Kernel#printf

printf(format_string [, arguments...]) -> nil

Returns: nil · Updated March 13, 2026 · Kernel Methods

output formatting strings printing

The printf method outputs a formatted string to STDOUT. It uses format specifiers similar to C’s printf function, allowing precise control over how numbers, strings, and other values are displayed.

Basic Format Specifiers

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

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

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

# %x - Hexadecimal
printf("%x", 255)       # => ff
printf("%X", 255)       # => FF

# %o - Octal
printf("%o", 64)        # => 100

# %b - Binary
printf("%b", 10)        # => 1010

Width and Precision

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

# Precision for floats
printf("%.3f", 3.14159) # => 3.142
printf("%.0f", 3.7)    # => 4

# Minimum and maximum width
printf("%8.2f", 3.14)   # => "    3.14"

Multiple Arguments

# Multiple values
printf("Name: %s, Age: %d", "Alice", 30)
# => Name: Alice, Age: 30

printf("%d + %d = %d", 1, 2, 3)
# => 1 + 2 = 3

Practical Examples

Table Formatting

# Align columns
printf("%-15s %10s\n", "Product", "Price")
printf("%-15s %10.2f\n", "Apple", 1.50)
printf("%-15s %10.2f\n", "Banana", 0.75)
printf("%-15s %10.2f\n", "Cherry", 12.99)

# Output:
# Product                   Price
# Apple                     1.50
# Banana                    0.75
# Cherry                   12.99

Hex Dump

# Memory dump style output
data = [72, 101, 108, 108, 111]  # "Hello"
data.each { |b| printf("%02X ", b) }
# => 48 65 6C 6C 6F

Number Formatting

# Thousand separators
printf("%,d", 1234567)  # => 1,234,567

# Percentage
printf("%.1f%%", 0.75) # => 75.0%

# Scientific notation
printf("%.2e", 1234)   # => 1.23e+03

Padding with Zeroes

printf("%05d", 42)     # => 00042
printf("%08.2f", 3.14) # => 00003.14

Using Named Arguments (Ruby 2.6+)

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

Comparison with Other Output Methods

puts "hello"   # Adds newline
print "hello"  # No newline
printf "hello" # With formatting

# printf returns nil
result = printf("Value: %d", 42)
puts result    # => nil

Special Characters

# Escape sequences
printf("Tab:\tNext\n")
printf("Newline:\nDone\n")

# Literal percent sign
printf("100%% complete\n")  # => 100% complete

The printf method is powerful for creating formatted output, especially when you need precise control over number formatting, alignment, or table-like displays.