# Overview: `NBest`

Operator¶

In addition to the `first`

and `last`

functions, which return first and last elements of a set, you can use the `NBest`

operator to obtain the first \(n\) elements of a set. This is based on criteria you provide to the `NBest`

operator as an additional argument.

Let’s take for example a transportation problem, where you have a variable `Transport(i,j)`

denoting how much is transported from factory `i`

to customer `j`

. You may want to know the three customers `j`

to which the most amount is transported per factory `i`

. Let’s use the `NBest`

operator to get these elements.

The first argument for the `NBest`

operator is the binding index; the result of the operator is a subset of elements from this binding index.

The second argument is the sort criteria you want to use. Note that a higher value returned by this sort-criteria indicates a better value, i.e. `NBest`

will sort the elements from largest to smallest.

The third argument is the number `n`

, indicating how many elements you are interested in (in this example, n = 3).

Note that you need to add a subset identifier to store the results (`LargestTransportCities`

in this case) and to set the *Order by* attribute to *user*.

In the code below, `LargestTransportCities`

is an indexed subset that is indexed over all factories `i`

and is a subset of the set `Customers`

:

```
LargestTransportCities(i) := NBest( j, Transport(i,j), 3 );
```

For each factory `i`

, the above indexed subset `LargestTransportCities(i)`

will contain the three customers to which the most amount is transported.

For more information about the *Order by* attribute and sorted sets, see the chapter Set Expressions in the AIMMS Language Reference.

Last Updated: November, 2019