Creating a file name by adding two NSStrings

I am trying to load a file. The file name depends on a string stored in an array. In the following example, I would like to load tabRed.png. @"Red" is stored in an array.

This is what I tried:

UIImage *tabImage = [UIImage imageNamed:(@"tab%@.png", [self.currentNoteBook.tabColours objectAtIndex:0])];

But I only got "Red" as an output and not 开发者_StackOverflow中文版"tabRed.png". What am I doing wrong?

UIImage *tabImage = [UIImage imageNamed:[NSString stringWithFormat:@"tab%@.png", [self.currentNoteBook.tabColours objectAtIndex:0]]];

You need to use NSString stringWithFormat. The comma operator (,) does quite a different thing.

From Wikipedia,

In the C and C++ programming languages, the comma operator (represented by the token ,) is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type).

So when you write (@"tab%@.png", [self.currentNoteBook.tabColours objectAtIndex:0]), it actually discards the first string and returns the object in array which is just "Red".

You need to make a new NSString for the file name. The string @"tab%@.png" won't automatically replace the placeholder for you. Use -stringWithFormat: as follows:

UIImage *tabImage = [UIImage imageNamed:[NSString stringWithFormat:@"tab%@.png", [self.currentNoteBook.tabColours objectAtIndex:0]]];