An array of linked lists in C

The pointer assignments are incorrect.

prev is assigned to a malloc of memory. Then prev is assigned to hold. So the newly allocated memory is lost.

prev = (Element *) malloc(sizeof(Element));
prev = hold;

Your code has more than one problem. First of all, note the create function:

else
    {
        setnum++;
        larray[setnum] = (Element *) malloc(sizeof(Element));
        larray[setnum]->next = NULL;
        larray[setnum]->numberOfValues = 0;
        // larray[setnum] practically becomes the head
    }

I'd say you want to increment setnum only after assigning to larray[setnum], otherwise, you will never use position larray[0]. Also, when setnum == MAXSETS, you will be accessing an out-of-bounds position (this is an off-by-one error). Thus, the test should check for setnum >= MAXSETS. create should look like this instead:

int create(void)
{
    if (setnum >= MAXSETS)
        printf("Error. Maximum value of sets reached.\n");
    else
        {
            larray[setnum] = (Element *) malloc(sizeof(Element));
            larray[setnum]->next = NULL;
            larray[setnum]->numberOfValues = 0;
            setnum++;
        }

    return setnum;
}

The add function is wrong. This piece of code is, for sure, not what you want:

else
{
    prev = (Element *) malloc(sizeof(Element));
    prev = hold;
    prev->next = hold;
    hold->next = NULL;
    hold->value = x;
    printf("Done!\n");
    larray[s]->numberOfValues++;
}

At this point, hold will point to NULL, after the assignment prev = hold;, you leak memory, because you lose reference to the memory you allocated immediately before, and, worse than that, you dereference a null pointer in the following statements. I suggest rewriting this function from scratch.

The display function won't work the way you expect. The for loop leaks memory in every iteration because of this assignment:

larray[s] = larray[s]->next;

Do you really want to destroy the set after displaying it? Also, the loop condition is wrong. When you add an element, you always increment numberOfValues in the first element on that array position, and you don't use numberOfValues in the other elements past the first. However, the loop condition to stop is that larray[s]->numberOfValues == 0, which, combined with the assignment on the previous iteration, makes your program behave completely arbitrarily, since larray[s]->numberOfValues was never initialized and will have some garbage value.

this code has too much problem ，　 I am not sure what you are suppose to do with this code，but I change part of your code， and test it with simple case, it works , hope it will be useful here

#define MAXSETS 5
#define MAXELEMENTS 3
#include <stdio.h>
#include <stdlib.h>
typedef struct element{
    int value;
    int numberOfValues;
    struct element * next;
} Element;


static int setnum = 0; // this is a key that identifies the set
int s, i; // s is input by the user to identify a set | i is a loop counter
Element * prev, * curr;

Element * larray[MAXSETS];

int create(void)
{
    while (setnum < MAXSETS)
        {

            larray[setnum] = (Element *) malloc(sizeof(Element));
            larray[setnum]->next = NULL;
            larray[setnum]->numberOfValues = 0;
            setnum++;
            // larray[setnum] practically becomes the head
        }
     printf("%d\n", setnum);
    return setnum;
}


void add(int s, int x)
{
static Element * hold = NULL;


if (larray[s]->numberOfValues >= MAXELEMENTS) // the set must be finite
{
    printf("Error. Set is full.\n");
}
else
    if (larray[s]->numberOfValues == 0)
    {
        larray[s]->value = x;
        larray[s]->numberOfValues++;
        printf("numberOfValues： %d\n", larray[s]->numberOfValues);
        hold = larray[s];
    }
    else
    {
        hold = larray[s];
        for(int i = 1; i < larray[s]->numberOfValues;i++)
        {
            hold = hold->next;
        }
        prev = (Element *) malloc(sizeof(Element));
        prev->next = hold;
        prev->value = x;
        larray[s]->numberOfValues++;
        printf("numberOfValues： %d\n", larray[s]->numberOfValues);
        hold->next = prev;
    }

}


void display(int s)
{
    curr = larray[s];
    for (i = 0; i < larray[s]->numberOfValues; i++)
    {
        printf("%d\n", curr->value);
        curr = curr->next;
    }
}

int main()
{
    create();
    add(0,0);
    add(0,1);
    add(0,2);
    display(0);

}