Coding classes in Flintshire

The number is invented, not a real book. The learner takes twelve digits starting 978, adds a thirteenth check digit using the standard sum with weights of one and three, and gets 9780000000019. Then comes the barcode. Each drawn digit becomes a pattern of seven narrow stripes, dark or light, written in Python as ones and zeros. There are three pattern tables, called L, G and R. The right-hand six digits always use R. The left-hand six use a mixture of L and G, and here is the trick: the choice of L or G for those six digits spells out the first digit. For a first digit of 9, the left six follow the pattern L, G, G, L, G, L.

How can a scanner tell an L pattern from a G pattern? The learner counts the dark stripes: every L code has an odd number, every G code an even number. So a scanner reads the six left-hand digits, notes odd or even for each, and looks up the first digit from that pattern of six. The program decodes its own barcode and gets back 9780000000019. Read backwards, the patterns do not match the tables, so our decoder refuses it rather than returning a wrong number.

Then the learner damages the barcode on purpose, flipping one stripe at a time, all 95 of them. Every single-stripe error is caught by the patterns themselves, because a flipped stripe changes the odd-or-even count or produces a pattern in no table, so the check digit is not even needed for these. It is a neat demonstration that careful design, not luck, lets a scanner say "I cannot read that" instead of guessing.

Breaking your own code on purpose, every way you can, is a habit worth learning, and a reason Flintshire teenagers should code in 2026. The longer argument is in why teenagers should still learn to code in 2026.