Array initialization at runtime

Question

code link

#include <stdio.h>

int main(void) {
    // your code goes here
    int a = 2;
    int b = 3;
    int c;
    c = a + b;
    int arr[c];
    arr[5] = 0;
    printf("%d",arr[5]);
    return 0;
}

Output is 0

How is it that at runtime it is taking the array number ? Is it a new feature ?

Answer 1

This is a variable length array. They were introduced in the 1999 revision of the C standard.

Sadly support for them came in slowly, so much that the 2011 revision made them an optional feature (but they are still standardized) ¹.

Despite looking cool, they have a major caveat. They can cause you to overflow the call stack if the size is "too big". As such, care needs to be taken when using them.

---

_{¹ Some compiler vendors were resistant, so it was made optional to appease them. Microsoft is an entire case study of this.}

Answer 2

This feature (Variable length array) has been introduced in C99. But currently this still is a compiler-dependent behavior. Some compiler(like gcc) supports it. Some(like msvc) doesn't.

BTW, arr[5] in your code, is out of range. Last element should be arr[4].

Answer 3

don't be confused in (static/fixed memory allocation) & (dynamic memory allocation) concepts :)

Let me clear your concept bro.

Relevant to following question,

                           C supports two type of array. 

1.Static Array - are allocated memory at "COMPILE TIME".

2.Dynamic Array - are allocated memory at "RUN TIME".

Ques. how to determine if an Array is static or dynamic?

Ans. Array declaration syntax:-

         int array_Name[size];     //size defines the size of block of memory for an array;

So, coming to the point-->

Point 1. if size is given at compile time to array, it's a "Static Memory Allocation". It is also called "fixed size memory allocation" because size is never changed. It's the LIMITATION of ARRAY in C.

ex. int arr[10]; //10 is size of arr which is staticly defined int brr[] = {1000, 2, 37, 755, 3}; //size is equal to the no. of values initilizes with.

point 2. If size is given at compile time to array, it's a Dynamic Memory Allocation. It is achieved by malloc() function defined in stdlib.h .

Now, its's the clarification of your code :-

 #include <stdio.h>

    int main(void) {
        // your code goes here
        int a = 2;
        int b = 3;
        int c;
        c = a + b;             //c is calculated at run time
        int arr[c];           //Compilor awaiting for the value of c which is given at run time but,
        arr[5] = 0;           //here arr is allocated the size of 5 at static(compile) time which is never be change further whether it is compile time in next statements or run time.
        printf("%d",arr[5]);  
        return 0;
    }

So, array(of size 5) holds value 0 at arr[5].
and ,other array indexes still show Garbage Values.

Hoping, you'll be satisfy with this solution to your problem :)