sprintf() / % Operator

# Use Kernel#sprintf (or format).
sprintf(format_string, value, ...)
format(format_string, value, ...)

# The % operator provides a shorthand.
format_string % value
format_string % [value1, value2, ...]

# Format an integer.
puts sprintf("%d", 42)       # 42
puts sprintf("%05d", 42)     # 00042 (zero-padded to 5 digits)
puts sprintf("%+d", 42)      # +42 (show sign)

# The % operator does the same thing.
puts "%05d" % 42             # 00042

# Format a floating-point number.
puts sprintf("%.2f", 3.14159)  # 3.14 (2 decimal places)
puts sprintf("%8.2f", 3.14)    # "    3.14" (8-character width)

# Format a string.
puts sprintf("%s scored %d points.", "Alice", 85)
# Alice scored 85 points.

# Pass multiple values using an array.
puts "Name: %s, Age: %d" % ["Bob", 30]
# Name: Bob, Age: 30

# Convert to hexadecimal and binary.
puts sprintf("%x", 255)    # ff
puts sprintf("%X", 255)    # FF
puts sprintf("%b", 10)     # 1010
puts sprintf("0x%08x", 255)  # 0x000000ff

# Display a percent sign.
puts sprintf("The tax rate is %d%%.", 10)  # The tax rate is 10%.

Ruby's sprintf (also known as format) uses C-style format specifiers to format numbers and strings into a specific layout. The % operator is syntactic sugar for sprintf, letting you write more concise code.

In modern Ruby, string interpolation ("#{variable}") is the common approach, but sprintf is better suited for zero-padding numbers and controlling decimal precision. When passing multiple values to the % operator, always use an array. A single value without an array ("%s" % variable) is fine, but for multiple values you must write "%s %s" % [value1, value2].

For string padding, you can also use center / ljust / rjust. For type conversion of strings, see to_i / to_f.