Don’t believe everything you learn - PHPSouthCoast

Don’t believe
everything you learn
@mheap at #phpsc17

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Things we believe are true

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Things we believe are true
But they’re not

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Falsehoods programmers
believe about X

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Michael

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Time

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@mheap
The number of days in every month
starts with a 3, so either 30 or 31?

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@mheap
The number of days in every month
starts with a 3, so either 30 or 31?
February has 28

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#phpsc17
Each month always has the same
number of days every year though?

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#phpsc17
Each month always has the same
number of days every year though?
February has 29 in leap years

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@mheap
But we can calculate leap years.
They’re every four years

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@mheap
Unless it’s divisible by 100

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#phpsc17
(Unless it’s divisible by 100)

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#phpsc17
(Unless it’s divisible by 100)
If it divides by 100 and 400, it
is a leap year

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@mheap
But that rule holds all the time

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@mheap
But that rule holds all the time
Unless you’re Sweden

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#phpsc17
February is 28 or 29 days
February the 30th doesn’t exist

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#phpsc17
Unless you’re Sweden

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#phpsc17

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#phpsc17
Or the Soviet Union

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@mheap
A month starts and ends in
the same year

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@mheap
the same year
Not in ancient Rome

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@mheap
the same year

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@mheap
the same year
Or a modern Ethiopia

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#phpsc17
Excluding DST, time always
goes forwards

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#phpsc17
Excluding DST, time always
goes forwards
Shanghai, 1927
1927-12-31 23:54:08 happened twice

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Modern Time

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@mheap
The day before Saturday
is always Friday

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@mheap
The day before Saturday
is always Friday
Samoa, 2011

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#phpsc17
The offsets between two timezones
will always be the same

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#phpsc17
The offsets between two timezones
will always be the same
10 changes in 2016
2 changes so far in 2017

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@mheap
Timezones are always +/-
an integer number of hours

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@mheap
Timezones are always +/-
an integer number of hours
Afghanistan, +04:30
New Zealand (Chatham Islands), +12:45

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#phpsc17
15 minutes is safe though?

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#phpsc17
15 minutes is safe though?
UTC-00:25:21 - Dublin Mean Time

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@mheap
If they do change, there’ll be
plenty of notice

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@mheap
If they do change, there’ll be
plenty of notice
North Korea, 2015
8 days notice of a 30 minute change

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Audience Participation

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@mheap
There are always 24 hours in a day

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@mheap
There are always 24 hours in a day
Daylight Savings Time
Means 23 or 25 hours in a day

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#phpsc17
Is the time 23:59:60 invalid?

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#phpsc17
Is the time 23:59:60 invalid?
Leap seconds
A minute can have 61 seconds

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Names
HELLO
, M
Y
N
AM
E
IS

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@mheap
A child will have the same surname as
either their mother or father (or both)

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@mheap
A child will have the same surname as
either their mother or father (or both)
Not in Iceland

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#phpsc17
People have at least 2 names

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#phpsc17
People have at least 2 names
Teller (of Penn & Teller fame)
Akihito (Emperor of Japan)

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@mheap
People’s names do not
contain numbers

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@mheap
contain numbers
John 5

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#phpsc17
contain numbers

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#phpsc17
contain numbers
K8lyn? (Kaitlyn)

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#phpsc17
contain numbers

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#phpsc17
contain numbers
KVIIIlyn

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@mheap
For every rule, there are
guaranteed to be exceptions

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@mheap
No spaces
Van Buren

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@mheap

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@mheap
Only letters
O’Reilly

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@mheap

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@mheap
One capital letter
McDonald

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HELLO
, M
Y
N
AM
E
IS

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@mheap
A name must contain one or
more alphanumeric characters

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@mheap
more alphanumeric characters
⽑毛泽东

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#phpsc17
more unicode characters

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@mheap
I can safely assume that this list
of bad words has no names in it

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@mheap
Ever been to Middlesex?

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@mheap

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@mheap
Eaten shittake mushrooms?

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@mheap

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@mheap
Been named Craig Cockburn?

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@mheap

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@mheap
Linda Callahan?

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@mheap

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#phpsc17
What about less obvious ones?

