Apply suggestions from code review

Co-authored-by: Lars Göttgens <lars.goettgens@rwth-aachen.de>

Author: thofma

src/matrix.jl

@@ -24,17 +24,18 @@ end
 # we overload some specific types to not hit the AA fallback
 for T in [ZZMatrix, QQMatrix]
   @eval begin
-    function sub(M::$T, r::AbstractVector{<:Integer}, c::AbstractVector{<:Integer})
+    # have to "duplicate" this, otherwise we get ambiguity errors
+    function sub(x::$T, r::AbstractUnitRange{Int}, c::AbstractUnitRange{Int})
+      return deepcopy(view(x, r, c))
+    end
+
+    function sub(M::$T, r::AbstractVector{Int}, c::AbstractVector{Int})
       N = zero_matrix(base_ring(M), length(r), length(c))
-      ii = 1
       GC.@preserve M N begin
-        for i in r
-          jj = 1
-          for j in c
+        for (ii, i) in enumerate(r)
+          for (jj, j) in enumerate(c)
             set!(mat_entry_ptr(N, ii, jj), mat_entry_ptr(M, i, j))
-            jj += 1
           end
-          ii += 1
         end
       end
       return N

test/flint/fmpz_mat-test.jl

@@ -259,6 +259,12 @@ end
   C[1, 1] = 20
   @test B == matrix_space(ZZ, 2, 2)([10 2; 4 5])
   @test A == S([1 2 3; 4 5 6; 7 8 9])
+
+  m = ZZ[1 2 4; 5 6 7]
+  n = m[[1], [1, 3]]
+  @test n == ZZ[1 4]
+  n = m[1:2, 1:2]
+  @test n == ZZ[1 2; 5 6]
 end
 
 @testset "ZZMatrix.unary_ops" begin