Basics: Index and NDIndex

Introduction

Index

The Index class represents a strided range of indices, and it is used to define the index extent of Field objects on construction.

Constructors:

• Index(): Creates a null interval with no elements.

• Index(n): Instantiates an Index object representing the range of integers from 0 to n-1 inclusive, with implied stride 1.

• Index(a,b): Instantiates an Index object representing the range of integers [a , b], with implied stride 1.

• Index(a,b,s): Instantiates an Index object representing the range of integers [a , b], with stride s.

Examples:

Index I(10);     // Integer set [0..9]
Index Low(5);    // Integer set [0..4]
Index High(5,9); // Integer set [5..9]
Index IOdd(1,9,2);   // Integer set [1..9] with stride 2
Index IEven(0,9,2);  // Integer set [0..9] with stride 2

Operations:

Given Index I(a,n,s) and an integer j, the following operations are possible:

   I+j  : a+j+i*s     // For i in [0..n-1]
   j-I  : j-a-i*s
   j*I  : j*a + i*j*s
   I/j  : a/j + i*s/j // Note: j/I is not defined due to non-uniform stride and fraction
prohibition.

NDIndex

NDIndex is a class that acts as a container for multiple Index instances. It simplifies operations such as intersections across N dimensions by forwarding requests to its contained Index objects.

Usage

The following example demonstrates the use of Index and NDIndex classes.

std::array<int, dim> pt = {64, 64, 64};
ippl::Index Iinput(pt[0]);
ippl::Index Jinput(pt[1]);
ippl::Index Kinput(pt[2]);
ippl::NDIndex<dim> ownedInput(Iinput, Jinput, Kinput);
 
std::array<bool, dim> isParallel;  // Specifies SERIAL, PARALLEL dims
isParallel.fill(true);
 
ippl::FieldLayout<dim> layoutInput(MPI_COMM_WORLD, ownedInput, isParallel);

In this example, we define an NDIndex object ownedInput with three Index objects Iinput, Jinput, and Kinput. We then create a FieldLayout object layoutInput using the NDIndex object and a boolean array isParallel that specifies the parallel dimensions.