This chapter introduces readers to another type of iteration and shows them how to create conditional controlled loops (using while loops). The seven challenges include nested code utilising and building on the skills they have learnt in previous chapters.

In this challenge the reader must create a graphical user interface, which will display the times tables and will need to use the following skills: loops (while and for), subprograms and the Tkinter library.

Iteration allows code to be repeated and this chapter shows the reader how to create count-controlled loops (using for loops) that implement the range expression. They complete ten challenges ensuring they are familiar with using iteration in their coding.

For this challenge, the reader needs to create a program that will store the user ID and passwords for the users of a system in a .csv file. The skills they need include: input and display data, lists, if statements, loops (while and for), subprograms and saving to and reading from a .csv file.

This is the first of the five chunky challenges that are created to allow the reader to practice all the skills they have learnt in a larger program that will take longer to complete. In this challenge the reader must create a program that will encode or decode a message using a shift code. The skills they will need include: input and display data, lists, splitting and joining strings, if statements, loops (while and for) and subprograms.

In many programming languages subprograms incorporate both functions and procedures, however, in Python these are combined into a simple single subprogram and it is not necessary to define the type. The reader is shown how to call subprograms, return values from a subprogram and use variables in subprograms. They complete six challenges and use menus to make the programs user-friendly.

In this chunky challenge the reader makes an on-screen version of the board game “Mastermind”. The computer will automatically generate four colours from a list of possible colours, which the user must guess correctly to win the game. The skills they need include: input and display data, lists, random choice from a list, if statements, loops (while and for) and subprograms.

Drawing shapes and patterns with the Python turtle allows readers to practise using loops and helps them learn computational thinking skills where they have to spot repeating patterns. The nine challenges allow them to put these skills into practice.