Incorrect output in an array inside a struct

Question

I have a struct with an int array inside that I'm passing to a function for the array to be initialized

array struct like so..

typedef struct Container{
    struct intArray *P;
    int length;
} Container;

typedef struct intArray{
    int *array;
    int length;
} intArray;

function to initialize the array like so...

int Initializer(intArray *myStruct, int n)
{
    myStruct->array = malloc(sizeof(int) * (lengthOfint);
                                              ^
                                        If n=55 then length would be 2
    //let's just say n=5
    myStruct->array[0] = n;

    //return 1 if successful
    return 1;
}

In another function I am calling the initializer function like so...

Container *myContainer = malloc(sizeof(Container));

myContainer->P = malloc(sizeof(intArray *) * Some_Defined_Value);

Initializer(&myContainer, 5);

printf("the data that should be at index 0 -> %d\n", myContainer->P.array[0];

I would think that the printf statement would print out 5 but it prints out garbage of varying numbers every time I compile it. I don't think I have a grasp of the array within a struct within a struct idea.

Please post code that could compile...myStruct->array = malloc(sizeof(int) * (lengthOfint); is missing a close parenthesis. Also, I'm puzzled how n=55 leads to lengthOfint == 2; are you measuring in bits and assuming sizeof(int) == 4 and CHAR_BIT == 8? Not unreasonable assumptions, but the use of a bit count is not so obvious. — Jonathan Leffler
– Jonathan Leffler, Commented Jun 12, 2013 at 23:37
I think, op means something to the likes of lengthOfint == ceil(log10(abs(n))). — David Foerster
– David Foerster, Commented Jun 12, 2013 at 23:46
The idea behind that is that I need to store a number into a int array so 12345 would be in an intArray[5] = {1,2,3,4,5}...So 55 would yield an array of size 2. — A.T.
– A.T., Commented Jun 12, 2013 at 23:49

user268396 · Accepted Answer · 2013-06-12 23:30:51Z

1

You pass the container to the initializer function, but if you look closely you'll see that the you are passing a pointer to a pointer to the "outer" container instead of a pointer to the desired struct intArray. You'd want something like Initializer(myContainer->P, 5);

answered Jun 12, 2013 at 23:30

user268396

12.1k2 gold badges33 silver badges26 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

A.T. Over a year ago

the compiler tells me that's an incompatible type argument for Initializer when I do that, that's the only reason I changed it to &myContainer. of course the code I have sampled here is not the complete thing but it's pretty accurate.

David Foerster Over a year ago

It's an incompatible pointer type because the types Container and intArray are indeed incompatible. Trying to cast Container** to intarray* instead doesn't make it better. These warnings exist for a reason!

A.T. Over a year ago

So what's the work around, no matter what this is the infrastructure I'm trying to implement -> A container which holds multiple intArray structs and each intArray struct holds an int array with different amounts of data.

Ingo Blackman · Accepted Answer · 2013-06-13 03:11:31Z

0

Container holds a pointer to struct intArray, which in this case is the start of an array of struct intArray. But then you initialize this pointer with

malloc(sizeof(intArray *) * Some_Defined_Value);

So malloc returns a pointer to a memory space which holds pointers to struct intArray because you used sizeof(intArray *) and not sizeof(intArray) (you allocated space for Some_Defined_Value number of pointers).

You need sizeof(intArray) * Some_Defined_Value here to allocate space for Some_Defined_Value number of struct intArray's.

Then you use

Initializer(&myContainer, 5);

which should at least give a warning because you pass a pointer to a pointer to a struct Container, but Initializer expects a pointer to a struct intArray, so this is not what you want. To initialize the first element of your array of struct intArray's use:

Initializer(&(myContainter->P[0]), 5);

Then:

printf("the data that should be at index 0 -> %d\n", myContainer->P.array[0]));

This code does not compile because it should be:

printf("the data that should be at index 0 -> %d\n", myContainer->P[0].array[0]);

myContainer->P accesses a pointer to a struct intArray. The code right above means you access the first element (element number 0) of the array of struct intArray's .

To initialize/access the second element of the array of struct intArray's use:

Initializer(&(myContainter->P[1]), 5);
printf("the data that should be at index 0 -> %d\n", myContainer->P[1].array[0]);

edited Jun 13, 2013 at 3:11

answered Jun 13, 2013 at 0:28

Ingo Blackman

9908 silver badges13 bronze badges

2 Comments

A.T. Over a year ago

I am trying to achieve A single container which holds an array of structs. Inside of each of these structs are an int array which holds an integer represented in array form -> 12345 is an array[5]={1,2,3,4,5}

Ingo Blackman Over a year ago

Then you should be fine with the end of what I wrote. If you want to initialize the 2nd intArray inside your Container, you need Initializer(&(myContainer->P[1])); and to access it (after initialization), myContainter->P[1].array[0].

Gauthier Boaglio · Accepted Answer · 2013-06-12 23:43:54Z

0

Also be aware that malloc returns a void * so most of the time you'll need a cast like so:

myStruct->array = (int*)malloc(sizeof(int) * (length);
//                ^^^^^^

Assuming length is the number of int you want to store in your array.

answered Jun 12, 2013 at 23:43

Gauthier Boaglio

10.3k5 gold badges52 silver badges85 bronze badges

Collectives™ on Stack Overflow

Incorrect output in an array inside a struct

3 Answers 3

3 Comments

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related