Thread: Linked List and Dynamic Memory Allocation

Hi,

I'm a newbie to C programming and currently very confused. We are studying linked lists right now and our assignment is to create a dictionary program variables.c that stores and retrieves name-value pairs.

I have been working on this for hours and I'm not even sure what I have is correct. Could someone please help me? Even if it's just to tell me the I'm on the right track.

Here's what I've written so far:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <string.h>
#include "variables.h"
        
typedef struct Variable
{
    double value;
    char* name;
    struct Variable* next;
    struct Variable* prev;
} Variable_t;
        
void set_variable(const char* name, double value)
{
    Variable_t* new_variable_node(const char* name, double value)
    {
        Variable_t* new_node = malloc(size of(Variable_t));
        assert(new_node != NULL);

        new_node->next = NULL;
        new_node->prev = NULL;
        new_node->value = value;

        new_node->name = malloc(strlen(name) + 1);
        assert(new_node->name != NULL);
        strcpy(new_node->name, name);

        return new_node;
    }
}
double variable_value(const char* name)
{

The files my professor gave are

variables.h

Code:

#define MAX_VARIABLE_LENGTH 15

/* Set the variable with the specified name to the specified value.
* If there is not a variable with that name in the dictionary,
* add one. */
void set_variable (const char* name, double value);


/* Return the value of the variable with the specified name.
* If variable does not exist, add it to the dictionary
* with a default value of 0.0, and return that value. */
double variable_value(const char* name);


/* Display the names and values of all variables in the dictionary. */
void display_variables();

dictionary_test.c

Code:

#include <stdio.h>
#include "variables.h"

void verify (double result, double expected_value)
{
    if (result != expected_value)
    {
        printf ("Incorrect result.  Should be %8.4f\n\n", expected_value);
    }
}

int main()
{
    double result;

    printf ("Starting dictionary test\n\n");
    set_variable("A1", 1.0);
    set_variable("A2", 2.0);
    set_variable("A3_3333", 3.3333);

    result = variable_value("A1");
    printf ("A1 is %8.4f.\n", result);
    verify(result, 1.0);

    result = variable_value("A2");
    printf ("A2 is %8.4f.\n", result);
    verify(result, 2.0);

    result = variable_value("A3_3333");

    printf ("A3_333 is %8.4f\n", result);
    verify(result, 3.3333);

    set_variable("A long variable", -1.0);
    result = variable_value("A long variable");
    printf ("A long variable is %8.4f\n", result);
    verify(result, -1.0);

    result = variable_value("Unset Variable ");
    printf ("Unset Variable  is %8.4f\n", result);
    verify(result, 0.0);

    printf ("\n");
    printf ("All variables:\n");
    display_variables();

    printf ("Dictionary test complete\n");
    getchar();
    return 0;
}

Any help will be GREATLY appreciated.

Did you mess up pasting the code you had written so far? You currently have a function within another function which isn't right. Also, you cut off the variable_value() function.

Your professor sure leaves a lot of blanks to be filled in. As noted before you cannot have nested function definitions and you need a starting place for doubly-linked list. See the faq which has a very good writeup on linked lists.

The way it's currently setup, the root node has to be a global variable unless the function declarations can be altered to accept the root node as an argument and return it as well (instead of void).