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use a variable for array index
- Thread starter tgel76
- Start date
You cannot make a variable constant at runtime.
You can change the value of a constant variable
but this won't help. Look at this mini program:
main()
{
const c = 1, *p = &c;
int * q = const_cast<int *>(p);
*q = 10;
char a[c] = {0}; // q == 10 but sizeof a == 1
return 0;
}
When the array is defined, the value of c is 10,
but the size of the array is one anyway.
To create a dynamic array you have to use the new
operator:
main()
{
int n = 10;
char * a = new char[n]; // create array
a[0] = 1;
a[1] = 2; // ...
delete[] a; // free memory
return 0;
}
Hope this helps.
-- ralf
You can change the value of a constant variable
but this won't help. Look at this mini program:
main()
{
const c = 1, *p = &c;
int * q = const_cast<int *>(p);
*q = 10;
char a[c] = {0}; // q == 10 but sizeof a == 1
return 0;
}
When the array is defined, the value of c is 10,
but the size of the array is one anyway.
To create a dynamic array you have to use the new
operator:
main()
{
int n = 10;
char * a = new char[n]; // create array
a[0] = 1;
a[1] = 2; // ...
delete[] a; // free memory
return 0;
}
Hope this helps.
-- ralf
- Thread starter
- #3
> sorry I haven't express my problem correct...
You have, but I think I haven't express my answer
correct. ;-)
> I just want to use the variables value to declare
> the size of the array.
I understand, but you cannot do this. The compiler
has to know the exact size of the array _before_
the program is executed because the memory for
a static array is reserved when the program is
loaded. And the only way to do this is to use a
constant value.
If you do not know the exact size of your array
you may use the operator "new".
Another 'workaround' is to define an array with
a maximum size, calculate and save its exact size
somewhere and use it as an upper bound in your
iterations.
-- ralf
You have, but I think I haven't express my answer
correct. ;-)
> I just want to use the variables value to declare
> the size of the array.
I understand, but you cannot do this. The compiler
has to know the exact size of the array _before_
the program is executed because the memory for
a static array is reserved when the program is
loaded. And the only way to do this is to use a
constant value.
If you do not know the exact size of your array
you may use the operator "new".
Another 'workaround' is to define an array with
a maximum size, calculate and save its exact size
somewhere and use it as an upper bound in your
iterations.
-- ralf
- Thread starter
- #5
- Thread starter
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