字符串格式化技巧/文档 [英] String Formatting Tricks/Docs

问题描述

正在阅读Shaggy Frog对此信息的回复，并对以下代码段感兴趣：

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]]);

我知道字符串格式化是一个古老的艺术，但我有点结尾到Cocoa / Obj-C编程，并跳过了几个等级沿途。在哪里可以学习NSString的 stringWithFormat 中允许的所有字符串格式化技巧？我熟悉Apple的字符串格式说明符页面，但从我可以告诉它没有阐明发生在％* s 或％5.2f （更不要说3个明显的占位符，后面是4个参数）上面？！

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?!?

推荐答案

-stringWithFormat 的文档导致您访问字符串格式说明符，它会将您发送到 IEEE printf specification 。

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.

唯一值得注意的例外情况：

The only notable exception:

％@

Objective-C对象，以
返回的
字符串打印 descriptionWithLocale： if可用，
或描述。还可以通过 CFTypeRef 对象来运行
，返回 CFCopyDescription
函数的
结果。

%@

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.



• 会转换为（null），这就是为什么 NSLog（someObject）。NSLog（@％@，someObject）比 是 nil ：



• 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:

您可能还对 wikipedia page about string formatting 。

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