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            //字符串转变为数组1
            NSMutableString * str=[[NSMutableString alloc]initWithFormat:@"1,1,1,1,1,1"];
            //字符串转变为数组2
            NSMutableArray * array=[NSMutableArray arrayWithArray:[str   componentsSeparatedByString:@","]];
            //替换数组中的某个值
            [array replaceObjectAtIndex:4 withObject:@"2"];
            //把数组转换成字符串
            NSString *ns=[array componentsJoinedByString:@","];