pointer to 2d array

[Mark]

put most simply, what is the c++ equivalent to this java code?

int map[][];
map = new int[16][16];
map[5][5] = 3;
System.out.prin tln(map[5][5]);

thanks in advance.

[Jonathan Mcdougall]

Re: pointer to 2d array

Mark wrote:[color=blue]
> put most simply, what is the c++ equivalent to this java code?
>
> int map[][];
> map = new int[16][16];
> map[5][5] = 3;
> System.out.prin tln(map[5][5]);[/color]

# include <iostream>

int main()
{
int map[16][16] = {0};
map[5][5] = 3;
std::cout << map[5][5] << "\n";
}


Jonathan

[Axter]

Re: pointer to 2d array


Mark wrote:[color=blue]
> put most simply, what is the c++ equivalent to this java code?
>
> int map[][];
> map = new int[16][16];
> map[5][5] = 3;
> System.out.prin tln(map[5][5]);
>
> thanks in advance.[/color]

You can use vector to simulate this dynamically.
#include <iostream>
#include <vector>
using namspace std;
.....

vector<vector<i nt> > MyMap(16, vector<int>(16) );
MyMap[5][5] = 3;
cout << MyMap[5][5] << endl;

[James Juno]

Re: pointer to 2d array

Mark wrote:[color=blue]
> put most simply, what is the c++ equivalent to this java code?
>
> int map[][];
> map = new int[16][16];
> map[5][5] = 3;
> System.out.prin tln(map[5][5]);
>
> thanks in advance.
>[/color]

Personally, I would

vector< vector<int> > map;
map.resize( 16 );
for ( int i = 0; i < 16; ++i )
map[i].resize( 16 );
map[5][5] = 3;
cout << map[5][5] << endl;

But that might not be "most simply" since I'm using STL.

-JJ

[Mark]

Re: pointer to 2d array

Jonathan Mcdougall, the idea was so that i could reassign map to
another matrix of a different size. perhaps i should have specified
that

thanks Axter. i'll try that.

[James Juno]

Re: pointer to 2d array

Axter wrote:[color=blue]
> Mark wrote:[color=green]
>> put most simply, what is the c++ equivalent to this java code?
>>
>> int map[][];
>> map = new int[16][16];
>> map[5][5] = 3;
>> System.out.prin tln(map[5][5]);
>>
>> thanks in advance.[/color]
>
> You can use vector to simulate this dynamically.
> #include <iostream>
> #include <vector>
> using namspace std;
> ....
>
> vector<vector<i nt> > MyMap(16, vector<int>(16) );[/color]

Nice way to initialize! Amazing how you can program for years and not
notice something obvious like this. Thanks!

-JJ

[Axter]

Re: pointer to 2d array


James Juno wrote:[color=blue]
> Mark wrote:[color=green]
> > put most simply, what is the c++ equivalent to this java code?
> >
> > int map[][];
> > map = new int[16][16];
> > map[5][5] = 3;
> > System.out.prin tln(map[5][5]);
> >
> > thanks in advance.
> >[/color]
>
> Personally, I would
>
> vector< vector<int> > map;
> map.resize( 16 );
> for ( int i = 0; i < 16; ++i )
> map[i].resize( 16 );
> map[5][5] = 3;
> cout << map[5][5] << endl;
>
> But that might not be "most simply" since I'm using STL.
>[/color]

You can resize after the vector has been initialize by using following
method:
map.resize(16, vector<int>(16) );

Also, to the original questioner, I would advise avoiding naming
variables that have the same name as objects in the STL library. map
is one of the containers in the C++ library.
Using the same name can be confusing to any one maintaining the code.
Also check out methods in the following links:

http://code.axter.com/dynamic_2d_array.h

http://www.codeguru.com/forum/showthread.php?t=231046
http://www.codeguru.com/forum/showth...hreadid=297838

C-Style compatible method:

http://code.axter.com/allocate2darray.h

http://code.axter.com/allocate2darray.c

[Mark]

Re: pointer to 2d array

i actually prefer your way James, at least for this scenario, since I
don't know what size I'll be initializing it to anyway.

Now quick question,

void map_resize(int x, int y)
{
map.resize(y);
for(int i=0; i<y; i++)
map[i].resize(x);
}

have I got my x and y correct, or should they be switched? It's hard
to visualize.

I tried running

map_resize(10,2 0);
map[15][5] = 5;
printf("%i", map[15][5]);

which works, but 15 > 10 should be out of bounds, if I wrote my
function correctly. however it is a container class and may be
resizing my map...

