tianjara.net | Andrew Harvey's Blog

Its pointless repeating what John put in the lecture slides, so this is just my additions that he mentioned but are not in the slides.

Aggregates

Initial condition is defaulted to NULL. So sometimes you will need to define,

  initcond = '';

This is different to NULL, because,

  null || 'abc' --> null

(where || is append) but

  '' || 'abc' --> 'abc'

PHP

$x = 2;
myFunc() {
  global $x;
}

If we omit the global $x, then any references to $x in myFunc will refer to a new local x, not the first x that is set to 2. To avoid this and force any references to x inside myFunc to refer to the first x that is equal to 2, we need this global $x line.

strpos

1. $i = strpos('abc', 'a') --> 0
2. $i = strpos('abc', 'b') --> 1
3. $i = strpos('abc', 'z') --> false

if($i) will only be true in case 2 (false in case 1 and 3). So if we want to test if the second string was at all in the first string we must use,

if($i !== false)

This one will be true in case 1 and 2, but not 3.