sprintf() / % Operator

sprintf(format_string, value, ...)
format(format_string, value, ...)

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

sample_sprintf_percent.rb

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 score: %d.", "user_1", 220)
# user_1 score: 220.

# Pass multiple values using an array.
puts "Name: %s, Role: %s" % ["user_2", "editor"]
# Name: user_2, Role: editor

# 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%.

Running the above produces the following output:

ruby sprintf_percent.rb
42
00042
+42
00042
3.14
    3.14
user_1 score: 220.
Name: user_2, Role: editor
ff
FF
1010
0x000000ff
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.