setString NSMutableString

I am new to programming in XCode, and as a first familiarisation exercise, I would like to make a small Calculator app. So obviously I came upon a few problems: I try to have the user use buttons to type the digits and operators into a string, which he then sees. When I use

displayString = [NSString stringWithFormat:@"%@%@", displayString, operatorString];

everything works fine. Then I decided to work with NSMutableStrings in order to get deletebuttons. I changed the headerfile appropriately, after which I still the earlier NSString functionality (? because NSMutableString inherits from NSString?) And then I got the following problem:

[displayString setString:@"test"];
NSLog(@"%@", displayString);

Even when this is the first method I call, I still get nul. I What am I doing wrong?

Unrelate开发者_StackOverflow社区d, is there a way to have this string write to a sort of function: I would like to be able to do

result = contentof:displayString

or something a like. Any ideas in how I could do this?

NSString doesn't have a setString: method. (Documentation)

For this case, you'd need an NSMutableString. Here's how you'd implement it:

NSMutableString *mutableString = [[NSMutableString alloc] initWithFormat:"%@%@", displayString, operatorString];

... and then to change it:

[mutableString setString:@"test"];