NSString* str_C; // 结果字符串
NSString* str_A, str_B; //已存在的字符串，需要将str_A和str_B连接起来

//方法1
str_C = [NSString initWithFormat:@"%@,%@", str_A, str_B ];

//方法2
str_C = [str_A stringByAppendingString: str_B];

//方法3
str_C = [string stringByAppendingFormat:@"%@,%@", str_A, str_B];