???l-value???

[Garfield]

I was just programming a cross-reference program incorporation linked lists into the program. Well, the line of code that gave me a problem was with a pointer to the struct:

list_ptr->data = line; /* line is gotten from command with fgets */

Well, the error was that the left operand was not an l-value, or something like that. Something to do with "l-value". What is L-value? What is my error and how can I change it? Thanks.

[-KEN-]

what kind of variables are you tring to assign? if it's a string, then stry using sprintf() or something. and L-Value is a number - an integer, double, float, etc.

[Garfield]

Yes, my variable 'line' is a string. Why is this a problem? How can I fix this? Thanks.

[Garfield]

So, how would I implement sprintf with my problem? Wouldn't that write to the output stream? Why would I need to do this? How would this fix my problem? Thanks.

[River21]

An L-value is any legal value that can be placed on the left side of the equal sign......

For example:

MyVariable = 5;

Because you can assign a value to a variable in C, this is legal.

5 = MyVariable;

NOT LEGAL because you are trying to alter the value of the
number 5, and you cannot do that. Let's say that MyVariable
was hypothetically equal to 6 in the above statement, then
you are trying to make the statement...

5 = 6;

and this is untrue.


If you change your assignment statement to:

line = list_ptr->data;

it should compile with no problems, assuming all your other code
is good, because the variable line IS a valid L-value;


NOTE -- Review operator associativity if this is still unclear.

Hope this helps!

[Garfield]

But, why would I want to do this:

> line = list_ptr->data;

I don't want to assign the value of the data field from a struct to the char * line. I want to do the opposite. I want assign the char * line to list_ptr->data. These are two totally different statements. The first one (the one you said) changes line's value and the second changes list_ptr->data's value. I want to do this:

> list_ptr->data = list;

I want to assign data the value of list. Not the opposite. Is there a way to do this without getting the l-value error? Thanks.

[Garfield]

Why wouldn't I be able to assign list_ptr->data the value of line? For the example you gave (with 5), I'm NOT changing a constant such as 5. I'm changing a pointer's field. Why is this not legal? Thanks again.

[no-one]

sprintf(list_ptr->data,"%s",line);

[Garfield]

And this sprintf function you gave me will just put the string into list_ptr->data as a string? I don't understand why this

list_ptr->data = line;

is illegal. data isn't a constant, so why can't I change it? Thanks.

[Garfield]

Okay, I tried the strcpy function and I tried sprintf. They both compiled with no errors, but then I had a runtime error. I think I tracked the error down to the functions strcpy (the first time) and sprintf (the second time). Here is the code. Tell me if you find the error.

/**************************************************
 ****	A cross-reference program illustrating	****
 ****	the use of linked lists			****
 ***************************************************/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define NAMESIZE 50

struct list {
	char name[NAMESIZE];		/* Holds the name of reference */
	struct list *next_list;	/* The next reference to name */
};

/***************************************************
 *	fndnm - searches through the linked list to
 *			find the given name
 *		
 *	parameters - pointer to first struct and name
 *						
 *	return - 0 if null, 1 if found
****************************************************/
int fndnm(struct list *first_list, char *search)
{
	struct list *look_list;
	/* start with the first struct */
	look_list = first_list;

	while (1)
	{
		if (look_list == NULL)
			return 0;
		else if (look_list->name == search)
			return 1;
		else
			look_list = look_list->next_list;
	}
}

int main()
{
	int counter, yesno;
	struct list *init_list;
	char line[100];

	for (counter = 0; counter < 5; ++counter)
	{
		printf("Enter a name: ");
		fgets(line, sizeof(line), stdin);
		line[strlen(line)-1] = '\0';
		sprintf(init_list->name, "%s", line);
		/* goto next list */
		init_list = init_list->next_list;
	}
	printf("Now, enter a name to search: ");
	fgets(line, sizeof(line), stdin);
	line[strlen(line)-1] = '\0';
	yesno = fndnm(init_list, line);
	if (yesno)
		printf("Found = TRUE");
	else
		printf("Found = FALSE");
	
	return (0);
}

This is it. The first time I tried, I replaced sprintf(init_list->name, "%s", line) with strcpy(init_list->name, line). Thanks.

[Stoned_Coder]

You need to use strcmp() to compare strings not the == operator.

[Garfield]

Is this why I'm getting the runtime error? Why would I have to use strcmp instead of a boolean evaluation? Thanks.

[Garfield]

I don't think that comparing with the == operator is the problem. The code didn't even get to that function. I tested (with printf) and I think the problem was with strcpy and sprintf. What do I do know? I'll try your answer anyway. It might work. I'm not really good at debugging...yet. Thanks.

[zen]

You are getting runtime errors due to your list pointer (it doesn't point to anything). It is like trying to do

int* a;
a*=10;

See my response in your other thread.

[Garfield]

Zen, this code

> int* a;
a*=10; <

For it to be right (I understand you comparison) if should be

int *a;
a = 10;

Right? Thanks.