Initialize 2d Array at Runtime and Allow user input

Question

printf("\nNow its time to make a custom array. Please enter the number of rows: ");
int rows = scanf_s("%d", &rows);
printf("Now enter the number of columns: ");
int cols = scanf_s("%d", &cols);

int **custom2d;

custom2d = malloc(sizeof(int) * rows);

for (int i = 0; i < rows; i++)
{
    *(custom2d + i) = malloc(sizeof(int) * cols);
}

for (int i = 0; i < rows; i++)
{
    for (int j = 0; j < cols; j++)
    {
        printf("Enter value for [%d][%d]: ", i, j);
        scanf_s("%d", custom2d[i][j]);
    }
}

I am very new to C but I know several other higher level languages. I can't understand why this code isn't working. When I get to the prompt for entering the array value at the index, I get an exception(access violation writing location). I am very confused. All I'm trying to do is allow the user to specify rows, columns, and then input a value at each location of the array.

int ** is not a 2D array, nor can it point to one. It is a completely different construct. — too honest for this site
– too honest for this site, Commented Apr 24, 2017 at 10:54
The best way to ensure you're allocating the correct type/size is to write ptr_var = malloc(num * sizeof *ptr_var), in your case that would be custom2d = malloc(rows *sizeof *custom2d); and custom2d[i] = malloc(cols * sizeof *custom2d[i]);. — Elias Van Ootegem
– Elias Van Ootegem, Commented Apr 24, 2017 at 10:59
Consider learning to use a debugger to trace through the program line by line inspecting the values of the relevant variables to learn what is really going on. — alk
– alk, Commented Apr 24, 2017 at 11:02
int rows = scanf_s("%d", &rows); → int rows; scanf_s("%d", &rows);, int cols = scanf_s("%d", &cols); → int cols; scanf_s("%d", &cols);. scanf returns the number of items successfully matched and assigned, not the input value. — Spikatrix
– Spikatrix, Commented Apr 24, 2017 at 11:03
@CoolGuy: This catch should make all of us bow our heads in shame ... ;-} — alk
– alk, Commented Apr 24, 2017 at 11:04

Community · Accepted Answer · 2017-05-23 11:33:13Z

3

Change this part :

int **custom2d;

custom2d = malloc(sizeof(int) * rows);

for (int i = 0; i < rows; i++)
{
    *(custom2d + i) = malloc(sizeof(int) * cols);
}

to :

int **custom2d;
custom2d = malloc(sizeof(int*) * rows);
if (custom2d == NULL)
{ 
    printf ("Error");
    return;
}
for (int i = 0; i < rows; i++)
{
    *(custom2d + i) = malloc(sizeof(int) * cols);
    if (custom2d[i] == NULL)
    {
        printf ("Error");
        return;
    }
}

See this link on why you should check the result of malloc. Also change :

scanf_s("%d", custom2d[i][j]);

to :

scanf_s("%d", &custom2d[i][j]);

Finally change :

int rows = scanf_s("%d", &rows);

and

int cols = scanf_s("%d", &cols);

to :

int rows;
scanf_s("%d", &rows);

and

int cols;
scanf_s("%d", &cols);

respectively, as right now you are using the return value of scanf_s and not the values read.

edited May 23, 2017 at 11:33

CommunityBot

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answered Apr 24, 2017 at 11:05

Marievi

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Comments

granmirupa · Accepted Answer · 2017-04-24 11:17:12Z

1

You need to allocate an array of pointers (int *) instead of int

custom2d = malloc(sizeof(int *) * rows);

and then for more readability, imho, you can also do:

custom2d[i] = malloc(sizeof(int) * cols);

in both cases you should check if the allocation has been done:

   if (custom2d == NULL)
        printf ("No allocation made");

and

   if (custom2d[i] == NULL)
        printf ("No allocation made");

Moreover you need to pass the pointer to scanf_s:

scanf_s("%d", &custom2d[i][j]);

UPDATE

You need to change also this because scanf_s returns the number of fields successfully converted and assigned

int rows;
scanf_s("%d", &rows);

int cols;
scanf_s("%d", &cols);

edited Apr 24, 2017 at 11:17

answered Apr 24, 2017 at 11:10

granmirupa

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