尝试将变量分配为C中数组的长度

Question

I'm trying to get n specific number of values from the user, to do this I went the easy way and asked the user to enter a specific number of values to be entered. This amount of values is saved in a variable, for example " x ":

int x;
printf("Enter the number of values to be entered: ");
scanf("%d", &x);
int array[x];

So when I try to run the code I get the following message:

Constant Expression Required

Why does this happens? How could I fix it?

Answer 1

int array[x];

x is not a compile time constant and the compiler is complaining about it. Variable Length Arrays are part of C99 standard. Looks like your compiler is not adhering to the standard.

You can use malloc to dynamically allocate the array and later free it. Or use a compiler that supports VLA.

Answer 2

This declaration:

int array[x];

is a Variable length array( VLA ) and it is a C99 feature. Without VLA support array sizes must be compile time constants. This DrDobb's article on VLA is a good reference.

You mentioned you were using both Borland C++ and Xcode , as far as I can tell Borland s last version was 1997 which would mean it is unlikely to support this feature although I can not find much info. If you are using a more modern branch of the tool it claims to suporrt C99 so there may be a way to enable support.

Xcode as far as I understand can use either clang or gcc both of which support VLA in c99 mode, gcc supports VLA as an extension in non-C99 mode and in C++ and clang supports this as well in limited cases

If you need to develop in both compilers you may have to abandon VLA and use an alternative method for dynamically sized arrays such as malloc in C and in C++ std::vector is probably the right choice for many situations.

Answer 3

Disclaimer: There is a lot of ambiguity with respect to the difference of the following two terms: "declaration" and "definition". Note the example I use to illustrate the difference if you happen to interpret the ambiguity differently than I do.

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For sake of completeness:

Declaring a variable length array ( VLA ) is entirely valid in C99, as you have done:

int n;

scanf( "%d", &n );

int arr[ n ];

... the issue is likely with the compiler you are using or the flags you have specified. An alternative approach is to use the memory pool and malloc a block of memory sufficiently large ( n * sizeof( int ) ) then free the block of memory when you are done with it.

Going back to your original code, it is very important to remark that: only declaring a variable length array is valid, defining one is not. Say for example you wanted to declare a variable length array and zero-fill all of the elements. You could not do the following :

int n;

scanf( "%d", &n );

int arr[ n ] = { 0 }; // Invalid.

... as you are defining arr . You would need to do the following:

int n;

scanf( "%d", &n );

int arr[ n ]; // Valid.

// Zero-fills the array.
for ( int i = 0; i < n; i++ ) {
  arr[ i ] = 0;
}

Answer 4

<!-- language:C -->
/*maybe you can like this*/
int x;
int *p;
printf("Enter the number of value to be Entered:");
scanf("%d", &x);
p = (int *)malloc(sizeof(int) * x);

//p[0 - (x-1)] can be used