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This chapter is from the book

2.9 Formatting a String

Formatting a string is done in Ruby as it is in C: with the sprintf method. It takes a string and a list of expressions as parameters and returns a string. The format string contains essentially the same set of specifiers available with C’s sprintf (or printf):

name = "Bob"
age = 28
str = sprintf("Hi, %s... I see you're %d years old.", name, age)

You might ask why we would use this instead of simply interpolating values into a string using the #{expr} notation. The answer is that sprintf makes it possible to do extra formatting, such as specifying a maximum width, specifying a maximum number of decimal places, adding or suppressing leading zeroes, left-justifying, right-justifying, and more:

str = sprintf("%-20s  %3d", name, age)

The String class has the method %, which does much the same thing. It takes a single value or an array of values of any type:

str = "%-20s  %3d" % [name, age]  # Same as previous example

We also have the methods ljust, rjust, and center; these take a length for the destination string and pad with spaces as needed:

str = "Moby-Dick"
s1 = str.ljust(13)           # "Moby-Dick"
s2 = str.center(13)          # "  Moby-Dick  "
s3 = str.rjust(13)           # "    Moby-Dick"

If a second parameter is specified, it is used as the pad string (which may possibly be truncated as needed):

str = "Captain Ahab"
s1 = str.ljust(20,"+")       # "Captain Ahab++++++++"
s2 = str.center(20,"-")      # "----Captain Ahab----"
s3 = str.rjust(20,"123")     # "12312312Captain Ahab"
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