_N_是什么意思？

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Example Here are the flower sales data consisting of customer ID, date of sale, variety of flower, and quantity.
240W 02-07-2008 Ginger 120
240W 02-07-2008 Protea 180
356W 02-08-2008 Heliconia 60
356W 02-08-2008 Anthurium 300
188R 02-11-2008 Ginger 24
188R 02-11-2008 Anthurium 24
240W 02-12-2008 Heliconia 48
240W 02-12-2008 Protea 48
356W 02-12-2008 Ginger 240
In this example, the grower wants a program that will find the customer with the single largest order, and print all the orders for that customer.
* Read the raw data;
DATA flowersales;
INFILE 'c:\MySASLib\TropicalSales.dat';
INPUT CustomerID $4. @6 SaleDate MMDDYY10. @17 Variety $9. Quantity;
PROC SORT DATA = flowersales;
BY DESCENDING Quantity;
RUN;
* Find biggest order and pass the customer id to a macro variable;
DATA _NULL_;
SET flowersales;
IF _N_ = 1 THEN CALL SYMPUT("selectedcustomer",CustomerID);
ELSE STOP;
RUN;
PROC PRINT DATA = flowersales;
WHERE CustomerID = "&selectedcustomer";
FORMAT SaleDate WORDDATE18.;
TITLE "Customer &selectedcustomer Had the Single Largest Order";
RUN;

请问各位高手，IF _N_ = 1 是什么意思？
网上查到是：An internal SAS counter in the data step which contains the current iteration number of the data step.
能不能具体点？好像还没理解
谢谢大虾

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关键词：是什么意思 Data step iteration Customer Selected customer example single

_n_是SAS系统变量，是在运行过程中产生的变量，作用就是记录当前是第几条record在运行

the first observation in the dataset.

与STATA中的_n,_N类似

_n_=1就是第一行。。。

data步的循环次数
某种意义上是第几条记录......

6# soporaeternus
如果循环1次的话，应该输出
Customer 356W Had the Single Largest Order
       Customer
Obs    ID               SaleDate             Variety    Quantity
1      356W      February 8, 2008     Anthurium    300

而书上是：
Customer 356W Had the Single Largest Order
       Customer
Obs    ID               SaleDate             Variety    Quantity
1      356W      February 8, 2008     Anthurium    300
2      356W      February 12, 2008   Ginger         240
5      356W      February 8, 2008     Heliconia       60
是我哪里弄错了吗？麻烦能不能回答一下？

没错吧，它只是把ID记住了。根据where语句的定义，它是找出ID，然后输出这个ID的所有记录

* Find biggest order and pass the customer id to a macro variable;
DATA _NULL_;
SET flowersales;
IF _N_ = 1 THEN CALL SYMPUT("selectedcustomer",CustomerID);这里找出了order最大的customerid；
ELSE STOP;
RUN;



PROC PRINT DATA = flowersales;
WHERE CustomerID = "&selectedcustomer"; 这里选出该ID的所有记录
FORMAT SaleDate WORDDATE18.;
TITLE "Customer &selectedcustomer Had the Single Largest Order";
RUN;

问题的要求是要“find the customer with the single largest order, and print all the orders for that customer”，楼主运行结果与书上不一致的主要问题在于：
INPUT CustomerID $4. @6 SaleDate MMDDYY10. @17 Variety $9. Quantity;
上面的输入格式是mixed！
楼主可以试着把“Variety $9. ”中“$”符号前面加“：”，变成“Variety ： $9.”再试试，输出的结果应该就一致了！