References to multidimensional array

[TLOlczyk]

I have a brain cramp and I need some help.

I have a chunk of code below which demonstrates
a problem I have with multidimensional arrays.
I want to keep it simple but something specific is getting in the way.

int a[10][10];
int b[10][10];
int **present;
int **next;

bool done=false;

while(!done)
{
// Loop which substitutes for complex calculation
for(int i=0;i<10;i++)
for(int j=0;j<10;j++)
next[j]=present[j];
int **temp=next;
next=present;
present=temp;
test_for_done(present);
}

Compare this to the one-dim version ( which works )
int a[10];
int b[10];
int *present;
int *next;

bool done=false;

while(!done)
{
// Loop which substitutes for complex calculation
for(int i=0;i<10;i++)
next=present;
int *temp=next;
next=present;
present=temp;
test_for_done(present);
}




The reply-to email address is (e-mail address removed).
This is an address I ignore.
To reply via email, remove 2002 and change yahoo to
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**
Thaddeus L. Olczyk, PhD

There is a difference between
*thinking* you know something,
and *knowing* you know something.

 

[Victor Bazarov]

I have a brain cramp and I need some help.

I have a chunk of code below which demonstrates
a problem I have with multidimensional arrays.
I want to keep it simple but something specific is getting in the way.

int a[10][10];
int b[10][10];
int **present;
int **next;

bool done=false;

while(!done)
{
// Loop which substitutes for complex calculation
for(int i=0;i<10;i++)
for(int j=0;j<10;j++)
next[j]=present[j];
int **temp=next;
next=present;
present=temp;
test_for_done(present);
}

Compare this to the one-dim version ( which works )
int a[10];
int b[10];
int *present;
int *next;

bool done=false;

while(!done)
{
// Loop which substitutes for complex calculation
for(int i=0;i<10;i++)
next=present;
int *temp=next;
next=present;
present=temp;
test_for_done(present);
}

The code you posted doesn't seem to use 'a' or 'b' in either variation.

I suspect that the problem you have is due to the fact that an array
of more than one dimension cannot be converted to a pointer to pointer.
They are incompatible types.

Perhaps we can come back to this when you post the actual code with which
you have the problem.

Victor

 

[Howard]

I have a brain cramp and I need some help.

I have a chunk of code below which demonstrates
a problem I have with multidimensional arrays.
I want to keep it simple but something specific is getting in the way.

int a[10][10];
int b[10][10];
int **present;
int **next;

bool done=false;

while(!done)
{
// Loop which substitutes for complex calculation
for(int i=0;i<10;i++)
for(int j=0;j<10;j++)
next[j]=present[j];

At this point, you have undefined behavior, since neither next nor present
have been initialized to anything! (Did you mean to initialize them to
point to the start of a or b?)

int **temp=next;
next=present;
present=temp;
test_for_done(present);
}

Compare this to the one-dim version ( which works )
int a[10];
int b[10];
int *present;
int *next;

bool done=false;

while(!done)
{
// Loop which substitutes for complex calculation
for(int i=0;i<10;i++)
next=present;

Same problem here. THis causes undefined behavior.

int *temp=next;
next=present;
present=temp;
test_for_done(present);
}

Perhaps if you explained what you're trying to do?

-Howard

 

[David Lindauer]

hi,

that is going to give compile errors most likely, and it *definitely*
won't work. The reason is the [] operator when used to create or index a
static array does something different than it does when being used with a
pointer variable.

Technically the reason is that no matter how many [] you use to define a
multidimensional array, the array is always a flat rendition with one row
contiguously following the last in memory. But if you declare a pointer
with multiple stars, what happens is that the pointer points to an array
of pointers, and each pointer in the indexed arrays points to another
array of pointers, and so on until you get to the last star where the
pointers each point to some data corresponding to rows given in the last
index.

The way it works out, if you have a single dimensional array the array
name is equivalent to a pointer to the array because you are already at
the last level of the pointers. But as you add dimensions to the array,
the layout grows different than the flat rendition as found in the static
version of the array, so your code won't work. If the compiler doesn't
tell you you can't make the assignment to temp, it is probably broken.

I'm drawing a blank as to how to fix it without resorting to something
like:

int *a[10]

and then allocating the elements for the other array index dynamically

David

 

[JKop]

int a[10][10];
int b[10][10];
int **present;

"present" points to the third ring of Saturn.

int **next;

"next" points to the Pluto's smaller moon.

bool done=false;

while(!done)
{
// Loop which substitutes for complex calculation
for(int i=0;i<10;i++)
for(int j=0;j<10;j++)
next[j]=present[j];

So you're setting the contents of Saturn's third ring to the contents of
Pluto's smaller moon.

int **temp=next;

"temp" now points to Saturn's third ring.

next=present;

"next" now points to Pluto's smaller moon.

present=temp;
etc.

test_for_done(present);
}

Compare this to the one-dim version ( which works )
int a[10];
int b[10];
int *present;
int *next;

bool done=false;

while(!done)
{
// Loop which substitutes for complex calculation
for(int i=0;i<10;i++)
next=present;
int *temp=next;
next=present;
present=temp;
test_for_done(present);
}

Why are you messing with these pointers?

Anyway, just for your information, here's how you define a reference to a
multidimensional array:

int a[10][10];
int b[10][10];

int (&x)[10][10] = a;
int (&y)[10][10] = b;


-JKop

 

[Ron Natalie]

I have a brain cramp and I need some help.

I have a chunk of code below which demonstrates
a problem I have with multidimensional arrays.
I want to keep it simple but something specific is getting in the way.

Other than declaring them, your code doesn't create use arrays
at all.

First thing you must realize. ARRAYS and POINTERS are not
the same thing. Further you can't even convert between
int array[10][10] and int**
they are not related types at all.

Second, let me further point out, that C++ really only has
single dimensioned arrays. A multidimension array array
is really an array of arrays.

int array[5][3]

array above is a 5 element array of 3 element arrays of int.

The implicit conversion of int[5][3] to a pointer is not int**,
it is int (*ptr)[3] (or in English: a pointer to a three element
array of type int). Every time ptr is incremented, it points at
the next 3 element array.


Chew over this and if you have further questions, try to be more
speicifc.