SAS Data Step’s Built-in Loop: An illustrated Example

Some newbie SAS programmers take SAS as their first programming language even learned. Sometimes they are confused by the concept of “data step’s built-in loop” even after reading the well-written The Little SAS Book: A Primer:

DATA steps also have an underlying structure, an implicit, built-in loop. You don’t tell SAS to execute this loop: SAS does it automatically. Memorize this:

DATA steps execute line by line and observation by observation.

Programmers could memorize the statement above and apply it well in their programming practices, but still find it hard to get the vivid idea about the so called implicit built-in loop. –This post would make it easy.

The following will show an explicit loop example in C++. Note that you do not need to know any about C++ to get the idea. Suppose that a data file data.dat in D driver holds three numbers


The question is how to (read and) print out these numbers and their sums.  Following is the C++ approach (just read the bold session):

#include <iostream>
#include <fstream>
using namespace std;
int main()
int x;
int sum=0;
ifstream inFile;“d:data.dat”);
inFile >> x;

while (!inFile.eof( ))
sum = sum + x;
inFile >> x;

inFile.close( );
cout << “Sum = ” << sum << endl;
return 0;

There is an explicit loop in these C++ codes: while (!inFile.eof( )) .  While it is not at the end of infile, the codes above will keep print out the numbers and do the accumulation. The final output is


The following SAS codes produce the exactly same output:

data _null_;
infile “d:data.dat” end=eof;
input x;
put x;
if eof then put sum=;

Note that SAS codes do not need an explicit loop to reach to the end of file. There is a so called implicit built-in loop.<the end>


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