Defining and initializing arrays

[John]

Is this a valid C++ program that will not crash on any machine?

#include <iostream>
using namespace std;
int main( void ) {
int i;
cin >i;
double X[i];

X[i-1] = 1123;
cout << X[i-1] << endl;

return 0;
}

What does the standard say about defining and initializing arrays of
length
non-const? Is it illegal? Or is that legal in C++?

Thanks,
--j

[Frederick Gotham]

John posted:

What does the standard say about defining and initializing arrays of
length
non-const? Is it illegal? Or is that legal in C++?

According to the current C++ Standard, the length of an array must be a
compile-time constant.

Many compilers provide an extension whereby the programmer can define an
array using a runtime value.

If you're writing portable code, use "new" or "vector" or "malloc" instead.

--

Frederick Gotham

[S S]

John wrote:

Is this a valid C++ program that will not crash on any machine?

#include <iostream>
using namespace std;
int main( void ) {
int i;
cin >i;
double X[i];

X[i-1] = 1123;
cout << X[i-1] << endl;

return 0;
}

What does the standard say about defining and initializing arrays of
length
non-const? Is it illegal? Or is that legal in C++?

Did you try it even on one machine before posting?

Thanks,
--j

[Frederick Gotham]

S S posted:

Did you try it even on one machine before posting?

Most compilers will compile it no problem -- if they're not in Standard C++
mode, that is.

--

Frederick Gotham

[Rolf Magnus]

John wrote:

What does the standard say about defining and initializing arrays of
length non-const? Is it illegal? Or is that legal in C++?

It would be legal in C, but it's illegal in C++.

[Howard]

"John" <we**********@y ahoo.comwrote in message
news:11******** **************@ m7g2000cwm.goog legroups.com...

Is this a valid C++ program that will not crash on any machine?

Well, I've seen a few machines that had a tendency to crash, regardless of
what you ran on them. :-) Of course, that's not what you're really
asking...

#include <iostream>
using namespace std;
int main( void ) {
int i;
cin >i;
double X[i];

X[i-1] = 1123;
cout << X[i-1] << endl;

return 0;
}

What does the standard say about defining and initializing arrays of
length
non-const? Is it illegal? Or is that legal in C++?

The standard says that the array size has to be a compile-time constant.
The above code should not compile (and if it does, then that's a compiler
extension, and is non-standard).

If you need to specify the size at run-time, you can define X as a double*,
and dynamically allocate using new[] (and later delete[] to clean up).
Better still, use std::vector!

-Howard

[amparikh]

John wrote:

Is this a valid C++ program that will not crash on any machine?

#include <iostream>
using namespace std;
int main( void ) {
int i;
cin >i;
double X[i];

X[i-1] = 1123;
cout << X[i-1] << endl;

return 0;
}

What does the standard say about defining and initializing arrays of
length
non-const? Is it illegal? Or is that legal in C++?

Thanks,
--j

It needs to be a constant. Think about it for a second. How can the
compiler possibly guess as to how much memory it needs to reserve ?

[Frederick Gotham]

Rolf Magnus posted:

John wrote:

>What does the standard say about defining and initializing arrays of
length non-const? Is it illegal? Or is that legal in C++?

It would be legal in C, but it's illegal in C++.

Only in C99, which has yet to take the place of C89.

--

Frederick Gotham

[Gianni Mariani]

am******@gmail. com wrote:
....

It needs to be a constant. Think about it for a second. How can the
compiler possibly guess as to how much memory it needs to reserve ?

Some compilers actually implement that feature as an extension so your
question obviously has an answer. The C99 standard actually allows this
and it is one way in which an array can be dynamically allocated.

[twomers]

If you need to specify the size at run-time, you can define X as a double*,

and dynamically allocate using new[] (and later delete[] to clean up).
Better still, use std::vector!

-Howard

Don't mix up new and delete syntax!

int num;
cout<< "Enter a number: ";
cin >num;

int *i = new int[num];

// do something

// if the pointer i was not an dynamically allocated array, you can use
this
// delete i;

// however it IS an array (probably), so you have to do this
delete []i;

Just thought I'd clarify in case the OP didn't know about the syntax,
odd as it is.