Conditionals and Loops in Ruby

Direct Answer

Ruby uses if/elsif/else, unless, and case to choose which expression to evaluate. It supports while and until, but collection work is usually clearer with iterators such as each, map, select, and times. Only false and nil are falsey in Ruby; values such as 0 and an empty string are truthy.

Deep Dive & Architecture

The important Ruby idea is that control-flow constructs evaluate to an object. In status = if paid then :ready else :waiting end, the chosen symbol becomes the assignment value. If no condition matches and there is no else, the result is nil. This makes a separate pre-declared result variable unnecessary in many cases.

A case expression sends === to each when candidate. A Range therefore checks membership, a Class checks whether the value is an instance, and a regular expression checks for a match. This is more flexible than treating case as a disguised chain of equality tests.

Iteration also has return-value semantics. each is for side effects and returns its receiver. map returns one transformed item for each input. select keeps matching items, while find stops at the first match. Picking the right iterator communicates intent and often removes bookkeeping variables.

Inside a loop or iterator, next skips the rest of the current iteration and break stops the nearest loop. redo starts the same iteration again without advancing, so it must be guarded carefully to avoid an endless loop. A manual while remains useful when progress depends on changing state rather than traversing a known collection.

Code Example

score = 86
grade = if score >= 90
          'A'
        elsif score >= 80
          'B'
        else
          'C'
        end

kind = case score
       when 90..100 then :excellent
       when 60..89  then :pass
       else :retry
       end

prices = [10, 25, 40]
discounted = prices.map { |price| price * 0.9 }
large = prices.select { |price| price >= 25 }

[grade, kind, discounted, large]

Common Mistakes & Pitfalls

Using a while counter when each/times would do (un-idiomatic), or confusing unless with if (unless = if not).