The Ruby Way: Solutions and Techniques in Ruby Programming, Second 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 ...