The Ruby Way, Second Edition: Solutions and Techniques in Ruby Programming (2nd Edition)

5.5. Formatting Numbers for Output

To output numbers in a specific format, you can use the printf method in the Kernel module. It is virtually identical to its C counterpart. For more information, see the documentation for the printf method.

x = 345.6789 i = 123 printf("x = %6.2f\n", x) # x = 345.68 printf("x = %9.2e\n", x) # x = 3.457e+02 printf("i = %5d\n", i) # i = 123 printf("i = %05d\n", i) # i = 00123 printf("i = %-5d\n", i) # i = 123

To store a result in a string rather than printing it immediately, sprintf can be used in much the same way. The following method returns a string:

str = sprintf("%5.1f",x) # "345.7"

Finally, the String class has a % method that performs this same task. The % method has a format string as a receiver; it takes a single argument (or an array of values) and returns a string.

# Usage is 'format % value' str = "%5.1f" % x # "345.7" str = "%6.2f, %05d" % [x,i] # "345.68, 00123"