[Jonathan Mcdougall]

Re: pointer to 2d array

Mark wrote:[color=blue]
> Jonathan Mcdougall, the idea was so that i could reassign map to
> another matrix of a different size. perhaps i should have specified
> that[/color]

Please 1) answer to the message you are answering to.. and 2) quote it.

You can assign arrays by hand:

void f(int source[10], int target[10])
{
for (int i=0; i<10; ++i)
target[i] = source[i];
}

But arrays are not value-types, meaning they cannot be assigned
directly:

target = source; // invalid;

You should use standard containers in this case, such as std::vector.
You've already been proposed good solutions.

You could also define a class yourself with the corresponding
operators. Search for matrix classes on google for some free
implementations .

Jonathan

[James Juno]

Re: pointer to 2d array

Mark wrote:[color=blue]
> i actually prefer your way James, at least for this scenario, since I
> don't know what size I'll be initializing it to anyway.
>
> Now quick question,
>
> void map_resize(int x, int y)
> {
> map.resize(y);
> for(int i=0; i<y; i++)
> map[i].resize(x);
> }
>
> have I got my x and y correct, or should they be switched? It's hard
> to visualize.
>
> I tried running
>
> map_resize(10,2 0);
> map[15][5] = 5;
> printf("%i", map[15][5]);
>
> which works, but 15 > 10 should be out of bounds, if I wrote my
> function correctly. however it is a container class and may be
> resizing my map...
>[/color]

I like to visualize it this way: In your function, the first resize
creates y columns of empty vectors. The loop then goes through the
columns and sets the length of the vectors to x rows. So, map[15][5] is
the 15th column (zero-based), 5th row, which is fine by the way you've
written your function. If you try to address a non-existent element
(for instance, map[5][15]), you'll have a problem. It won't resize
itself automatically.

-JJ

[BobR]

Re: pointer to 2d array


James Juno wrote in message ...[color=blue]
>Mark wrote:[color=green]
>> put most simply, what is the c++ equivalent to this java code?
>>
>> int map[][];
>> map = new int[16][16];
>> map[5][5] = 3;
>> System.out.prin tln(map[5][5]);
>>
>> thanks in advance.
>>[/color]
>
>Personally, I would
>
> vector< vector<int> > map;
> map.resize( 16 );
> for ( int i = 0; i < 16; ++i )
> map[i].resize( 16 );
> map[5][5] = 3;
> cout << map[5][5] << endl;
>
>But that might not be "most simply" since I'm using STL.
>-JJ[/color]

Try this :

// int main(){
std::vector<std ::vector<int> > Vvmap(16, 16);
std::cout << Vvmap.size()<<s td::endl;
std::cout << Vvmap.at(0).siz e()<<std::endl;
Vvmap.at(5).at( 5) = 3;
Vvmap.at(5).pus h_back(5);
std::cout <<Vvmap.at(5).a t(5)<<std::endl ;
for( size_t i(0); i < Vvmap.size(); ++i ){
std::cout<<Vvma p.at( i ).size()<<std:: endl;;
}
// return 0;
// } // main() end
--
Bob R
POVrookie

[BobR]

Re: pointer to 2d array


James Juno wrote in message ...[color=blue]
>[snip]
>I like to visualize it this way: In your function, the first resize
>creates y columns of empty vectors. The loop then goes through the
>columns and sets the length of the vectors to x rows.
>** So, map[15][5] is the 15th column (zero-based), 5th row, **[/color]

No. It's the 16th col, 6th row. <G>
[color=blue]
>which is fine by the way you've
>written your function. If you try to address a non-existent element
>(for instance, map[5][15]), you'll have a problem. It won't resize
>itself automatically.[/color]

So, use the at() to access. That way you'll get an out_of_range exception
(which you can 'catch'):
map.at(5).at(15 ) = 7;

/ -----------------
size_t rows(7);
size_t cols(7);
std::vector<std ::vector<int> > Vvmap( rows, cols);
try{
Vvmap.at(6).at( 26) = 32;
}
catch(std::out_ of_range &Oor){
std::cout<<"cau ght "<<Oor.what()<< std::endl;
}
catch(...){
std::cout<<"cau ght something"<<std ::endl;
}
// output: caught vector [] access out of range
/ -----------------

Only use the [] operator if you are *positive* the index is in range(...and
actually need the *tiny* speed increase). Like in this for loop:
for(size_t i(0); i < map.size(); ++i){
map[ i ].push_back( i );
}

[corrections always welcome ]
--
Bob R
POVrookie