Kernel#format

format(format_string [, arguments...]) -> string

The format method (alias for sprintf) creates a formatted string using specifiers similar to C’s printf. It provides precise control over string formatting.

Basic Format Specifiers

# %d - Integer
format("%d", 42)          # => "42"

# %f - Float
format("%.2f", 3.14159)  # => "3.14"

# %s - String
format("%s", "hello")    # => "hello"

# %x - Hex
format("%x", 255)        # => "ff"

Width and Precision

# Minimum width
format("%5d", 42)       # => "   42"

# Left align
format("%-5d", 42)       # => "42   "

# Precision
format("%.3f", 3.14159) # => "3.142"

Multiple Arguments

format("Name: %s, Age: %d", "Alice", 30)
# => "Name: Alice, Age: 30"

Practical Examples

Currency Formatting

def format_currency(amount)
  format("$%.2f", amount)
end

format_currency(19.99)   # => "$19.99"
format_currency(1000.00)  # => "$1000.00"

Table Generation

headers = ["Name", "Score"]
rows = [["Alice", 95], ["Bob", 87]]

puts format("%-10s %5s", *headers)
rows.each { |r| puts format("%-10s %5d", *r) }

Padding

# Zero-padded
format("%05d", 42)     # => "00042"

# Hex with padding
format("%08X", 255)   # => "000000FF"

Named Parameters (Ruby 2.6+)

format("%{name} is %{age} years old", name: "Bob", age: 25)
# => "Bob is 25 years old"

Flags

# +: Always show sign
format("%+d", 5)       # => "+5"
format("%+d", -5)     # => "-5"

# Space: Space for positive
format("% d", 5)      # => " 5"

# #: Alternate form
format("%#x", 255)    # => "0xff"
format("%#o", 64)     # => "0100"

Array Arguments

# Using * to expand array
args = ["Hello", 42]
format("Message: %s, Code: %d", *args)

Percentage

# Literal percent
format("100%%")        # => "100%"
format("%.0f%%", 75.5) # => "76%"

Common Use Cases

• Generating formatted output
• String interpolation with control
• Building reports
• Internationalization

See Also

• Kernel#caller
• Kernel#local_variables
• Kernel#eval