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dynamic arrays - pointer problem

i'm trying to setup a dynamic array ... but its fighting with me all the way ....

in the unit header.......
int dbfieldsSize;
int **MappedFields;

in the constructor .....
dbfieldsSize = sizeof(dbfields) / sizeof(dbfields[0];
MappedFields = new int *[dbfieldsize];

I get this when building.... E2034 Cannot convert 'const int' to 'int *'

Here is the block that gives me the problem .....

for (int i = 0; i < ENDROW; i++){
if (MappedFields[i] == sgData->Col){
Application->MessageBox("The data column has already been mapped to the
database.","OOPS!",MB_OK|MB_ICONWARNING);
return;
}
}

More specifically, it happens at if (MappedFields[i] == sgData->Col)

(sgData is a StringGrid by the way)

I'm assuming this error message is because the "i" from the for loop isn't a pointer, but I'm just not sure how to account for this ....

Thanks!

ASKER CERTIFIED SOLUTION

jkr

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jkr

To elaborate - using

int **MappedFields;

MappedFields = new int *[dbfieldsize];

you are creating an array of arrays (actually an array of 'int*'), which is probably not what you want to do. A sregular array is created using

int *MappedFields;

MappedFields = new int [dbfieldsize];

Now, if you apply 'operator[]' to 'int **MappedFields;' as you did, you get an 'int*, thus the error the compiler is complaining about.

jkr

BTW, see also

http://www.cplusplus.com/doc/tutorial/dynamic.html ("Dynamic Memory")

and

http://www.cplusplus.com/doc/tutorial/arrays.html ("Arrays")

jkr

... and some more details on that very issue here: http://www.fredosaurus.com/notes-cpp/newdelete/50dynamalloc.html ("C++ Notes: Dynamic Allocation of Arrays")

trs28

ASKER

I did what you suggested in the first post and it seems to be working...i think
(which I thought I initially tried that but I guess not)

Tell me if this makes sense, when I'm debugging (i'm using BCB5 by the way), if I inspect the array, it only shows me one element (i.e. it only shows [0]) .

This has me a little confused .... I've tried two different values for dbfieldsSize: 3 & 15 ...but both only show one element when inspected !?!?!

why is this? being that the application isn't 100% complete, its a little hard if its seeing everything properly w/o inspecting the arrays

trs28

ASKER

i think i understand ... .its because MappedFields is a pointer, correct?

trs28

ASKER

forget that last post ... i was thinking of it as a pointer to an element, not a pointer to an array (which is the direction i'm trying to take)

jkr

Yes, and you need to fill the array first in order to see more. I am not too familiar with BCB, but you should be able to instepct the contents when you use the memory address.

trs28

ASKER

well, i saw this after I had already populated it .... so you're telling me that because MappedFields is a pointer to an array, it'll only show me the element that the pointer is currently on??

jkr

No, pointers and arrays are interchangeable in C/C++ - the name of an array is aplso a pointer to the array's 1st element. You can check that quite easily using

int* a = NULL; // Pointer to int, initialize to nothing.
int n; // Size needed for array
cin >> n; // Read in the size
a = new int[n]; // Allocate n ints and save ptr in a.
for (int i=0; i<n; i++) {
a[i] = 0; // Initialize all elements to zero.
}
for (int j=0; j<n; j++) {
cout << a[j] << endl; // print contents
}
. . . // Use a as a normal array
delete [] a; // When done, free memory pointed to by a.
a = NULL; // Clear a to prevent using invalid memory reference.

What BCB5 as a debugger will display is a different story, though.