4.1

.gitignore

@@ -0,0 +1 @@
+*.o

4.1/sum.cpp

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+#include <iostream>
+
+int sum(int n)
+{
+    return (n != 0)
+         ?  n + sum(n - 1)
+         :  n;
+}
+
+int main()
+{
+    int n = 5;
+    std::cout << "For n = " << n << " sum is " << sum(n) << '\n';
+}

4.1/sum_tail.cpp

@@ -0,0 +1,19 @@
+#include <iostream>
+
+int sumStep(int n, int s)
+{
+    return (n != 0)
+         ?  sumStep(n - 1, n + s)
+         :  s;
+}
+
+int sum(int n)
+{
+    return sumStep(n, 0);
+}
+
+int main()
+{
+    int n = 5;
+    std::cout << "For n = " << n << " sum is " << sum(n) << '\n';
+}