Expression must have a constant value problem

Question

I used C in Visual Studio to make a code for a user to input size of array.

The code does not work in Visual Studio and gives errors.

But on a site like replit it works.

I don't understand what to do to make it work in Visual Studio.

#include <stdio.h>
#include <time.h>
#include <string.h>
#include <math.h>

int main()
{
    int m;
    do
    {
        printf("please enter array size--> ");
        scanf_s("%d", &m);
    } while (m <= 1);

    int arry[m];

    for (int i = 0 + 1; i < m + 1; i++)
    {
        printf("%d,", arry[i] = i);
    }

    return 0;
}

Answer 1

You can try my sanity check code below to test the usefulness and standard compliance of your compiler:

#include <stdio.h>

int main()
{
  #if !defined(__STDC__) || !defined(__STDC_VERSION__)
    puts("This compiler is garbage.");
  #elif (__STDC_VERSION__ >= 201112L)
     #if (__STDC_NO_VLA__==1)
       puts("This compiler is mighty strange but compliant.");
     #else
       puts("This compiler is modern and useful.");
       int m = 5;
       int array[m];
     #endif
  #elif (__STDC_VERSION__ == 199901L)
    puts("This compiler is old but useful.");
    int m = 5;
    int array[m];
  #endif

  return 0;
}

Compilers giving the output "mighty strange" or "garbage" will not support variable-length arrays, if supporting the C language at all.

Output from various common x86 compilers below.

Default settings

• clang 14.0.0 x86: This compiler is modern and useful.
• gcc 12.1 x86: This compiler is modern and useful.
• icc 2021.5.0 x86: This compiler is modern and useful.
• icx 2022.0.0 x86: This compiler is modern and useful.
• MSVC 19.32 x86: This compiler is garbage.

-std=c99

• clang 14.0.0 x86 -std=c99: This compiler is old but useful.
• gcc 12.1 x86 -std=c99: This compiler is old but useful.
• icc 2021.5.0 x86: -std=c99: This compiler is old but useful.
• icx 2022.0.0 x86: -std=c99: This compiler is old but useful.
• MSVC 19.32 x86 /std:c99: This compiler is garbage.

-std=c11

• clang 14.0.0 x86 -std=c11: This compiler is modern and useful.
• gcc 12.1 x86 -std=c11: This compiler is modern and useful.
• icc 2021.5.0 x86 -std=c11: This compiler is modern and useful.
• icx 2022.0.0 x86 -std=c11: This compiler is modern and useful.
• MSVC 19.32 x86 /std:c11: This compiler is garbage.

-std=c17

• clang 14.0.0 x86 -std=c17: This compiler is modern and useful.
• gcc 12.1 x86 -std=c17: This compiler is modern and useful.
• icc 2021.5.0 x86 -std=c17: This compiler is modern and useful.
• icx 2022.0.0 x86 -std=c17: This compiler is modern and useful.
• MSVC 19.32 x86 /std:c17: This compiler is garbage.

Answer 2

This declaration

int arry[m];

is a declaration of a variable length array that is conditionally supported by C compilers because m is not an integer constant expression.

It seems you are using a version of MS VS the C compiler of which does not support variable length arrays or you need to change properties of your project and select the last version of the C compiler.

Otherwise declare an array of a fixed size that is not less than the possible value of the variable m or allocate it dynamically.

In any case pay attention to that this for loop

for (int i = 0 + 1; i < m + 1; i++)
{

printf("%d,", arry[i] = i);

}

results in accessing memory beyond the array because the valid range of indices is [0, m).

It must look like

for (int i = 0; i < m; i++)
{
    printf("%d,", arry[i] = i + 1 );
}

Answer 3

By default, when MSVC compiles code as C, it implements ANSI C89 with Microsoft-specific language extensions. Some of these MSVC extensions are standardized in ISO C99 and later.

__STDC_NO_VLA__ is defined as 1 if the implementation doesn't support standard variable length arrays. The MSVC implementation defines it as 1 when compiled as C and one of the /std C11 or C17 options is specified.

https://clang.llvm.org/c_status.html#c99

https://gcc.gnu.org/c99status.html

MS says VLA is dangerous for you & less efficient so they are not supported