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#phpsc17
What about less obvious ones?
Super Bowl XXX

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@mheap
Did you hear about planning
permission emails bouncing?

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@mheap
Did you hear about planning
permission emails bouncing?
Erection of a structure

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#phpsc17
The sad story of

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#phpsc17
The sad story of
Christopher Null

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Addresses

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#phpsc17
A building number is numeric

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#phpsc17
A building number is numeric
1A Egmont Road, Middlesbrough,
TS4 2HT

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@mheap
No buildings are numbered zero

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@mheap
No buildings are numbered zero
0 Egmont Road, Middlesbrough,
TS4 2HT

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#phpsc17
If the addresses on the left are even,
addresses on the right must be odd

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#phpsc17
If the addresses on the left are even,
addresses on the right must be odd
Seine-Saint-Denis, France
Even numbers on both sides, no odds

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@mheap
You can omit leading zeros

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@mheap
You can omit leading zeros
101 Alma St, Palo Alto
Apartments 1 and 001 on different floors.

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#phpsc17
A street name won't include a number

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#phpsc17
A street name won't include a number
8 Seven Gardens Burgh, Woodbridge,
IP13 6SU

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@mheap
You wouldn’t name a town Street

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@mheap
You wouldn’t name a town Street
Street, Somerset, UK

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#phpsc17
You can’t have two towns/cities with
the same name in a country

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#phpsc17
You can’t have two towns/cities with
the same name in a country
The UK has three places
named Newport

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@mheap
Those cities don’t have
duplicate street names

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@mheap
Those cities don’t have
duplicate street names
10 High Street, Newport, PO30 1SS
10 High Street, Newport, NP20 1FQ
10 High Street, Newport, TF10 7AN

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#phpsc17
Street names will be
reasonably short

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#phpsc17
Street names will be
reasonably short
Aleja Alije Izetbegovića Prvig
Predsjednika Predsjedništva Republika
Bosna i Hercegovina

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@mheap
A building has a single postcode

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@mheap
A building has a single postcode
DVLA Swansea:
V5Cs are processed at SA99 1BA,
Driving licences at SA99 1AB

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#phpsc17
Addresses are a fixed point
in space

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#phpsc17
Addresses are a fixed point
in space
BF1 4FB

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@mheap
Concrete buildings are a fixed
point in space

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@mheap
Concrete buildings are a fixed
point in space
In Zürich, a 6200 ton building was
moved by 60 meters to make way for
railway tracks

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#phpsc17
Do all countries have a
capital city?

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#phpsc17
Do all countries have a
capital city?
Switzerland does not. The government
is currently in Bern, but the city is not
the capital

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Commerce

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#phpsc17
A product has one price

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#phpsc17
A product has one price
Price with tax, price without tax, sale
price, list price, minimum price

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@mheap
A product has a price

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@mheap
A product has a price
Products sold on auction sites
do not have a price until it’s sold

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#phpsc17
Currencies symbols uniquely
identify a currency

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#phpsc17
identify a currency
The peso and dollar sign $ is
used by many countries

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@mheap
Currencies have a unicode symbol
that represents them

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@mheap
Currencies have a unicode symbol
that represents them
The Swiss franc does not
Until 2010, neither did the Indian rupee

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#phpsc17
identify a currency

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#phpsc17
identify a currency
The peso and dollar sign $ is
used by many countries

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@mheap
Locale doesn’t matter for currency

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@mheap
Locale doesn’t matter for currency
1.000

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#phpsc17
The currency symbol always precedes
the amount e.g. £1.50

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#phpsc17
The currency symbol always precedes
the amount e.g. £1.50
1.50€ in Spain

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@mheap
The currency symbol always
precedes or follows the amount

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@mheap
The currency symbol always
precedes or follows the amount
1€50 in France

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Computers

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The network is reliable.
Latency is zero.
Bandwidth is infinite.
The network is secure.
Topology doesn't change.
There is one administrator.
Transport cost is zero.
The network is homogeneous.

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Don’t believe everything you learn - PHPSouthCoast

Don’t believe everything you learn - PHPSouthCoast

Michael Heap

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