variable lenght arrays

[Gkendoyan]

How do you prompt the user to enter the number of elements for the array and use that information to creatr a variable length array?
And then how do you prompt the user to enter in a number for each element of the array and scan in the appropriate numbers? the numbers are double precision floating point.

for example,
Enter the numbe of elements in the array: 3

Enter element 0: 3
Enter element 1: -1
Enter element 2: 4

I know it starts with

int main()
{
double N;
int a[size];

printf("Enter the number of elements in the array:" );
scanf("%f", &size); //I'm pretty sure this is wrong

[camel-man]

declare size as an int
declare array of floats
prompt user to enter in size of array
scan in size
for loop i = 0
prompt user to enter in float for array value
scan in array i
loop back:

[Gkendoyan]

int main()
{
double a[];
int n, i;
printf("Enter the number of elements in an array: ");
scanf("%d", &n);
for (i=0; i<n; i++)
{
    printf("Enter element %d: ", n);
    scanf("%d", &i);
}

Is this correct?

[camel-man]

Let me be more precise, follow the steps in order

declare n as an int
declare array of floats with n in the brackets
prompt user to enter in size of array
scan in n

for loop i = 0
prompt user to enter in float for array value
scan in array i
loop back:

[grumpy]

Let me be more precise, follow the steps in order

declare n as an int
declare array of floats with n in the brackets
prompt user to enter in size of array
scan in n

for loop i = 0
prompt user to enter in float for array value
scan in array i
loop back:

You have given a precise description of a method to achieve undefined behaviour.

When declaring a VLA, using the syntax

    int n = some_value_obtained_from_somewhere();
    float array[n];

the result is an array with number of elements equal to the value of n at that time. There is no magical association established between the array and n. Subsequently changing n (for example, reading a new value from the user) therefore does NOT magically resize the array.

[Alpo]

int main()
{
double a[]; /// For a VLA, the declaration should come after you have the length

int n, i;
printf("Enter the number of elements in an array: ");
scanf("%d", &n);

/// Array declaration goes here, include [n]

for (i=0; i<n; i++)
{
    printf("Enter element %d: ", n);
    scanf("%d", &i);  /// This is not a good idea, 'i' controls the loop

    /// Assign values to VLA around here
}

Is this correct?

I would really recommend just using malloc() rather than a VLA, it's not as scary as it looks. You just have to remember that the value you give it is the number of bytes you will get in return (which is why you always see the sizeof operator being used with it). Aside from that you just need to remember the symbols associated with the pointers malloc() returns so you can free() it later.

[camel-man]

What am I remembering.. for some reason I thought VLA were valid after C99.

[jimblumberg]

for some reason I thought VLA were valid after C99.

No VLA are only guaranteed to be valid with C99. Before C99 they were not supported by the standard. After C99 they became optional, meaning a compiler no longer needs to support this feature (and several others) to claim standard compliance.

If you are going to use this "feature" you should really find and study the documentation for this "feature". Remember VLA are limited to the stack space, as are ordinary statically allocated arrays. There are also differences between VLA and "normal" arrays. For example a VLA can not be declared static.

Jim

[grumpy]

What am I remembering.. for some reason I thought VLA were valid after C99.

As Jim said, the 1999 C standard was the first C standard that specified VLAs, but some compilers supported VLA's (or comparable extensions) before then.

Your preceding description in this thread, however, of how to use VLAs is incorrect.