topic Re: do while in SAS Programming

do while

chouchou — Wed, 15 Oct 2014 19:32:30 GMT

Hi guys, I thought Square is the same thing as n**2 because they are just the opposite side of an equation.

but I really cannot figure our why it shows different result when I run the following two codes;

data square;

do n = 1 to 1000 while (n**2 le 100);

Square = n**2;

output;
end;

put n= ; The result is N=11

run;

data square;

do n = 1 to 1000 while (Square le 100);

Square = n**2;

output;
end;

put n= ; The result is N=12 :smileyshocked:

run;

n Square

1 1

2 4

... ...

10 100

11 121

12 144

Re: do while

ballardw — Wed, 15 Oct 2014 19:50:52 GMT

Has nothing to do with the ** but the behavior of DO WHILE and evaluation of Missing values.

DO while evaluates at the start of the loop. In the first case when n**2 is used n has a value (1) and is evaluated. With the second case Square does not have any value until AFTER the first execution of the comparison (Square le 100) and since Missing is Always less than any given value it evaluates as true. Do a proc print after each of the data steps and you'll see that behavior and the second version being one step "later" for values of Square.

Re: do while

stat_sas — Wed, 15 Oct 2014 19:52:49 GMT

Values N=11 or N=12 are the loop terminators when condition was evaluated as false. In your dataset square you will have the right numbers.

Re: do while

stat_sas — Wed, 15 Oct 2014 20:11:00 GMT

Hi chouchou,

Using this, I am getting 11 values in dataset square.

data square;
do n = 1 to 1000 while (Square le 100);
Square = n**2;
output;
end;
run;

proc print data=square;
run;

Re: do while

ballardw — Wed, 15 Oct 2014 20:16:32 GMT

But since the OP was Putting the value of n After the execution of the loop it has n=12. The do set the value of N then evaluates Square. So after leaving the loop n is one larger.

Re: do while

stat_sas — Wed, 15 Oct 2014 20:33:16 GMT

@ ballardw: Agreed, dataset square will have 11 values and n will have maximum value of 11 while square will be 121 against this. After the termination of while loop now n has value of 12 which is being used in put statement and log shown in log as 12. If we use following, it will give us wrong numbers

data square;
do n = 1 to 1000 while (Square le 100);
Square = n**2;
end;
run;

proc print data=square;
run;

Re: do while

Tom — Thu, 16 Oct 2014 00:10:06 GMT

Because you are referencing the square of different values of N in the WHILE () condition.

In the first WHILE(n**2 <= 100) you are referencing the N that you just incremented and in the second WHILE(square<=100) you are referencing the square of the N from the previous iteration of the loop.

The value AFTER the loop is larger than the value in the last iteration of the loop because you are using a WHILE condition instead of an UNTIL condition.

Try :

do n = 1 to 1000 until (n**2 > 100);

do n = 1 to 1000 until (square > 100);