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You are using an out of date browser. It may not display this or other websites correctly.You should upgrade or use an alternative browser. %#08x formatting of printf

• Thread starter Bilgehan.Balban
• Start date Jan 26, 2006

B

Bilgehan.Balban

Hi, I use %#08x to print unsigned integers in hexadecimal format. According to C ref. man. Harbison & Steele, #08 stands for "pad the number with up to 8 zeroes to complete it to 8 digit number". Is this correct understanding? However this is not always the case. I sometimes see 4, sometimes 2, that does not complete the whole number into 8 digits. I don't know what changes this, but do you have an alternative that definitely pads to 8 digits? Thanks, Bahadir R

Robert Harris

Hi, I use %#08x to print unsigned integers in hexadecimal format. According to C ref. man. Harbison & Steele, #08 stands for "pad the number with up to 8 zeroes to complete it to 8 digit number". Is this correct understanding? However this is not always the case. I sometimes see 4, sometimes 2, that does not complete the whole number into 8 digits. I don't know what changes this, but do you have an alternative that definitely pads to 8 digits? Thanks, Bahadir Click to expand...

The # is a flag prefixing "0x" to the result The 8 is the minimum field width (but some of the field may consist of spaces before the "0x"). You really need a precision of 8 and a field width of 10 (to include the "0x", so you need: "%#10.8x" Robert H

Henryk

No time to check, but as I recall I used something like printf("%08x", ....) or printf("0x%08", ...) for the prefix. I never used the # tag. J

Jordan Abel

No time to check, but as I recall I used something like printf("%08x", ....) or printf("0x%08", ...) for the prefix. I never used the # tag. Click to expand...

That's because the # tag doesn't apply to a value of zero. B

Bilgehan.Balban

Robert said: The # is a flag prefixing "0x" to the result The 8 is the minimum field width (but some of the field may consist of spaces before the "0x"). You really need a precision of 8 and a field width of 10 (to include the "0x", so you need: "%#10.8x" Robert Click to expand...

Thanks, I'll try this. Bahadir C

Chris Torek

Almost. See below ...

Robert Harris said: (e-mail address removed) wrote: The # is a flag prefixing "0x" to the result The 8 is the minimum field width (but some of the field may consist of spaces before the "0x"). You really need a precision of 8 and a field width of 10 (to include the "0x", so you need: "%#10.8x" Click to expand...

Both of these are correct but incomplete. As Jordan Abel pointed out elsethread, the "#" flag ("alternate form") only applies the prefix is the number is nonzero (for both octal and hex conversions, in fact). ("Alternate form" has different meanings for floating-point formats as well.) The "0" in "08" is also a flag, meaning "pad with zeros instead of blanks" (for numeric conversions -- the effect is undefined for conversions like %s). To include a zero flag, it must (obviously) appear before a numeric field width: "%09d" means "pad with zeros, 9 wide, decimal" but "%90d" means "pad with blanks, 90 wide, decimal". The zero flag only applies for right-justified fields; if you specify both the "-" flag and the "0" flag, the "0" flag is ignored: "%-9d" and "%-09d" mean the same thing (as does "%0-9d"). Using a precision, as in Robert Harris' example above, is roughly equivalent to specifying the zero flag. But it is not exactly the same. For instance: printf("5.2d: >%5.2d<\n", 5); printf("05d: >%05d<\n", 5); prints lines with "> 05<" and ">00005<" respectively. Using "%#10.8x" will do the Right Thing for all nonzero numbers: printf("*%#10.8x*\n", 0x4321); /* prints *0x00004321* */ but for 0 you get: printf("*%#10.8x*\n", 0); /* prints * 00000000* */ Note that the 0x has disappeared (as required for "%#x" format), so 10.8 -- a field width of 10, with a precision of 8 -- now pads with blanks. You could leave out the field width ("%#.8x"), but then nonzero numbers will occupy 10 character fields (8 digits plus the leading 0x) while zero will use 8 character (8 digits, no leading 0x). You could even write "%#010.8x" and hope for zero padding when the 0x is omitted, but this seems ... wrong. The two "best" alternatives are likely "0x%.8x" or "0x%08x", which will do exactly the same thing. Post reply

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