This Geometry nodes Selection collection simplifies some of the most used index selection methods. No need to think about math or compare nodes anymore to find the right index. There are seven nodes available at this moment.

Index selection: Find an specific index or inverted index.
Wrap selection: Determine the amount of indices and the amount of indices to skip sequentially.
Position select: Find index at a position in space or all indices above or below it.
Edge select: Find all edges with a certain angle.
XYZ selection: Find the indices at a particular normal direction.
Inside volume selection: Find the indices inside or outside a mesh volume.
Random island selection: Select random mesh islands after "Realize instances" node.