String Formatting Tricks/Docs

Question

Was reading the response by Shaggy Frog to this post and was intrigued by the following line of code:

NSLog(@"%@", [NSString stringWithFormat:@"%@:%*s%5.2f", key, padding, " ", [object floatValue]]);

I know string formatting is an age old art but I'm kinda doing the end around into Cocoa/Obj-C programming and skipped a few grades along the way. Where is a good (best) place to learn all the string formatting tricks allowed in NSString's stringWithFormat? I'm familiar with Apple's String Format Specifiers page but from what I can tell it doesn't shed light on whatever is happening with %*s or the %5.2f (not to mention the 3 apparent placeholders followed by 4 arguments) above?!?

I can see why NSLog(@"%@", [NSString stringWithFormat: @"...", ...]); was used in that answer (to parallel the question), but NSLog(@"...", ...); should work identically and it reads more easily. — Isaac
– Isaac, Commented Apr 23, 2010 at 23:06

J... · Accepted Answer · 2020-01-13 15:12:06Z

2

The documentation of -stringWithFormat leads you to String Format Specifier which in turn sends you to the IEEE printf specification. That's about as much information as you'll ever want.

The only notable exception:

%@

Objective-C object, printed as the string returned by descriptionWithLocale: if available, or description otherwise. Also works with CFTypeRef objects, returning the result of the CFCopyDescription function.

nil gets converted to (null), that's the reason why NSLog(@"%@", someObject) is safer than NSLog("someObject). The later might crash when someObject is nil:

You might also be interested in the wikipedia page about string formatting.

edited Jan 13, 2020 at 15:12

J...

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answered Apr 23, 2010 at 19:49

Georg Schölly

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2 Comments

Meltemi Over a year ago

ya. that i know.. what i don't know is a good source describing all the printf string argument manipulation tricks/techniques.

mipadi Over a year ago

man printf is a good place to start (the Cocoa methods follow the same practices, which the additions listed above), although the wiki page linked above is basically just a fancy form of the printf man page. For example, %*s uses two specifies, * and s; both are described on the man page (* in the "width" section, and s in the type section).

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