6.3-memory.c

#include <stdio.h>
#include <stdlib.h> //required for the memory functions
#include <stdbool.h>
#include <string.h>

typedef struct user
{
    char name[30];
    int age;
    bool isVerified;
} user;

user *createUser(char name[], int age, bool isVerified)
{
    user *newUser = malloc(sizeof(user));
    strcpy(newUser->name, name);
    newUser->age = age;
    newUser->isVerified = isVerified;
    return newUser;
};

int main()
{
    //We've been creating statically sized arrays like so:
    int data[] = {1, 2, 3};
    //This is permanently sized to 3.

    //Typically we want dynamically sized;

    int size;
    printf("How many elements do you want to store?: ");
    scanf("%d", &size); //ONLY USE WHAT WE NEED :) #StopGlobalWarming

    printf("Enter each number followed by the enter key:\n");
    int *arr = malloc(size * sizeof (int));
    for (int i = 0; i < size; i++)
    {
        scanf("%d", &arr[i]);
    }

    printf("Your array:\n");
    for (int i = 0; i < size; i++)
    {
        printf("arr[%d] = %d\n",i, arr[i]);
    }
    
    //When done, make sure you call free! Otherwise memory leak
    free(arr);

    ////////////    null vs void pointer   ////////////
    //Malloc will return either a null pointer, or void pointer
    //null pointer means it points nowhere
    //void pointer means it points to unknown type
    //void = good
    //null = bad = Unable to allocate memory
    //cast from void to int pointer is implicit (C is partially weakly typed)

    //You may want to check if allocation was successful: 
    int *grades = malloc(30000 * sizeof (int)); // 30000 ints
    
    if (grades == 0) // or == NULL
    {
        printf("Error Allocating Memory. Deleting Harddrive. 3...2...1...");
        return -1; 
    } else
    {
        //printf("allocation success!\n"); //Usually you don't tell the users this lol
    }

    //Dynamic memory also allows variables to persist past there scope.
    //specifically, think of variables defined within functions.

    user *newUser = createUser("Caleb Curryyyy", 18, false);

    printf("\nuser name: %s\nage: %d\nisVerified: %d\n", 
            newUser->name, 
            newUser->age, 
            newUser->isVerified
            );

    //REMEMBER TO FREE
    free(newUser);  


    //can see this concept being useful in making a library
    //to work with a data structure such as a linked list
    //saving that for intermediate C ;)

    //You can see the effects of not freeing if you do this:
    //while(true)
    //{
    //  printf(".");
    //    int *test = malloc (10000000 * sizeof(int));
    //}

    //Other functions you may want to learn

    //realloc - used to resize amount of memory (say an array needs to grow)
    //calloc - similar to malloc but initializes everything to 0 (no "cruft")
    //much more to learn - statt here:
    //https://en.wikipedia.org/wiki/C_dynamic_memory_allocation

    return 0;
}