /* extract words from the line and handle the words	*/

        nextWord = strtok (line, TS);

        /* if the first token is empty, then there are no words */

        if(nextWord == NULL){
            fprintf(stderr, "File contains no words.\n");
            return -2;
        }

        /* go through each word and build linked list with entries*/

        while(nextWord != NULL){
            entry *marker;
            entry *temp = append();

            marker = start;
            printf(nextWord); //this prints the word just fine

////////////////////////////// here is where the trouble starts

            temp->word = malloc(sizeof(strlen(nextWord)+1));
            strcpy(temp->word, nextWord);

////////////////////////////// if i remove the above 2 lines, my program runs and ends fine

            temp->count = 0;

            /* If the very first node is empty */
            if (start == NULL) {
                start = temp;
                start -> next = NULL;
            }
            else {
                while(marker -> next != NULL)
                    marker = marker -> next;

                temp -> next = NULL;
                marker -> next = temp;
            }

            nextWord = strtok (NULL, TS);
        }
    }

	/*	sorting and printing */

	fclose(fp);


    	return 0;
}

entry *append() {
    entry *temp = malloc(sizeof(entry));
    return temp;
 }

i'm so frustrated :[

**tabstop** · 09-27-2009

You don't want sizeof in your malloc statement.

**rasheemo** · 09-27-2009

oh wow. that was it. thank you so much.

**quzah** · 09-28-2009

Originally Posted by rasheemo

Code:

while(curr){
                printf("traversing!");
                curr = curr->next;
            }

I set curr to the next one linked to it until i get to the last node (NULL).

Code:

if ((curr = (struct wordEntry*)malloc(sizeof(entry))) == NULL)

I allocate space for the curr, which is located at the null entry.

What's the point of your loop, if you're just overwriting 'cur' anyway? For that matter, what's the point of your loop anyway? All it does is run to the end of the chain and end up with a NULL pointer. You may as well have just done:

Code:

cur = NULL;

...and saved yourself the trouble.

Quzah.

Thread: VERY simple problem, can't figure it out for the life of me. :[

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