97 Things Every Programmer Should Know

Chapter 89. Use the Right Algorithm and Data Structure

Jan Christiaan “JC” van Winkel

A big bank with many branch offices complained that the new computers it had bought for the tellers were too slow. This was in the time before everyone used electronic banking, and ATMs were not as widespread as they are now. People would visit the bank far more often, and the slow computers were making the people queue up. Consequently, the bank threatened to break its contract with the vendor.

The vendor sent a performance analysis and tuning specialist to determine the cause of the delays. He soon found one specific program running on the terminal that consumed almost all the CPU capacity. Using a profiling tool, he zoomed in on the program and he could see the function that was the culprit. The source code read:

       for (i=0; i<strlen(s); ++i) {
           if (... s[i] ...) ...
       }

And string s was, on average, thousands of characters long. The code (written by the bank) was quickly changed, and the bank tellers lived happily ever after.…

SHOULDN’T THE PROGRAMMER have done better than to use code that needlessly scaled quadratically?

Each call to strlen traversed every one of the many thousand characters in the string to find its terminating null character. The string, however, never changed. By determining its length in advance, the programmer could have saved thousands of calls to strlen (and millions of loop executions): ...