A new exercise illustrating C-functions

Author: jussienko

README.md

@@ -42,6 +42,7 @@ courses are stored in tags.
 ### Optimising with Cython
 
  - [Creating simple extension](cython/simple-extension)
+ - [Using C-functions](cython/c-functions)
  - [Optimising heat equation](cython/heat-equation)
 
 ### Interfacing with libraries

cython/c-functions/README.md

@@ -0,0 +1,36 @@
+## Using C-functions
+
+Fibonacci numbers are a sequence of integers defined by the recurrence 
+relation 
+   
+   F<sub>n</sub> = F<sub>n-1</sub> + F<sub>n-2</sub>
+ 
+with the initial values F<sub>0</sub>=0, F<sub>1</sub>=1.
+
+The module [fib.py](fib.py) contains a function `fibonacci(n)` that
+calculates recursively F<sub>n</sub>. The function can be used e.g. as
+
+```python
+from fib import fibonacci
+
+fibonacci(30)
+```
+
+Make a Cython version of the module, and investigate how adding type
+information and making `fibonacci` a C-function affects performance
+(hint: function needs to be called both from Python and C). Use
+`timeit` for performance measurements, either from command line
+
+```bash
+$ python3 -m timeit -s "from fib import fibonacci" "fibonacci(30)"
+```
+
+or within IPython
+
+```python
+In []: %timeit fibonacci(30)
+```
+
+**Note:** this recursive algorithm is very inefficient way of calculating
+Fibonacci numbers and pure Python implemention of better algorithm
+outperforms Cython implementation drastically.

cython/c-functions/fib.py

@@ -0,0 +1,4 @@
+def fibonacci(n):
+    if n < 2:
+        return n
+    return fibonacci(n-2) + fibonacci(n-1)

cython/c-functions/solution/fib.pyx

@@ -0,0 +1,14 @@
+cpdef int fibonacci(int n):
+    if n < 2:
+        return n
+    return fibonacci(n-2) + fibonacci(n-1)
+
+def fibonacci_py(int n):
+    if n < 2:
+        return n
+    return fibonacci_py(n-2) + fibonacci_py(n-1)
+
+def fibonacci_py2(n):
+    if n < 2:
+        return n
+    return fibonacci_py2(n-2) + fibonacci_py2(n-1)

cython/c-functions/solution/setup.py

@@ -0,0 +1,11 @@
+from distutils.core import setup, Extension
+from Cython.Build import cythonize
+
+ext = Extension("fib",
+                sources=["fib.pyx"],
+               )
+
+setup(
+     ext_modules=cythonize(ext)
+)
+