# Extracting entries at multiple indices from std::vectors in c++

## Alec Jacobson

## July 20, 2010

I've been doing some C++ coding, working with matrices. In a previous project it was convenient to think of our systems in block form. This translates easily into matlab's indexing. So say you have in matlab:
```
a = [10,11,12,13,14,15];
```

Then you can extract elements 1,3,5 (remember matlab is one-indexed) by calling
```
b = a([1,3,5]);
```

which reveals
```
b =
10 12 14
```

So in C++, I've made my first attempt at useful templating to extend the std::vector class to have this feature. It is no where near implemented the full features of matlab's operator(), but it seems to do this particular feature correctly.
```
#include <vector>
#include <stdio.h>
template<typename T>
class SmartVector : public std::vector<T>{
public:
// act like operator[]
T operator()(size_t _Pos){
return (*this)[_Pos];
}
// act like matlab operator()
SmartVector<T> operator()(std::vector<size_t>& positions){
SmartVector<T> sub;
sub.resize(positions.size());
size_t sub_i = 0;
for(
std::vector<size_t>::iterator pit = positions.begin();
pit != positions.end();
pit++,sub_i++){
sub[sub_i] = (*this)[*pit];
}
return sub;
}
};
int main(){
// make some vector
SmartVector<int> a;
a.push_back(10);
a.push_back(11);
a.push_back(12);
a.push_back(13);
a.push_back(14);
a.push_back(15);
for(int i =0;i<a.size();i++)
printf("a[%d] = %d\n",i,a[i]);
// make some list of indices
std::vector<size_t> sub_indices;
sub_indices.push_back(0);
sub_indices.push_back(2);
sub_indices.push_back(4);
// use operator() to extract entries at those indices
SmartVector<int> b = a(sub_indices);
printf("\nb = a([ ");
for(int i =0;i<sub_indices.size();i++)
printf("%d ",(int)sub_indices[i]);
printf("]);\n\n");
for(int i =0;i<b.size();i++)
printf("b[%d] = %d\n",i,b[i]);
return 0;
}
```

Then if you run ./main you should see:
```
a[0] = 10
a[1] = 11
a[2] = 12
a[3] = 13
a[4] = 14
a[5] = 15
b = a([ 0 2 4 ]);
b[0] = 10
b[1] = 12
b[2] = 14
```