The most critical distinction between DO WHILE and DO UNTIL is evaluation timing. A DO WHILE loop checks the condition at the top; if the condition isn't met initially, the code block is skipped entirely. In contrast, DO UNTIL checks the condition at the bottom, guaranteeing that the loop executes at least once.
When using these in SAS Viya or CAS, pay close attention to your "exit strategy." Because DO UNTIL evaluates at the end, an incorrect logical expression (like checking for balance=0 when calculations result in a tiny decimal like 0.00001) can trigger an infinite loop. This can hang your session and consume significant cloud resources. Always ensure your increment or decrement logic will eventually meet the exit criteria, especially when dealing with floating-point financial data.
Examples use generated data (datalines) or SASHELP.
1 Code Block
DATA STEP Data
Explanation : This example uses an iterative DO statement to repeatedly decrement the 'balance' variable values and write these values to the output dataset. The DATA step runs on the SAS Compute server.
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data loan;
balance=10000;
do payment_number=1 to 10;
balance=balance-1000;
output;
end;
run;
proc print data=loan;
run;
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DATA loan;
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balance=10000;
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DO payment_number=1 to 10;
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balance=balance-1000;
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OUTPUT;
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END;
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RUN;
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PROC PRINTDATA=loan;
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RUN;
2 Code Block
DATA STEP Data
Explanation : This example uses the same iterative DO loop to execute the DATA step in CAS and create an output CAS table.
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cas casauto sessopts=(caslib='casuser');
libname mylib cas;
data mylib.loan;
balance=10000;
do payment_number=1 to 10;
balance=balance-1000;
output;
end;
run;
proc print data=mylib.loan;
run;
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cas casauto sessopts=(caslib='casuser');
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LIBNAME mylib cas;
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DATA mylib.loan;
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balance=10000;
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DO payment_number=1 to 10;
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balance=balance-1000;
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OUTPUT;
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END;
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RUN;
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PROC PRINTDATA=mylib.loan;
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RUN;
3 Code Block
DATA STEP Data
Explanation : This example shows how to nest one DO loop within another to calculate the value of a one-year investment earning 7.5% annual interest, compounded monthly.
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data earn;
Capital=2000;
do Year=1 to 10;
do Month=1 to 12;
Interest=Capital*(.075/12);
Capital+Interest;
output;
end;
end;
run;
proc print data=earn; run;
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DATA earn;
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Capital=2000;
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DO Year=1 to 10;
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DO Month=1 to 12;
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Interest=Capital*(.075/12);
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Capital+Interest;
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OUTPUT;
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END;
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END;
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RUN;
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PROC PRINTDATA=earn; RUN;
4 Code Block
DATA STEP Data
Explanation : This example uses DOLIST syntax to iterate over a list of values, including single, multiple, expressions, and sequences with or without variables.
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data do_list;
x=-5;
do i=5, /*a single value*/
5, 4, /*multiple values*/
x + 10, /*an expression*/
80 to 90 by 5, /*a sequence*/
60 to 40 by x; /*a sequence with a variable*/
output;
end;
run;
proc print data=do_list; run;
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DATA do_list;
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x=-5;
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DO i=5, /*a single value*/
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5, 4, /*multiple values*/
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x + 10, /*an expression*/
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80 to 90BY5, /*a sequence*/
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60 to 40BY x; /*a sequence with a variable*/
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OUTPUT;
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END;
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RUN;
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PROC PRINTDATA=do_list; RUN;
5 Code Block
DATA STEP Data
Explanation : This example uses iterative DO TO syntax to repeatedly increment the values of 'x'. The loop also uses the OUTPUT statement to write each incremented 'x' value to the output dataset.
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data do_to;
x=0;
do i=0 to 10;
x=x+1;
output;
end;
run;
proc print data=do_to;
run;
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DATA do_to;
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x=0;
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DO i=0 to 10;
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x=x+1;
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OUTPUT;
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END;
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RUN;
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PROC PRINTDATA=do_to;
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RUN;
6 Code Block
DATA STEP Data
Explanation : This example uses iterative DO TO-value BY-increment syntax to repeatedly increment the values of 'x' by '1'. The BY-increment value specifies that the index variable increments by '2' with each loop execution. The loop also uses the OUTPUT statement to write each incremented 'x' value to the output dataset.
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data do_to_by;
x=0;
do i=0 to 10 by 2;
x=x+1;
output;
end;
run;
proc print data=do_to_by;
run;
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DATA do_to_by;
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x=0;
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DO i=0 to 10BY2;
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x=x+1;
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OUTPUT;
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END;
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RUN;
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PROC PRINTDATA=do_to_by;
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RUN;
7 Code Block
DATA STEP Data
Explanation : The first DATA step in this example uses a DO WHILE loop to repetitively execute a group of statements as long as a condition is true. The program calculates the number of payments required for a specified loan amount by repeatedly iterating to decrement 'Balance' as long as the balance is greater than zero. The DATA step runs on the SAS Compute server.
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data loan;
balance=10000;
payment=0;
do while (balance>0);
balance=balance-1000;
payment=payment+1;
output;
end;
run;
proc print data=mylib.loan;
run;
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DATA loan;
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balance=10000;
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payment=0;
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DO while (balance>0);
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balance=balance-1000;
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payment=payment+1;
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OUTPUT;
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END;
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RUN;
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PROC PRINTDATA=mylib.loan;
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RUN;
8 Code Block
DATA STEP Data
Explanation : This DATA step loads the same data into a table in a different library, 'mylib.loan', and then calculates the number of payments that must be made for the specified loan. It does this by repeatedly iterating and decrementing 'Balance' until the balance is zero.
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libname mylib;
data mylib.loan;
balance=10000;
payment=0;
do until (balance=0);
balance=balance-1000;
payment=payment+1;
output;
end;
run;
proc print data=mylib.loan;
run;
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LIBNAME mylib;
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DATA mylib.loan;
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balance=10000;
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payment=0;
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DO until (balance=0);
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balance=balance-1000;
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payment=payment+1;
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OUTPUT;
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END;
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RUN;
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PROC PRINTDATA=mylib.loan;
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RUN;
Pro Tip
The critical distinction between these loops lies in the timing of the evaluation: DO WHILE checks the condition at the top and may never execute if the condition is false initially, whereas DO UNTIL checks at the bottom and guarantees at least one iteration, which can cause logic errors or negative balances if your starting value (like 'balance') is already zero.
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