Editing Jackson structured programming (section)

== Motivation for the method ==
At the time that JSP was developed, most programs were batch COBOL programs that processed sequential files stored on tape.  A typical program read through its input file as a sequence of records, so that all programs had the same structure&mdash; a single main loop that processed all of the records in the file, one at a time.  Jackson asserted that this program structure was almost always wrong, and encouraged programmers to look for more complex data structures.  In Chapter 3 of ''Principles of Program Design''<ref name="PoPD"/> Jackson presents two versions of a program, one designed using JSP, the other using the traditional single-loop structure.  Here is his example, translated from COBOL into Java.  The purpose of these two programs is to recognize groups of repeated records (lines) in a sorted file, and to produce an output file listing each record and the number of times that it occurs in the file.

Here is the traditional, single-loop version of the program.

<syntaxhighlight lang="java">
String line;
int count = 0;
String firstLineOfGroup = null;

// begin single main loop
while ((line = in.readLine()) != null) {
    if (firstLineOfGroup == null || !line.equals(firstLineOfGroup)) {
        if (firstLineOfGroup != null) {
            System.out.println(firstLineOfGroup + " " + count);
        }
        count = 0;
        firstLineOfGroup = line;
    }
    count++;
}
if (firstLineOfGroup != null) {
    System.out.println(firstLineOfGroup + " " + count);
}
</syntaxhighlight>

Here is a JSP-style version of the same program.  Note that (unlike the traditional program) it has two loops, one nested inside the other.  The outer loop processes groups of repeating records, while the inner loop processes the individual records in a group.
<syntaxhighlight lang="java">
String line;
int numberOfLinesInGroup;

line = in.readLine();
// begin outer loop: process 1 group
while (line != null) {  
    numberOfLinesInGroup = 0;
    String firstLineOfGroup = line;

    // begin inner loop: process 1 record in the group
    while (line != null && line.equals(firstLineOfGroup)) {
        numberOfLinesInGroup++;
        line = in.readLine();
    }
    System.out.println(firstLineOfGroup + " " + numberOfLinesInGroup);
}
</syntaxhighlight>

Jackson criticises the traditional single-loop version for failing to process the structure of the input file (repeating groups of records containing repeating individual records) in a natural way.  One sign of its unnatural design is that, in order to work properly, it is forced to include special code for handling the first and last record of the file.