Vectors vs. dynamic arrays

Jimmy O'Regan [joregan at gmail.com]

Thu, 7 Aug 2008 17:35:21 +0100

My C++ sucks, so I need a second opinion

We have a user who's using some oddball compiler that doesn't support C99 type arrays, so I'm wondering if it's ok to replace this code:

  double classes_ocurrences[M]; //M = Number of ambiguity classes
  double classes_pair_ocurrences[M][M];
  double tags_estimate[N]; //N = Number of tags (states)
  double tags_pair_estimate[N][N];

with this:

  vector <double> classes_ocurrences(M); //M = Number of ambiguity classes
  vector <vector <double> > classes_pair_ocurrences(M, vector<double>(M));
  vector <double> tags_estimate(N); //N = Number of tags (states)
  vector <vector <double> > tags_pair_estimate(N, vector<double>(N));

As far as I know, it's functionally equivalent, and this test:

#include <vector>
#include <iostream>
 
using namespace std;
 
int main ()
{
        int N = 2;
        int a[N][N];
        vector < vector <int> > b (N, vector<int>(N));
 
 
        a[0][0] = 1;
        a[0][1] = 2;
        a[1][0] = 3;
        a[1][1] = 4;
 
        b[0][0] = 1;
        b[0][1] = 2;
        b[1][0] = 3;
        b[1][1] = 4;
 
        cout << "a: " << a[0][0] << " b: " << b[0][0] << endl;
        cout << "a: " << a[0][1] << " b: " << b[0][1] << endl;
        cout << "a: " << a[1][0] << " b: " << b[1][0] << endl;
        cout << "a: " << a[1][1] << " b: " << b[1][1] << endl;
 
        return 0;
}

gives:

a: 1 b: 1
a: 2 b: 2
a: 3 b: 3
a: 4 b: 4

as expected. I'm just wondering if there's some subtle nuance I'm missing.

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René Pfeiffer [lynx at luchs.at]

Fri, 8 Aug 2008 23:35:11 +0200

On Aug 07, 2008 at 1735 +0100, Jimmy O'Regan appeared and said:

> My C++ sucks, so I need a second opinion 

I fully understand. http://www.chunder.com/text/stroustrup.html

> We have a user who's using some oddball compiler that doesn't support
> C99 type arrays, so I'm wondering if it's ok to replace this code:
> [...]
> I'm just wondering if there's some subtle nuance I'm missing.

I got used to c++ vectors a while ago and so far I had no nasty surprises. I think you can switch to vectors without running into trouble.

Best, René, who is waiting for C++0x.

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Jimmy O'Regan [joregan at gmail.com]

Fri, 8 Aug 2008 22:50:04 +0100

2008/8/8 René Pfeiffer <[email protected]>:

> On Aug 07, 2008 at 1735 +0100, Jimmy O'Regan appeared and said:
>> My C++ sucks, so I need a second opinion 
>
> I fully understand. http://www.chunder.com/text/stroustrup.html
> 
>
>> We have a user who's using some oddball compiler that doesn't support
>> C99 type arrays, so I'm wondering if it's ok to replace this code:
>> [...]
>> I'm just wondering if there's some subtle nuance I'm missing.
>
> I got used to c++ vectors a while ago and so far I had no nasty
> surprises. I think you can switch to vectors without running into
> trouble.

Great

I think I should change the assignments to use push_back - from what I've read, it's more efficient.

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