字符串操作：
http://www.myexception.cn/mobile/455287.html
1,判断两字符串是否相同
NSString *str1 = @"hello pepe";
NSString *str2=[NSString stringWithFormat:@"hello %s","pepe"];
if([str1 isEqualToString:str2]){
NSLog(@"相同");
}
else{
NSLog(@"不同");
}
2,浮点型运算
CGRect screenRect=[UIScreenmainScreen].bounds;
CGFloat widthf= (screenRect.size.height-self.adView.frame.size.width)/2;
CGFloat heighf=screenRect.size.width-self.adView.frame.size.height-50.0f;