Is it possible create arrays without declaring the specific size in struct?

Question

Trying to create a struct that contains three arrays. I'm unsure of how large to make the arrays until the input is read.

How can i initialize these arrays in the struct if i'm unsure how large they are going to be in the struct init? I will know the total amount from the first line of input.

I would be the size that is read in from the use input. Should I just make i a huge number?

struct TaskSet
{
    float priority[i];
    float p[i];
    float w[i];
    float d[i];
};

herohuyongtao · Accepted Answer · 2014-02-26 16:48:44Z

5

You can either use a huge number or use T *arr (together with the size):

struct TaskSet
{
    float *priority;
    int size_priority;
    ...
    float *d;
    int size_d;
};

P.S.: You need to use malloc with the size once read from the user.

answered Feb 26, 2014 at 16:48

herohuyongtao

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5 Comments

jakebower Over a year ago

The solution here is MUCH better than using a huge number for i for several reasons. First, you can never be sure that i is "huge enough", and second, nearly all of the time you would be wasting "huge" amounts of space. Dynamic allocation, as outlined here, is exactly the correct solution.

user3287789 Over a year ago

Thanks for the tip. I've never actually used malloc before. So in my main program, i would have something like Taskset.priority = (int*) malloc (i), where i is the number i'm reading from the file? How would i navigate that? just like an array?

herohuyongtao Over a year ago

@user3287789 You should use Taskset.priority = (float*) malloc (i * sizeof(float)). And navigate it just like normal arrays.

user3287789 Over a year ago

thanks again! first time i've ever done that. I'm not sure what you guys get out of this, but this site is incredibly helpful. Really appreciate the input/help.

herohuyongtao Over a year ago

@user3287789 This site is fabulous! :P

Kevin · Accepted Answer · 2014-02-26 16:54:32Z

0

A struct needs to be a known size at compile-time so that the compiler knows how much space to give each one in memory. Consider the following struct:

struct pair{
    int a;
    int b;
};

When you create a struct as a function variable, the compiler creates enough space for two ints on the stack. If you create an array of five structs (see below) the compiler creates enough space for 2*5=10 contiguous ints on the stack:

struct pair p[5];

It is not possible for the compiler to create a struct on the stack if it does not know the size of the struct ahead of time.

Pointers are always the same size, so I recommend using a struct of pointers, like in herohuyongtao's sample code.

answered Feb 26, 2014 at 16:54

Kevin

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