Converting vectors to an array

Question

I am having trouble converting type vector< vector > to an array.So far, I've tried to do the following (with help from /u/ Robert Crovella)

pos_x_h=(double *)malloc(N*sizeof(double));
pos_y_h=(double *)malloc(N*sizeof(double));
for (int i = 0; i<N; i++){
  vector<double> temp = r[i];
  pos_x_h[i] = temp[i][0];
  pos_y_h[i] = temp[i][1];
}

Here, r is the position vector with N elements each having x and y components. I also tried doing

double arr[N];
std::copy(r.begin(), r.end(), arr);

Both attempts didn't work, and I'm not sure why. You can see the code here.

You don't need to index `temp` with two indices. Just use `temp[0]` and `temp[1]`. And if you `#include ` you don't need to (and shouldn't) cast the return value of `malloc`. That's "old" C (I know… I learnt that from the 1982 K&R book before I got wise.) — Floris, Apr 13 '14 at 04:46

Floris · Accepted Answer · 2014-04-13T05:08:43.440

2

The following ought to work. Note that I prefer sizeof *pos_x_h over sizeof(double) since the former makes sure the size is correct even if you change the type of the variable (which might be in another piece of code). updated* for C++ you need to cast the result of malloc. I was thinking with my C hat on...

second update

A bit more thought tells me you really don't want to have temp as a vector - that is just making things more confusing. Instead, point to the address of the first element of r[i]: this compiles without errors

    pos_x_h=(double *)malloc(N*sizeof(double));
    pos_y_h=(double *)malloc(N*sizeof(double));
    for (int i = 0; i<N; i++){
      double* temp;
      temp = &r[i][0];
      pos_x_h[i] = temp[0];
      pos_y_h[i] = temp[1];
    }

Of course you could simply do

    pos_x_h=(double *)malloc(N*sizeof(double));
    pos_y_h=(double *)malloc(N*sizeof(double));
    for (int i = 0; i<N; i++){
      pos_x_h[i] = r[i][0];
      pos_y_h[i] = r[i][1];
    }

and avoid the whole mess.

edited Apr 13 '14 at 05:08

answered Apr 13 '14 at 04:51

Floris

45,857
6
70
122

Can you please add a comment that it is better to use new in a c++ code, as tagged by the OP? – 0x90 Apr 13 '14 at 04:52
Gaahhh...this still doesn't work. This also gives me the same errors 1)incompatible types in assignment, 2)cannot convert ‘double’ to ‘double*’ in assignment. What is going on here? *cries* – jimmu Apr 13 '14 at 04:56
@0x90 I am not exactly sure where that would help in this case, as each iteration in the loop uses a new instance of `temp` (it is pointing to a new value). Could you clarify? – Floris Apr 13 '14 at 04:56
@MisterSpock Can you show the actual definition of `r` please - maybe I misunderstand what type it is. But I am guessing that you need `temp` to be of the type `double*`. I made the edit in my answer - see if that helps. – Floris Apr 13 '14 at 04:57
[here](http://pastebin.com/M2Sugxst) is the code. each item in r vector has (x,y) values which are both double. – jimmu Apr 13 '14 at 04:59
@MisterSpock - I made a further update. As 0x90 pointed out, in C++ it is better to use `new` and `delete[]` rather than `malloc` which is really a `C` hangover. I get it wrong myself all the time - being much more of a C person than a `C++` person, and frequently confounded by the differences. That comes from having two different languages that are superficially similar. Makes you think you know what you are doing. :-/ – Floris Apr 13 '14 at 05:10
Thanks.It compiled with the one without temp variable. It gave me segfault error, but at least it compiled. Thanks again! – jimmu Apr 13 '14 at 05:19
It doesn't seem like you call the `initialize` function before you start copying things - so the vector `r` doesn't point anywhere meaningful. – Floris Apr 13 '14 at 05:46

Converting vectors to an array

1 Answers1