NSSuperscriptAttributeName show error?

I am trying to create a subscripted text in iphone using NSMutableAttributedString. But the "NSSup开发者_开发百科erscriptAttributeName" show some error " undeclared identifier ". How can i solve it?

NSDictionary * superscriptAttrs = [NSDictionary dictionaryWithObject:[NSNumber numberWithInt:1] 
                                                                  forKey:NSSuperscriptAttributeName];
    NSAttributedString * st = [[NSAttributedString alloc] initWithString:@"st"
                                                              attributes:superscriptAttrs];

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Assuming that you're on iOS, you need to use a different set of constants which you can find here.

In your case NSSuperscrptAttributeName should be kCTSuperscriptAttributeName.