Ruby Programming

103 flashcards by MentisLibrary

Ruby the way Rubyists write it: truthiness and symbols, strings and collections, blocks, procs and lambdas, classes, modules and mixins, metaprogramming, exceptions, and the gem and Bundler tooling.

Which values are falsy in Ruby?

Only false and nil. Everything else is truthy, including 0, "", [] and {}.

What does it mean that everything in Ruby is an object?

Every value, including nil, integers, symbols and classes themselves, responds to methods. There are no primitive types outside the object model.

What distinguishes a symbol from a string?

A symbol such as :name is an immutable, interned identifier: every occurrence is the same object. A string is mutable and each literal creates a new object.

How does Ruby distinguish local, instance, class and global variables?

By sigil: a bare lowercase name is local, @name is an instance variable, @@name a class variable, $name a global, and a Capitalized name is a constant.

What does an unset instance variable evaluate to?

nil. Instance variables are never declared, so a typo produces nil instead of an error.

What do a trailing ? and ! on a method name mean?

Conventions, not syntax. ? marks a predicate returning true or false; ! marks the more surprising or destructive of a pair of methods.

What does a Ruby method return when there is no return?

The value of the last expression evaluated. Explicit return is used mainly for early exit.

puts versus print versus p

puts writes to_s plus a newline and flattens arrays onto separate lines. print writes to_s with no newline. p writes inspect plus a newline and RETURNS its argument.

How does string interpolation work?

"#{expression}" evaluates the expression, calls to_s on the result and splices it in. It works only in double-quoted strings, heredocs and %W.

What is the difference between && and and?

They compute the same thing but and has much lower precedence, below assignment. && binds tighter than =, and does not.

What does ||= do?

Assigns the right side only when the left side is nil or false. It expands roughly to x || x = value.

How do 1..5 and 1...5 differ?

Two dots include the end value, three dots exclude it. Both build a Range, which is enumerable and usable in case.

What comparison does case/when actually use?

The case equality operator === on the when value, so a Range tests inclusion, a Regexp tests a match, a Class tests is_a?, and a lambda is called.

What does the safe navigation operator &. do?

Calls the method only if the receiver is not nil; otherwise the whole expression is nil.

How do you write a guard clause, and why?

Return, raise or next at the top of the method for the cases you will not handle, so the main path is never indented.

When is unless the right choice?

For a single negative condition where if ! would read worse. Never with else, and never with a compound condition.

What does the ternary and the then keyword offer?

cond ? a : b for a short two-way choice. then is optional after if/when and lets a branch sit on one line.

How does integer division behave, and how do you get a float?

/ between two integers truncates toward negative infinity. Use fdiv, or make one operand a float.

What size can a Ruby integer reach?

Any size. Integer is arbitrary precision and grows beyond machine word size automatically.

How do multiple assignment and the splat operator work?

The left side is matched positionally; *name absorbs the remaining values into an array. It works in assignment, in block parameters and in method parameters.

How do keyword arguments differ from a trailing options hash?

Keyword arguments are declared by name with name:, are checked at call time, and are kept separate from positional arguments. A hash passed positionally is no longer converted into them.

What do *args, **opts and &blk collect in a method signature?

*args gathers extra positional arguments into an array, **opts gathers keyword arguments into a hash, and &blk captures the block as a Proc.

Single quotes versus double quotes in Ruby

Double quotes interpolate #{} and process escapes such as \n. Single quotes handle only \\ and \'.

What do %w, %i and %q build?

%w[a b c] an array of strings, %i[a b c] an array of symbols, %q() a single-quoted string and %Q() a double-quoted one.

What does the squiggly heredoc <<~ do?

Removes the common leading indentation from every line, so a multi-line string can be indented with the surrounding code.

How do String#+ and String#<< differ?

+ returns a new string and leaves both operands alone. << appends in place and returns the same object, mutating anything else holding a reference.

What does the # frozen_string_literal: true magic comment do?

Makes every string literal in that file frozen, so attempts to mutate one raise FrozenError and identical literals can be shared.

How does gsub differ from sub, and what can the replacement be?

sub replaces the first match, gsub every match. The replacement can be a string with \1 backreferences, a hash, or a block receiving the match.

How do you test whether a string matches a pattern?

Regexp#match? or String#match? returns a boolean and sets no globals, which makes it the fast, side-effect-free choice.

How do named captures work in a Ruby regex?

(?<name>...) labels a group. MatchData#[] accepts the name, and =~ against a LITERAL regex also assigns local variables.