I want to format a string to a const width. if the length of string is less than the const value, then padding it with space char at the tail.
For example, with const = 5,
"a" is formatted to "a "
"ab" is formatted to "ab "
"abc" is formatted to "abc "
"abcd" is formatted to "abcd "
"abcde" is formatted to "abcde"

thanks!