Can I declare an array in C without declaring its size and element?

Question

When I print the array element, the elements will be hm not constant?

Like if I input 1, 2, 3 sometimes when printed out it became 1, 2, 4 and sometimes another number just come up

#include <stdio.h>

int main()
{
    int i, j=0;
    int num[] = {};
    
    
    for (i=0; i<=3; i++)
    {
        
        printf("\nEnter a number: ");
        scanf("%d", &num[j]);
        j++;
    }
    
    printf("\nnum1: %d\n", num[0]); 
    printf("num2: %d\n", num[1]);   
    printf("num3: %d\n", num[2]);   
    printf("num4: %d", num[3]);
}

This is my code and this is the output Output

The short answer is: Your code is illegal. You can instead do: `int num[4] = {};` or do: `int num[] = {0, 0, 0, 0};` — Support Ukraine, Apr 17 '23 at 08:03
BTW: If your using gcc always do `gcc -Wall -Wextra -Werror ....` — Support Ukraine, Apr 17 '23 at 08:06
In theory Arrays means a list of elements with a **fixed** length. — Darth-CodeX, Apr 17 '23 at 08:07
Yep, i tried that also and it give me the right output. So, this means that i need to initialize the size of the array first however it is, right? Thankss — repivtsea, Apr 17 '23 at 08:07
And in case, you don't know length at compile time, consider making a vector using `malloc` and `realloc` — Darth-CodeX, Apr 17 '23 at 08:08
@repivtsea gcc is set of compilers, which includes compilers for languages such as `c`, `c++`, `d`, `ada` ,etc — Darth-CodeX, Apr 17 '23 at 08:09
`int num[] = ...` creates an array with size implied by the initializer. In your case 0, so you are using the array beyond its size. — nielsen, Apr 17 '23 at 08:09
_"Yep, i tried that also and it give me the right output"_: be aware that this only _appears_ to work as you're accessing an array out of bounds. Doing so results in _undefined behaviour_ (google that term) — Jabberwocky, Apr 17 '23 at 08:39

Eric Postpischil · Answer 1 · 2023-04-17T14:13:40.160

When an array is defined with [] for the dimension and an initialization is given, its size is deduced from the initializers. However, = {} has no initializers. This does not conform to the formal grammar in C, which specifies there shall be at least one initializer inside the braces (C 2018 6.7.9 1). Because of this, the behavior is not defined by the C standard, except that a conforming compiler must produce a diagnostic message.

Some compilers accept this declaration and treat it as an array of zero length. This is another reason the behavior is not defined by the C standard, as the standard defines an array to be a nonempty set of objects (C 2018 6.2.5 20).

The program then goes on to attempt to put values in num[0], num[1], num[2], and num[3]. Since the array has zero length, these elements do not exist in the C computing model, and no memory has been reserved for them. The scanf calls likely write these values into memory that the program is using for other purposes. And, since the program is using that memory for other purposes, it may change that memory, resulting in the values later accessed by num[0], num[1], num[2], or num[3] to be different from what scanf wrote there.

Can I declare an array in C without declaring its size and element?

You can declare an array without specifying its size. At file scope, outside of any function, int num[]; would be a valid declaration that tells the compiler an array named num may be defined somewhere else.

When you define an array, you should specify its size, either by explicitly giving the size inside [] (as with int num[3];) or by giving initial values from which the compiler can deduce the size (as with int num[] = { 9, 8, 7, 6 };). Inside the body of a function, a declaration of an array without extern is also a definition, so it should include the size.

score 0 · Answer 2 · answered Apr 17 '23 at 13:58

0

Declaration of an array without specified size in not valid in C language, as here compiler may be allocating an array of default size. And this is leading to the undefined behaviour of this program.

Also in this program stack smashing is hapenning, which is a kind of buffer overflow, which in case program is trying to write data beyond the allocated limit.

answered Apr 17 '23 at 13:58

Ujjawal Kumar

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_"as here compiler may be allocating an array of default size."_ huh?? – Jabberwocky Apr 17 '23 at 14:00

Can I declare an array in C without declaring its size and element?

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