Any simple way to pull multiple elements from an array?

[neuronet]

Say we have an array X with 100 elements.

To access elements 1, 3, and 5, in Matlab, I'd just enter:
y=X([1 3 5]);

Is there a built-in function in C++ to do that without using a for loop?

Obviously I tried

X[1 2] and such but the compiler gets angry.

[Anthony Hernandez]

Just

y = X[1];
y = X[3];
y = X[5];

When you're accessing elements in an array it needs an index to look at. So to access multiple values you need to have separate calls for each index.

2 5 8 12 11 1 3 <- Values
0 1 2 3 4 5 6 <- Index

Reference:
http://www.cplusplus.com/doc/tutorial/arrays/

[Stewbond]

I don't really understand the equation above. I'm guessing y is an array (maybe a vector?) of 3 and you are setting it to those three elements of the array X? If that's the case then you'd need to do something like:

y[0] = x[1]; 
y[1] = x[3]; 
y[2] = x[5];

Remember MATLAB m files are much higher level than C++. That means that many more things are done for you. Here, we are much closer to the processor and need to tell it every specific instruction.

You COULD make a class that would do something like this. But you'd have to define overloading functions and the syntax wouldn't be the same. It would be something like:

#include <cstdarg>
class Array
{
  double X[100];
public:
  double* operator[] (int size, ...)
  {
    va_list vl;
    va_start(vl,size);
    double* y = new double[size];

    for (int i = 0; i < size; ++i)
      y[i]=X[ va_arg(vl,int) ];

    va_end(vl);
  }
};

This will create an array with those elements in it,. used like so:

double* y;
Array X;
// X gets populated somehow
y = X[3, 1, 3, 5];

The first argument represents the size of y. The rest of the arguments represent the indices of X that are being sent to y.

Note that if you don't use the delete keyword here, you'll have memory leaks.

[neuronet]

Stebond, thanks that is very helpful.

You wrote:
"I don't really understand the equation above. I'm guessing y is an array (maybe a vector?) of 3 and you are setting it to those three elements of the array X?"

Yes, exactly.

Thanks for the help on what I'll need to do. I guess I need to get used to taking care of these kind of details, at least in making a class that does it which is a nice suggestion. Once I learn how to do classes :)