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