Perl undef

undef is not a constant. If you say

$a = undef;

you are not doing what you intend.

undef works like an operator

undef $a;

and can be used with return

return undef;

and can be used in an assignment

$a = undef;

Perl push/pop

NB that if you are using a Perl array as a stack, by pushing and popping with it, the top-of-stack item has the highest index, not index 0.

Perl character classes

\d \s \w and their inverses can be used in Perl regexp character classes.

Metacharacters in character classes are - ] \ ^ $

Perl if

The postfix if (and unless, while, until) in Perl aren't operators but statement modifiers. So operator precedence is not a concern when they follow an expression.

Circuit analysis with diodes

(Non-PDF notes on http://www.ittc.ku.edu/~jstiles/312/handouts/The%20Ideal%20Diode%20Circuit%20Analysis%20Guide.pdf)

1. Guess the bias of each diode, ie assume forward or reverse bias (FB/RB)
2. 'Enforce' the assumptions by replacing
(a) FB diodes with a short circuit (0 ohm link) so that there is zero voltage drop across them
(b) RB diodes with an open circuit (break) so that there is zero current through them.
3. Determine the circuit values in the usual way, and use them to check the assumptions you made:
(a) A short circuit (FB) should have positive current flowing through it
(b) An open circuit (RB) should have negative voltage across its two ends.
NB to check for these conditions, not for the 'zero' conditions in point 2.
4. Any assumptions that aren't met should be reversed, and the process gone through again.

Then, because real diodes aren't ideal diodes and do have a voltage drop across them, adjust the analysis to take the drops into account.

Win32 Perl

It appears to be SIGHUP that Perl gets when its console window is closed, and SIGINT when Ctrl-C is pressed.

This makes sense as 'SIGHUP is a signal sent to a process when its controlling terminal is closed' and 'SIGINT is the signal sent to a process by its controlling terminal when a user wishes to interrupt the process'.

.NET timespan

NB that Timespans have a sign (because they are the result of subtracting one DateTime from another).

.NET time and date

If you parse a string that only contains a time into a DateTime, the result is that time on the current day. Eg '11:14' parsed on Jan 6 2011 => 20119601T1114.

If you parse a string that only contains a date into a DateTime, the result is midnight (0000) on that day. Eg '01/01/2011' => 20110101T0000. You also get this if you take the Date property of any DateTime.

The difference between two DateTimes (including one returned from .Date) obtained with '-' is a TimeSpan. Similarly, if you want to add to a datetime with '+', the addend must be a TimeSpan.

C# is extremely picky about these definitions, which is a pain, but at least the system is consistent once you understand it. You aren't guessing like with SQL.

C# case

switch (expression)
{
case constant-expression:
statement
jump-statement
[default:
statement
jump-statement]
}

jump-statement is either break, or goto case-label, where default is also an acceptable label. Eg 'goto case "x"'.

Fall-through from case to case is not allowed, except when you precede a case with any number of empty cases.