Creating a dynamic array of structs

Question

I need to create an array of structs but I have to read the array size from a file. My problem is that I might be using the pointers all wrong. I can't use global variables. Here's the code I made :

#include <stdio.h>
#include <stdlib.h>
struct driver
{
       int *amount;
       int ADT1[9];
       int ADO1[9];
};

struct driver totaldrivers[*amount];

The errors(2) I get are "amount undeclared here(not in a function)" and "storage size of 'totaldrivers' isn't known". I tried giving the *amount a value directly but no luck and even so the amount must be taken by the file the user has created and the number is the first line. And that's where I have my issue. The (.txt)file format is:

4(amount of entries)
123456789 , 978675645 (ADT1,ADO1)
.
.
.
I am sure I've made rookie mistakes so forgive me if the question is silly.

amount is not a global variable, you need an initialized driver structure instance to access it. — Some programmer dude
– Some programmer dude, Commented Dec 30, 2013 at 19:09
I think the point is to read the first line and use that to size your array, not put it inside your struct. — Duck
– Duck, Commented Dec 30, 2013 at 19:09
if 'amount' is count of number of entries then dont put in the struct — pm100
– pm100, Commented Dec 30, 2013 at 19:10
Also, you don't need one int per digit. Instead a single unsigned int will be enough. — Some programmer dude
– Some programmer dude, Commented Dec 30, 2013 at 19:11

Fiddling Bits · Accepted Answer · 2013-12-30 19:30:17Z

3

I'm assuming the contents of your structure should be as follows (I've removed int *amount):

struct driver
{
    int ADT1[9];
    int ADO1[9];
};

To create a dynamic 1D array of these structures, first create a pointer to a 1D array of driver:

struct driver *pDriver;

Next, allocate memory for the number of elements you need in the array. This should be known (N):

pDriver = malloc(N * sizeof(*pDriver));

If you want to keep a record of the number of elements, you should do something like this:

struct driver
{
    int ADT1[9];
    int ADO1[9];
};

struct driverArray
{
    int numDrivers;
    struct driver *pDriver;
};

struct driverArray driverArr;

driverArr.numDrivers = N;
driverArr.pDriver = malloc(driverArr.numDrivers * sizeof(*driverArr.pDriver));

edited Dec 30, 2013 at 19:30

answered Dec 30, 2013 at 19:19

Fiddling Bits

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4 Comments

Roaf Beast Over a year ago

this one seems like it works. will update soon when i finish the rest of the code.

Roaf Beast Over a year ago

code works like a charm but i have another issue. I got informed that i need to use functions and not just one main. how do i pass this array in the function?

Roaf Beast Over a year ago

despite this answer being correct I couldn't figure out how to pass the arrays in the function. asking my mates i got a different way that worked better for me. thanks anyway!

Fiddling Bits Over a year ago

Ask another question if you need more help. Godd luck.

keshlam · Accepted Answer · 2013-12-30 19:26:16Z

1

Rather than

struct driver totaldrivers[*amount];

Set up a pointer to refer to it, then allocate the array from the heap when you know the size you'll need.

struct driver totaldrivers[];
int amount=7; /* for example */
totaldrivers=malloc(sizeof(struct driver)*amount);
/* Apology for the typo; I wrote "driver=" the first time. Good catch! */

You also probably don't want that "int *amount" inside the struct.

edited Dec 30, 2013 at 19:26

answered Dec 30, 2013 at 19:12

keshlam

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2 Comments

Fiddling Bits Over a year ago

This is not correct. You must create memory for the variable, totaldrivers, not the type, driver.

keshlam Over a year ago

Good catch. Pure finger-check and sloppy proofreading. Fixed. Thanks.

Anis_Stack · Accepted Answer · 2013-12-30 20:32:07Z

0

struct driver totaldrivers[*amount];

here, you use array (first method):

* amount is the size the array of driver struct. this is a wrong way to define size of array !! there is many way to define the size of array some of them:

1) preprocessor using define directive: #define size 12

2) global variable : int size 12

you can also use pointer (second method):

struct driver *totaldrivers;

totaldrivers= malloc(nb * sizeof(*totaldrivers)); // nb : number of elements

you should not complicated your code take it easy !

edited Dec 30, 2013 at 20:32

answered Dec 30, 2013 at 20:09

Anis_Stack

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Comments

cup · Accepted Answer · 2013-12-30 21:21:02Z

0

Another way of doing this is to use the dynamic stack allocation facility introduced in C99. Just make sure your compiler is C99 compilant.

As the others have said, dispense with amount.

struct driver
{
    int ADT1[9];
    int ADO1[9];
};

Introduce a function to do whatever you want. Allocate the array here

void RunWith(int amount)
{
    struct driver totaldrivers[amount];

    ... 
}

In the caller, pass in the amount

int main ()
{
    RunWith(25);
    ...
    return 0;
}

There is no need to malloc or free.

answered Dec 30, 2013 at 21:21

cup

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