Add components method to Abelian group

docs/src/manual/abelian/elements.md

@@ -19,6 +19,7 @@ parent(x::FinGenAbGroupElem)
 ### Access
 
 ```@docs
+getindex(x::FinGenAbGroupElem, v::AbstractVector{Int})
 getindex(x::FinGenAbGroupElem, i::Int)
 ```
 

src/GrpAb/Elem.jl

@@ -146,6 +146,36 @@
   return x.coeff[1, i]
 end
 
+@doc raw"""
+    getindex(x::FinGenAbGroupElem, v::AbstractVector{Int}) -> Vector{ZZRingElem}
+
+Returns the $i$-th components of the element $x$ where $i \in v$.
+
+!!! note
+    This function is inefficient since the elements are internally stored using ZZMatrix but this function outputs a vector.
+"""
+function getindex(x::FinGenAbGroupElem, v::AbstractVector{Int})
+  return [x.coeff[1, i] for i in v]
+end
+
+@doc raw"""
+    firstindex(x::FinGenAbGroupElem) -> Int64
+
+Returns the first component of the element $x$.
+"""
+function Base.firstindex(x::FinGenAbGroupElem)
+  return Int64(1)
+end
+
+@doc raw"""
+    lastindex(x::FinGenAbGroupElem) -> Int64
+
+Returns the last component of the element $x$.
+"""
+function Base.lastindex(x::FinGenAbGroupElem)
+  return ngens(parent(x))
+end
+
 ################################################################################
 #
 #  Comparison

src/exports.jl

@@ -206,6 +206,7 @@ export codifferent
 export codomain
 export coefficient_ideals
 export coefficients
+export components
 export coinvariant_lattice
 export cokernel
 export collapse_top_layer