???l-value???

This is a discussion on ???l-value??? within the C Programming forums, part of the General Programming Boards category; I was just programming a cross-reference program incorporation linked lists into the program. Well, the line of code that gave ...

  1. #1
    the Corvetter
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    ???l-value???

    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:
    Code:
    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.
    1978 Silver Anniversary Corvette

  2. #2
    Just one more wrong move. -KEN-'s Avatar
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    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.

  3. #3
    the Corvetter
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    Yes, my variable 'line' is a string. Why is this a problem? How can I fix this? Thanks.
    1978 Silver Anniversary Corvette

  4. #4
    the Corvetter
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    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.
    1978 Silver Anniversary Corvette

  5. #5
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    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!

  6. #6
    the Corvetter
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    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.
    1978 Silver Anniversary Corvette

  7. #7
    the Corvetter
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    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.
    1978 Silver Anniversary Corvette

  8. #8
    Has a Masters in B.S.
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    sprintf(list_ptr->data,"%s",line);
    ADVISORY: This users posts are rated CP-MA, for Mature Audiences only.

  9. #9
    the Corvetter
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    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
    Code:
    list_ptr->data = line;
    is illegal. data isn't a constant, so why can't I change it? Thanks.
    1978 Silver Anniversary Corvette

  10. #10
    the Corvetter
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    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.
    Code:
    /**************************************************
     ****	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.
    1978 Silver Anniversary Corvette

  11. #11
    Skunkmeister Stoned_Coder's Avatar
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    You need to use strcmp() to compare strings not the == operator.
    Free the weed!! Class B to class C is not good enough!!
    And the FAQ is here :- http://faq.cprogramming.com/cgi-bin/smartfaq.cgi

  12. #12
    the Corvetter
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    Is this why I'm getting the runtime error? Why would I have to use strcmp instead of a boolean evaluation? Thanks.
    1978 Silver Anniversary Corvette

  13. #13
    the Corvetter
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    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.
    1978 Silver Anniversary Corvette

  14. #14
    zen
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    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.

  15. #15
    the Corvetter
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    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.
    1978 Silver Anniversary Corvette

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