update readme with example

.gitignore

@@ -1,3 +1,4 @@
 .DS_Store
 build/
 *.swp
+.ipynb_checkpoints

README.md

@@ -80,7 +80,10 @@ To view the solution, modify the `poisson.cc` file to include
     outfile.close();
 ```
 
-Then `make`.  Here there will be `np` different solution files `solution-*.txt`.  This can be viewed in Python:
+Then `make`.  Here there will be `np` different solution files `solution-*.txt`.  This can be viewed in Python for example in [example_output.ipynb](tools/example_output.ipynb), which gives the following for the processor layout (`mpirun -np 6`) and the solution:
+
+![](tools/example_output.png)
+
 
 # Citing
 

tools/example_output.ipynb

@@ -0,0 +1,103 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true,
+    "deletable": true,
+    "editable": true
+   },
+   "outputs": [],
+   "source": [
+    "import numpy as np\n",
+    "import matplotlib.pyplot as plt\n",
+    "%matplotlib inline"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false,
+    "deletable": true,
+    "editable": true
+   },
+   "outputs": [],
+   "source": [
+    "nproc = 6\n",
+    "data = []\n",
+    "for p in range(nproc):\n",
+    "    d = np.loadtxt('../build/solution-%d.txt'%p, delimiter=',',\n",
+    "                   usecols=[1,2,3], dtype='int,int,float', unpack=True)\n",
+    "    d[0] -= 1  # zero-based\n",
+    "    d[1] -= 1  # zero-based\n",
+    "    data.append(d)\n",
+    "xmin = min([d[0].min() for d in data])\n",
+    "ymin = min([d[1].min() for d in data])\n",
+    "xmax = max([d[0].max() for d in data])\n",
+    "ymax = max([d[1].max() for d in data])\n",
+    "nx = xmax - xmin + 1 + 2 # for boundaries\n",
+    "ny = ymax - ymin + 1 + 2 # for boundaries\n",
+    "print(nx, ny)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false,
+    "deletable": true,
+    "editable": true
+   },
+   "outputs": [],
+   "source": [
+    "X, Y = np.mgrid[0:1:nx*1j,0:1:ny*1j]\n",
+    "Z = np.zeros(X.shape)\n",
+    "P = np.zeros(X.shape)\n",
+    "for p in range(nproc):\n",
+    "    I = data[p][0]\n",
+    "    J = data[p][1]\n",
+    "    Z[I,J] = data[p][2]\n",
+    "    P[I,J] = p"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false,
+    "deletable": true,
+    "editable": true
+   },
+   "outputs": [],
+   "source": [
+    "f, (ax1, ax2) = plt.subplots(1, 2, figsize=(10, 5))\n",
+    "ax1.pcolormesh(X, Y, P)\n",
+    "ax2.pcolormesh(X, Y, Z)\n",
+    "plt.savefig('example_output.png')"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.6.1"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 2
+}