7.4. How Long Can a Routine Be?

On their way to America, the Pilgrims argued about the best maximum length for a routine. After arguing about it for the entire trip, they arrived at Plymouth Rock and started to draft the Mayflower Compact. They still hadn't settled the maximum-length question, and since they couldn't disembark until they'd signed the compact, they gave up and didn't include it. The result has been an interminable debate ever since about how long a routine can be.

The theoretical best maximum length is often described as one screen or one or two pages of program listing, approximately 50 to 150 lines. In this spirit, IBM once limited routines to 50 lines, and TRW limited them to two pages (McCabe 1976). Modern programs tend to have volumes of extremely short routines mixed in with a few longer routines. Long routines are far from extinct, however. Shortly before finishing this book, I visited two client sites within a month. Programmers at one site were wrestling with a routine that was about 4,000 lines of code long, and programmers at the other site were trying to tame a routine that was more than 12,000 lines long!

A mountain of research on routine length has accumulated over the years, some of which is applicable to modern programs, and some of which isn't:

• A study by Basili and Perricone found that routine size was inversely correlated with errors: as the size of routines increased (up to 200 lines of code), the number of errors per line of code decreased (Basili and Perricone 1984).

• Another study found that routine size was not correlated with errors, even though structural complexity and amount of data were correlated with errors (Shen et al. 1985).

• A 1986 study found that small routines (32 lines of code or fewer) were not correlated with lower cost or fault rate (Card, Church, and Agresti 1986; Card and Glass 1990). The evidence suggested that larger routines (65 lines of code or more) were cheaper to develop per line of code.

• An empirical study of 450 routines found that small routines (those with fewer than 143 source statements, including comments) had 23 percent more errors per line of code than larger routines but were 2.4 times less expensive to fix than larger routines (Selby and Basili 1991).

• Another study found that code needed to be changed least when routines averaged 100 to 150 lines of code (Lind and Vairavan 1989).

• A study at IBM found that the most error-prone routines were those that were larger than 500 lines of code. Beyond 500 lines, the error rate tended to be proportional to the size of the routine (Jones 1986a).

Where does all this leave the question of routine length in object-oriented programs? A large percentage of routines in object-oriented programs will be accessor routines, which will be very short. From time to time, a complex algorithm will lead to a longer routine, and in those circumstances, the routine should be allowed to grow organically up to 100 200 lines. (A line is a noncomment, nonblank line of source code.) Decades of evidence say that routines of such length are no more error prone than shorter routines. Let issues such as the routine's cohesion, depth of nesting, number of variables, number of decision points, number of comments needed to explain the routine, and other complexity-related considerations dictate the length of the routine rather than imposing a length restriction per se.

That said, if you want to write routines longer than about 200 lines, be careful. None of the studies that reported decreased cost, decreased error rates, or both with larger routines distinguished among sizes larger than 200 lines, and you're bound to run into an upper limit of understandability as you pass 200 lines of code.