Numeric.abs / round / ceil / floor / truncate

# Returns the absolute value.
number.abs

# Rounds to the nearest integer (n specifies the number of decimal places).
number.round
number.round(n)

# Rounds up (ceiling).
number.ceil
number.ceil(n)

# Rounds down (floor).
number.floor
number.floor(n)

# Truncates toward zero.
number.truncate
number.truncate(n)

# abs: Get the absolute value.
puts (-5).abs    # 5
puts 3.14.abs    # 3.14
puts (-2.7).abs  # 2.7

# round: Round to the nearest integer.
puts 3.14159.round     # 3
puts 3.14159.round(2)  # 3.14
puts 3.14159.round(4)  # 3.1416
puts 2.5.round         # 3 (rounds up)

# ceil: Round up.
puts 3.1.ceil      # 4
puts (-3.1).ceil   # -3 (rounds toward zero)
puts 3.14.ceil(1)  # 3.2

# floor: Round down.
puts 3.9.floor      # 3
puts (-3.9).floor   # -4 (rounds away from zero)
puts 3.96.floor(1)  # 3.9

# truncate: Truncate toward zero.
puts 3.9.truncate    # 3
puts (-3.9).truncate  # -3 (different from floor!)

# Practical example: Calculate tax (truncate to the nearest integer).
price = 1234
total = (price * 1.1).floor
puts "#{price} yen → with tax: #{total} yen"  # 1234 yen → with tax: 1357 yen

Calling round, ceil, or floor without an argument returns an integer. Passing an argument n returns a floating-point number rounded to n decimal places. You can also pass a negative number to round at the integer level (e.g., 1234.round(-2) returns 1200).

Pay attention to behavior with negative numbers. floor(-3.9) returns -4 (toward negative infinity), ceil(-3.9) returns -3 (toward positive infinity), and truncate(-3.9) returns -3 (toward zero) — each producing a different result.

Floating-point numbers have internal representation errors, so 0.1 + 0.2 may not equal exactly 0.3. For financial calculations or other precision-sensitive work, use the BigDecimal class instead.

For type conversion of numbers, see number.to_i / to_f / to_s. For math functions, see Math.sqrt / log / sin / cos.