Constructing DO Loops (3)

Generating Data with DO Loops

Constructing DO Loops

DO Loop Execution

Using the form of the DO loop just presented, let's see how the DO loop executes in the DATA step. This example calculates the interest earned each month for a one-year investment.

     data finance.earnings;
        Amount=1000;
        Rate=.075/12;
        do month=1 to 12;
           Earned+(amount+earned)*(rate);
        end;
     run;

This DATA step does not read data from another source. When submitted, it compiles and then executes only once to generate data. During compilation, the program data vector is created for the Finance.Earnings data set.

Program Data Vector

When the DATA step executes, the values of Amount and Rate are assigned.

Program Data Vector

Next, the DO loop executes. During each execution of the DO loop, the value of Earned is calculated and added to its previous value. On the twelfth execution of the DO loop, the program data vector looks like this:

Program Data Vector

After the twelfth execution of the DO loop, the value of month is incremented to 13. Because 13 exceeds the stop value of the iterative DO statement, the DO loop stops executing, and processing continues to the next DATA step statement. The end of the DATA step is reached, the values are written to the Finance.Earnings data set, and in this example, the DATA step ends. Only one observation is written to the data set.

SAS Data Set Finance.Earnings

Amount Rate month Earned

1000 0.00625 13 77.6326

Amount	Rate	month	Earned
1000	0.00625	13	77.6326

Notice that the index variable month is also stored in the data set. In most cases, the index variable is only needed to process the DO loop and can be dropped from the data set.