🚀 go-pugleaf

In article <1436@fisher.UUCP>, njt@fisher.UUCP (Nathaniel Thurston) writes:
- In article <219@analog.UUCP> kim@analog.UUCP (Kim Helliwell) writes:
- >> 	for (i = ntables; i--; ) <statement>
- >> 	if (i == -1) <more statements>
- >> 
- >> I would expect this to get the value of i, decrement the stored value,
- >> and then exit or execute the statement.  Rather, it tests the value
- >> before it decrements, and the loop exits with i = 0.  Is this a bug
- >> in the code generation, or is C supposed to do this?
- >> 
- >> 	Greg
- >
- >	for (i = ntables; ; i--) <statement>
- >	if (i = -1) <more statements>
- >
- >Kim Helliwell
- 
- You're both wrong.  The first loop should perform the action Greg describes,
- using the value before decrementing.  To have C test the value after
- decrementing, use --i.  In the "fix" that Kim gives, the loop will never
- exit without a break statement.
- The correct code is:
- 	for (i = ntables; --i; ) <statement>
- 	if (i == -1) <more statements>
- -- 
- 

I believe Greg is right.  When the loop falls through, `i' should end
up as -1.  If in fact it does not, the C compiler forgot to generate
the trailing DEC instruction on that branch of the test.  This seems
serious, since checking a post-condition like this is fairly common in
C; are you sure?

Kim's code is wrong, and would never exit (empty condition == true).
Also the trailing test has an assignment (i = -1) instead of a
conditional (i == -1).  Thus, that test would always succeed.

Nathaniel's code is wrong, since I presume Greg intended to execute the
<more statements> line if the loop fell through (vs. a `break');
these statements would not be executed if the loop fell through at
i==0.  The correct condition here (if it were the right test, which it
ISN'T for traversing arrays) would be `if (i == 0) ...'.

Sigh.

						- Matt
------- 
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