What does a hash/pound (#) represent in a format string?

Question

I have some code to format a file size string:

NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init]; 
[numberFormatter setPositiveFormat: @"#,##0.## bytes"];

Is the @"#,##0.## bytes" the same kind of format string as I'd use in stringWithFormat? What do the hash/pound symbols mean here?

Thanks!

Answer 1

# will usually be replaced by a number if one exists, and nothing if it doesn't. 0 will be replaced by a number if one exists, and zero if it does not.

So for the following formatting '##00.00##' you would get the following outputs:

1 => 01.00
12.1 => 12.10
1234.5 => 1234.50
1.2345 => 01.2345

Answer 2

They are called placeholders.

Placeholders

You use the pound sign character (#) to represent numeric characters that will be input by the user. For example, for the positive pattern "$#,##0.00", if the characters 76329 were entered into a cell to which the pattern has been applied, they would be displayed as $76,329.00. Strictly speaking, however, you don't need to use placeholders. The format strings ",0.00", "#,#0.00", and "#,##0.00" are functionally equivalent. In other words, including separator characters in a pattern string signals NSNumberFormatter to use the separators, regardless of whether you use (or where you put) placeholders. The placeholder character's chief virtue lies in its ability to make pattern strings more human-readable, which is especially useful for displaying patterns in the user interface.

Source: http://developer.apple.com/documentation/InternetWeb/Reference/WO542Reference/com/webobjects/foundation/NSNumberFormatter.html

Answer 3

'#' represents a not mandatory digit place that will not appear in case of a 0 digit in that position, whereas 0 means the digit will aleays apprear.

lets take an example : 345.5

#,##0.## = 345.5

0,000.00 = 0,345.50

Answer 4

'#' is in most languages used as an optional digit, as opposed to '0', which is a mandatory digit (to be used to get leading/trailing zeroes).