Basic Objective-C syntax: "%@"?

Question

I'm working through the Stanford iPhone podcasts and have some basic questions.

The first: why is there no easy string concatenation? (or am I just missing it?)

I needed help with the NSLog below, and have no idea what it's currently doing (the %@ part). Do you just substitute those in wherever you need concatenation, and then comma separate the values at the end?

NSString *path = @"~";
NSString *absolutePath = [path stringByExpandingTildeInPath];

NSLog(@"My home folder is at '%@'", absolutePath);

whereas with any other programing language I'd have done it like this:

NSLog(@"My home folder is at " + absolutePath);

Thanks! (Additionally, any good guides/references for someone familiar with Java/C#/etc style syntax transitioning to Objective-C?)

Answer 1

%@ is a placeholder in a format string, for a NSString instance.

When you do something like:

NSLog(@"My home folder is at '%@'", absolutePath);

You are telling NSLog to replace the %@ placeholder with the string called absolutePath.

Likewise, if you put more placeholders, you can specify more values to replace those placeholders like this:

NSString *absolutePath = @"/home/whatever";
NSLog(@"My home #%d folder is at '%@'", 5, absolutePath);

Will print:

My home #5 is at /home/whatever

An easy way to do string concatenation:

NSString *s1 = @"Hello, ";
NSString *s2 = @"world.";
NSString *s = [NSString stringWithFormat:@"%@%@", s1, s2];
// s will be "Hello, world."

You can't have a + sign as a string concatenate operator, since there is no operator overloading in Objective-C.

Hope it helps.

Answer 2

That is a string format specifier. Basically it allows you to specify a placeholder in the string and the values that are to be inserted into the placeholder's spot. The link I reference above lists the different notations for the placeholders and each placeholder's specific format.

It's just like C#'s String.Format method:

NSLog(String.Format("My home folder is at '{0}'", absolutePath));

Answer 3

You can use NSString +stringWithFormat to do concatenation:

NSString* a = // ...
NSString* b = // ...
NSString* a_concatenated_with_b = [NSString stringWithFormat:@"%@%@",a,b];

The reason for the "%@" is that the string formatting is based off of and extends the printf format strings syntax. These functions take a variable number of arguments, and anything beginning with a percent sign (%) is interpreted as a place holder. The subsequent characters determine the type of the place holder. The standard printf does not use "%@", and since "@" is the symbol commonly used for things that Objective-C adds to the C language, it makes sense that the "@" would symbolize "an Objective-C object".

There is no automatic concatentation using the plus sign (+), because NSString* is a pointer type, and Objective-C is a strict superset of C, and so, consequently, adding to an NSString* object does pointer manipulation. Objective-C does not have any operator overloading feature as in the C++ language.