Add exercise: difference of squares (JuliaLang#11)

config.json

@@ -5,6 +5,13 @@
   "active": false,
   "test_pattern": "TODO",
   "exercises": [
+    {
+      "slug": "difference-of-squares",
+      "difficulty": 1,
+      "topics": [
+        "generators"
+      ]
+    },
     {
       "slug": "word-count",
       "difficulty": 1,

exercises/difference-of-squares/difference-of-squares.jl

@@ -0,0 +1,14 @@
+"Square the sum of the numbers up to the given number"
+function sum_of_squares(n::Int)
+
+end
+
+"Sum the squares of the numbers up to the given number"
+function square_of_sum(n::Int)
+
+end
+
+"Subtract sum of squares from square of sums"
+function difference(n::Int)
+
+end

exercises/difference-of-squares/example.jl

@@ -0,0 +1,9 @@
+# This uses reduce instead of sum because sum can't handle empty collections properly.
+# There is no universal zero-element, so it has to be specified manually and cannot
+# be determined automatically. Otherwise, n=0 will cause an error.
+sum_of_squares(n::Int) = reduce(+, 0, i^2 for i in 1:n)
+
+# However, sum{T<:Real}(r::Range{T}) can handle "empty" ranges.
+square_of_sum(n::Int) = sum(1:n)^2
+
+difference(n::Int) = square_of_sum(n) - sum_of_squares(n)

exercises/difference-of-squares/runtests.jl

@@ -0,0 +1,22 @@
+using Base.Test
+
+include("difference-of-squares.jl")
+
+@testset "Square the sum of the numbers up to the given number" begin
+    @test square_of_sum(5) == 225
+    @test square_of_sum(10) == 3025
+    @test square_of_sum(100) == 25502500
+end
+
+@testset "Sum the squares of the numbers up to the given number" begin
+    @test sum_of_squares(5) == 55
+    @test sum_of_squares(10) == 385
+    @test sum_of_squares(100) == 338350
+end
+
+@testset "Subtract sum of squares from square of sums" begin
+    @test difference(0) == 0
+    @test difference(5) == 170
+    @test difference(10) == 2640
+    @test difference(100) == 25164150
+end