Ruby Data & Collections
Array, Hash, Set and Enumerable in earnest. Bang methods that return nil, Hash defaults that share one object between every key, fetch against [], grouping and tally, and lazy enumerators that are not always cheaper.
Who it is for
Comfortable, but surprised by how containers behave.
The behaviour people get wrong after they are already productive in the language.
You will get the most out of it if you have written Ruby recently enough to read it without effort, and have been surprised by it at least once. If you are picking the language up for the first time, start with Ruby Essentials and come back to this one.
How long it takes
90 graded questions at roughly 45 seconds each is about 68 minutes of answering in total.
At the default five questions a day, and because the session mixes in your other courses, expect around 18 days — call it 3 weeks — to see everything once, plus a few weeks of thinning reviews after that before it is all mastered.
A question is mastered on the third correct answer, once the gap between reviews has grown past a week — so a course cannot be finished in a weekend however long you sit with it.
Question mix
- 60 multiple choice
- 10 concept cards
- 10 predict the output
- 10 put it in order
- 10 matching questions
Syllabus
What you will be asked about
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01
Bang Methods & nil
Predict when a bang method returns nil instead of the array.
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02
each, map and Accumulation
Choose between each, map, inject and each_with_object when building a value.
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03
Selecting and Rejecting
Filter a collection with select, reject, partition, find and filter_map.
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04
Hash Defaults
Distinguish a default value from a default block, and avoid the shared-object bug.
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05
fetch, dig and Missing Keys
Read nested data with [], fetch and dig, and know which one complains.
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06
group_by, tally and Counting
Summarise a collection with tally, group_by and the *_by family.
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07
sort, sort_by and <=>
Order a collection with sort, sort_by and the spaceship operator.
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08
Flattening, zip and Windows
Reshape nested data with flat_map, zip, transpose and the each_* windows.
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09
Sets and uniq
Remove duplicates with uniq, a block, or a Set - and know what counts as equal.
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10
Lazy Enumerators
Use lazy to work with infinite sequences and to stop computing early.
8 more topics, and all 90 questions
Ruby Data & Collections is a Pro course. One subscription opens it and every other course in the catalogue, including the ones still being written.
A question from this course
Bang Methods & nil
What does this print?
p [1, 2, 3].uniq!Show the answer
A nil
You may have expected the array back, because most methods hand you something useful. uniq! returns nil when it removed no duplicates; the return value is a report on whether it changed anything, not the data.
The rest of the track
Ruby at every depth
Ruby Essentials
The behaviour underneath the friendly syntax. Only nil and false are falsy, symbols are not strings, string literals are mutable, and a method returns its last expression whether or not you meant it to.
Ruby Core
Where Ruby trips people who use it daily. Blocks against procs against lambdas, the ancestor chain and method lookup, the three kinds of equality, and what happens when two names point at the same object.
Ruby Errors & Edge Cases
Exceptions and their edges. Rescue ordering, StandardError against Exception, what an ensure does to a return value, retry, and the difference between catch/throw and raising at all.
Ruby Standard Library
The library people reimplement by hand: Struct and Data, Set, the Comparable and Enumerable mixins, Forwardable, JSON, Digest and BigDecimal. Ten places where a built-in already does the job, and one behaviour in each that is not what you assumed.
Ruby Idioms & Pitfalls
The Ruby people write out of habit that is wrong or merely un-Ruby: for loops that leak their variable, and versus &&, unless with an else, ||= on a false value, map followed by compact, and rescuing Exception. Ten habits, and the behaviour that punishes each one.
Ruby Advanced
The metaprogramming that still catches experienced Rubyists: singleton classes, method_missing, refinements, prepend, and the two searches behind constant lookup. Every snippet is one you could be handed in a code review.
Ruby Performance
What actually costs in MRI: object allocation, frozen and deduplicated strings, symbol keys, array and string growth, lazy enumerators, and the generational collector. Every claim is one you can measure with GC.stat rather than guess at.
Ruby Testing
What a test actually asserts, and where that claim is weaker than it looks. Argument order and failure messages, the three kinds of equality, exceptions, isolation between examples, and the difference between a failure and an error.
Ruby Interview Questions
The Ruby questions interviewers actually ask, and the answers that need a reason attached. Proc and lambda return semantics, the ancestor chain, mixins, duck typing, shared references, symbols, method_missing, memoisation and Enumerable built by hand.
Start Ruby Data & Collections
in tomorrow’s session.
Enrol and this course joins your daily five, mixed in with everything else you are studying — 90 questions spread over about 18 days rather than dropped on you at once.
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