Linked List and Dynamic Memory Allocation

This is a discussion on Linked List and Dynamic Memory Allocation within the C Programming forums, part of the General Programming Boards category; Hi, I'm a newbie to C programming and currently very confused. We are studying linked lists right now and our ...

  1. #1
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    Unhappy 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.

  2. #2
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    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.

  3. #3
    CSharpener vart's Avatar
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    assert should be used to catch the programmers errors during debugging.

    malloc returning NULL is not a programmers error - it is possible condition indicating out of memory

    so it should be handled in the release version as well.

    in your case program will just crash in release
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  4. #4
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    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.

  5. #5
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    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).

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