String.* / center / ljust / rjust

string.center(width)
string.center(width, pad_char)

# Left-aligns the string (pads on the right).
string.ljust(width)
string.ljust(width, pad_char)

# Right-aligns the string (pads on the left).
string.rjust(width)
string.rjust(width, pad_char)

# Repeats the string n times.
string * n

sample_string_center_ljust_rjust.rb

text = "Ruby"

# Center the string.
puts text.center(10) # "   Ruby   "
puts text.center(10, "*") # "***Ruby***"

# Left-align (pad on the right).
puts text.ljust(10) # "Ruby      "
puts text.ljust(10, "-") # "Ruby------"

# Right-align (pad on the left).
puts text.rjust(10) # "      Ruby"
puts text.rjust(10, "0") # "000000Ruby"

# Zero-pad numbers (useful for dates, sequential numbers, etc.).
puts "5".rjust(3, "0") # 005
puts "12".rjust(3, "0") # 012
puts "123".rjust(3, "0") # 123 (no change if already at or beyond the width)

# Repeat a string with the * operator.
puts "=-" * 10 # =-=-=-=-=-=-=-=-=-=-
puts "Ruby " * 3 # Ruby Ruby Ruby

# Useful for formatting table output.
["Item", "Price", "Qty"].each do |header|
  print header.ljust(10)
end
puts

Running the above produces the following output:

ruby string_center_ljust_rjust.rb
   Ruby   
***Ruby***
Ruby      
Ruby------
      Ruby
000000Ruby
005
012
123
=-=-=-=-=-=-=-=-=-=-
Ruby Ruby Ruby 
Product       Price        Qty        

center, ljust, and rjust are used for formatting text output. They are handy for aligning columns in command-line tools or decorating log headings. If the specified width is already less than or equal to the string's length, the string is returned unchanged.

You can use rjust(n, "0") to zero-pad numbers, but sprintf or string formatting (e.g., "%03d") works directly with numeric values, so it may be more appropriate when dealing with numbers specifically.

For string formatting, see sprintf() / % operator. To get the length of a string, use length / size.