Nowadays supermarket customers can check out themselves and scan products themselves. Many obviously pay by card but I was interested to see how a cash dispensing till would work.

I have written some Python code to simulate the whole setup. We start with a till that has a distibution of coins (1000 1p etc). The key function is change(), which given a price and total paid (difference rem) has to issue the right coin change. It finds the largest coin below the rem and then works from largest to smallest to issue correct change. This way we try to avoid running out of small coins. This large-to-small algorithm is very simple and probably close how humans would do it. One improvement could be to under-sample coins that we have few off in the till.

The trickiest bit (draw()) is simulating what a customer would give in change. I had to add some randomness to it to stock up small coins (10% of time customer gives exact change in 1, 2 or 5p coins). Without this part the till will run out of change very quickly.

I work a lot with global variables in this example as I don’t have to handle function IO so much.


In this example I want to show the principles of dynamic programming and recursion in a simple example. Imagine you have coins of different values and you want to count the many ways that those coins can make up 200 pennies/cents. To solve this problem we use a recursive function which grows like a tree. You start with the biggest coins first.

Define a function add() which first loops through all coins. If we have an empty current set or the current coin is less or equal the last coin in the current set, then continue: if the sum of the set plus the proposed coin is less than 200, then add the current coin to the set and pass the new set to add(); if the sum of the set plus the current coin equals 200, add the coin to the set and add the new set to the set of solutions (do nothing if 200 is exceeded). Finally print the length of all solutions which is 73682.

This a recursive setup where add() references itself – you pass a current unfinished set to this function.

Here is the code in Python (v3) – it takes 7 seconds on

I was interested to see how to match 2 different sets with slightly different distributions. This can be relevant when you want to test the differences between 2 groups.

Assume you have a long (1) and short set (2). First I sort set 2 by its values. I also estimate the mean difference between consecutive sorted values (mean diff).

My algorithm passes once through set 1 and tries to find a match for every set 1 item in set 2. If the current difference/distance is better than the previous or the next then we have a match (because it’s sorted), and we remove the matched item from set 2. I added a condition where the difference has to be between X multiples of the mean difference. This ensures that I don’t match some remaining large value/distance just because few match items remain. I also added another break condition: if distances get bigger, stop.

I matched 93% in my test. It takes 6% of n1*n2 possible iterations in my test with n1=60k and n2=10k.

The resulting distributions of the matches is an average between distribution 1 and 2. This means you cannot longer assume that the matched items represent the full sets 1 or 2. However you can compare matches to each other.

 The code can be seen and run here

We used Ocado for many years for our food shopping and got very used to their website. The Waitrose website despite some improvements is still behind. It is quite slow and the login page has some bugs.

It doesn’t even have an auto-fill basket option. This github page has some simple Python code to predict items that should be auto-filled: Github Hopefully a Waitrose developer will get inspired.