map (alias collect) returns a new array of the block's return values. select (alias filter) keeps elements where the block is truthy; reject keeps elements where it is falsy. The &:method shorthand turns a symbol into a proc that calls the method on each element, equivalent to { |x| x.to_s } but shorter. Chain them freely: nums.map(&:to_s).select { |s| s.size > 1 } reads top-to-bottom as a pipeline. None of these mutate the original array; their ! counterparts (map!, select!) do, but prefer the non-mutating forms unless you have a specific reason.

inject (alias reduce) walks the collection with an accumulator. With a seed, the block runs once per element starting from the seed; without a seed, the first element IS the seed and the block runs from the second element. The symbol form nums.inject(:+) is sugar for nums.inject { |a, b| a + b } and is the idiomatic way to sum or product. The hash-building idiom shows up everywhere; on Ruby 2.6+ you can replace it with pairs.to_h for the simple [[k, v], ...] shape, but inject is still the right tool when the value depends on prior accumulator state.

group_by returns a hash keyed by the block's return value, with each value being the array of elements that produced that key. It is the Ruby equivalent of Collectors.groupingBy in Java. each_slice(n) iterates in fixed-size chunks and is the right tool for batched processing (paginated API calls, fixed-size database inserts). partition is select and reject fused into one pass, returning both arrays in a destructuring assignment. Together with chunk_while and slice_when for run-based grouping, these cover almost every array reorganisation you encounter without writing a manual loop.