Ruby quick reference

The smallest Ruby program, and the difference between puts, print and p.

Numbers, strings, symbols, true / false, nil, and converting with to_i, to_f, to_s and Integer().

if / elsif / unless, case, iterating with each and times, while, and next / break.

Define methods with def, keyword arguments, implicit return, blocks with yield, and lambdas.

Arrays and hashes, map / select / sort_by, fetch with a default, and other everyday Enumerable methods.

Define classes with initialize and attr_reader, inherit with <, mix in modules, and use Struct.

Interpolation, common string methods, format, and regular expressions with scan, =~ and [].

Read what you type in the IDE's Input tab with gets or $stdin.each_line.

Read project files with File.read and CSV, write JSON, and keep temporary files under /tmp.

Load your own files with require_relative, and what Ruby offers without installing gems.

What Run does with your project, the time limit, input, tests, and what a run cannot do.

What NoMethodError on nil, NameError, ArgumentError, TypeError, ZeroDivisionError and SyntaxError mean and how to fix them.

Where to read more: the Ruby documentation site, the quick start, and the core and standard library reference.