Divide & Conquer: An Iterative Approach To Writing Software

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Divide & Conquer: An Iterative Approach To Writing Software Mark Crossfield, July 2018 whose pronoun.is/he

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Structure Introduction Feedback loops Requirements Slicing Turning requirements into code Summary 2

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Requirements • Users must supply a username and a password to authenticate • Their username and password must be checked against the database table of known users • If the username is found and the password matches then they must be authenticated • If the username is not found then the user must not be authenticated • If the user is found but the password does not match then the user must not be authenticated 4

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Slicing: Getting a (little) bit of everything 6

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Slicing tip: Cheat—start with a simpler problem

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Slicing tip: Start with empty inputs

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Slicing tip: Ignore part of the requirements

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Slicing tip: Work towards the ‘happy path’

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Do literally anything you can to simplify the problem Section title

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Converting Requirements into Code 12

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Coding Tip: Keep focussed on the next simplest bit of the requirements

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘james’ and password ‘sesame’ Then the user is authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘james’ and password ‘letmein’ Then the user is not authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘mark’ and password ‘sesame’ Then the user is not authenticated

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What can our first step be? 18

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘mark’ and password ‘sesame’ Then the user is not authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘mark’ and password ‘sesame’ Then the user is not authenticated

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Coding Tip: Start at the outside

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Coding Tip: Test each stage against your expectations

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘mark’ and password ‘sesame’ Then the user is not authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘james’ and password ‘sesame’ Then the user is authenticated

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Coding Tip: Let the code guide you—start with intent

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Don’t jump ahead, call code before you define it, so that you know what it needs to do

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Coding tip: Fake bits of the implementation… hard code stuff!

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘james’ and password ‘sesame’ Then the user is authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘mark’ and password ‘sesame’ Then the user is not authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘mark’ and password ‘sesame’ Then the user is not authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘james’ and password ‘sesame’ Then the user is authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘james’ and password ‘sesame’ Then the user is authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘mark’ and password ‘sesame’ Then the user is not authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘mark’ and password ‘sesame’ Then the user is not authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘james’ and password ‘letmein’ Then the user is not authenticated

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Given a user exists with username ‘james’ and password ‘sesame’ When a user logs in with username ‘james’ and password ‘letmein’ Then the user is not authenticated

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Summary Shorten Feedback Loops Start with the smallest thing… cheat if necessary Work from the outside towards the happy path Do one thing at once Fake things Call code before writing it Test all the time Practice your practices! 76

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Learning Resources Structured exercises—work locally • http://exercism.io/ Structured exercises you can do in the browser • https://www.hackerrank.com/ • http://www.codewars.com/ Generic problems you can try to solve in any language • http://codingdojo.org/kata/ • Book: 57 Challenges to Develop Your Coding Skills https://pragprog.com/book/bhwb/exercises-for-programmers 77

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Any questions? Section title 78