Can arrays provide and after defined length in C? And Why it doesn't work on VS Community

Question

So I have C programing homework and what I suppose to do is get a input from user and make that input length of an array. I basically thought that would work:

int main()
{
    int a, b;
    int array[a];
    scanf("%d", &a);
    
    array[0] = 0;
    printf("%d", array[0]);
    return 0;
}

It worked on an online compiler but it doesn't work on my Visual Studio Community IDE.

What is the problem? Is it my code or IDE?

Yes in C++ use [std::vector](https://en.cppreference.com/w/cpp/container/vector). This also shows you why you should tag questions with C or C++ they are different languages — Pepijn Kramer, Mar 18 '23 at 10:11
@Rynxie Exchange these lines int array[a]; scanf("%d", &a) The variable a shall be initialized with a positive value before the array declaration. Also pay attention to that MS VS C depending on the used version can not support variable length arrays. — Vlad from Moscow, Mar 18 '23 at 10:22
@VladfromMoscow it doesn't work either way in MS VS C but what do u think? does my teacher accept it that way or should I use linked list? — Rynxie, Mar 18 '23 at 10:31
@Rynxie What is the version of VS Community? Try to change the properties of the project selecting the support of C17. — Vlad from Moscow, Mar 18 '23 at 10:34
VLA's (variable length arrays) are C99 feature, made optional in all following updates like C11 or C17, and **never** supported by Microsoft compiler. You can use different techniques as allocating dynamic memory or using _malloca(). See https://stackoverflow.com/questions/56555406/creating-dynamic-sized-array-using-msvc-c-compiler — Frankie_C, Mar 18 '23 at 11:08
Does this answer your question? [Creating dynamic sized array using MSVC C++ compiler](https://stackoverflow.com/questions/56555406/creating-dynamic-sized-array-using-msvc-c-compiler) — Frankie_C, Mar 18 '23 at 11:09

Cardel Emanuel · Answer 1 · 2023-04-26T03:31:09.487

I'm sure that this is late, but I just wanted to let you know I was facing the same issue, and this was the solution I came up with. I couldn't achieve this using an actual array [] so had to make a manual work around.

#include<stdio.h>

int main(void)
{
//set variables
int a, b, i, sum = 0, avg;


//how many scores to average?
printf("How many scores do you want to average?: ");
 scanf("%d", &a);


//collect scores
for (i = 0; i < a; i++)
{
printf("Enter scores: ");
scanf("%d", &b);
sum = sum + b;
}
// print sum
   printf("The Sum of your array = %d \n", sum);
//results
    avg = sum / a;
    printf("The Average of your array = %d", avg);
}

I'm new to coding so forgive me if this is full of extra unnecessary fluff, but with this code, you can get the users input to determine the "array length", and it also asks the user to fill in those arrays and displays the total sum of all the values along with the average.

NicknEma · Answer 2 · 2023-03-18T12:18:06.337

Apart from the fact that a is uninitialized at the time it's first used (which means it will contain garbage bytes - the compiler should tell you if you enable warnings), no, you can't really do this in MSVC (the compiler that comes with Visual Studio).

If you have to do that on Windows, you could try compiling with MinGW as an alternative, or if you have to use Visual Studio, your only option is to use malloc to dynamically allocate the necessary memory. If you can't do that, hope your teacher will understand...

score 0 · Answer 3 · answered Mar 18 '23 at 15:52

Can arrays provide and after defined length in C? And Why it doesn't work on VS Community

Yes, an array with length determined at run-time can be defined after a is defined.
int array[a]; is a variable length array (VLA).

It is a standard feature first in C99 and optional in later versions.

VS Community never fully supported C99.
VS versions that support C11 and later do not implement this optional feature.

What is the problem?

Assuming VLAs are always available.
Code should assign a before int array[a];.

Can arrays provide and after defined length in C? And Why it doesn't work on VS Community

3 Answers3