Zero [was Re: What is considered an "advanced" topic in Python?]

[Marko Rauhamaa]

Steven D'Aprano <steve at pearwood.info>:

> What does zero mean, and how do engineers misunderstand it?

Off the top of my head:

 * Code that does:

      if elements:
          for element in elements:
              ...

   instead of:

      for elements in elements:
          ...

 * C++ insists that all objects have nonzero sizes. This program:

   ====================================================================
   #include <stdio.h>
   struct S {};
   int main()
   {
       printf("%zd\n", sizeof(struct S));
       return 0;
   }
   ====================================================================

   prints out 1 when compiled with c++ but 0 when compiled with cc.

 * In C, it used to be illegal to define a struct without a dummy field
   or a zero-length array. At one point, that created irritating trouble
   for compilers that generated C.

 * malloc(0) is allowed to return NULL as a successful return value
   inviting application programming errors. Calling free(NULL) didn't
   use to be guaranteed to work.

 * Numerous shell commands assign special meaning to zero arguments.
   For example:

      $ ls a b c
      a b c
      $ ls a b
      a b
      $ ls a
      a
      $ ls
      a b c d e f g

      $ ln -s a b c
      # creates two links under c, which must be a directory
      $ ln -s c
      # creates one link under .

 * Wild-card expansion always produces at least one result:

      for src in *.py; do
          # executed even in the absence of a .py file
      done

 * Python insists that a block contain at least one statement even
   though technically, it is not necessary for syntactic reasons. Not a
   biggie but can cause a minor annoyance when commenting out a line.

 * Numerous timer implementations take 0 to mean infinity. For example,
   in Java:

       public final void wait(long timeout)
                throws InterruptedException

       [...] If timeout is zero, however, then real time is not taken
       into consideration and the thread simply waits until notified.

       <URL: https://docs.oracle.com/javase/7/docs/api/java/lan
       g/Object.html#wait%28long%29>

   A nasty surprise for those who'd naively expect zero to mean zero,
   especially if zero is the result of downward rounding.


Marko