tungwaiyip.info

home

about me

links

my software

Media

Yucatán Photos

St Lucia Photos

Photo Album

Videos

Blog

< June 2005 >
SuMoTuWeThFrSa
    1 2 3 4
5 6 7 8 91011
12131415161718
19202122232425
2627282930  

past articles »

Click for San Francisco, California Forecast

San Francisco, USA

 

Python's half open index notation

Beginner programmers often wonder about Python's sequence indexing and slicing notation. Array index starts from 0. Slicing uses half open notations, where L[a:b] is a subsequence with index x where a <= x < b.

Why is the endpoint excluded? Isn't it more intuitive if array index starts from 1 and the endpoint is included, so that a 3 elements array is referenced as L[1:3] with items L[1], L[2], L[3]?

It turns out this notation is an elegant and deliberate design and it has some excellent properties.

We write programs to operate on arrays, to find their length, traverse the subsequences, split them or join them. The half open notation always show a simple pattern. But the inclusive notation often requires adding 1 or substracting 1 to the indexes in many operations. Thus it is more vulnerable to off-by-one-error. This article One True Way of array indexing discuss this at length. I have reproduce its example (with corrections) below:

Operation Half open Inclusive
length of a slice L[a:b] b-a (b-a+1)
first n characters of L L[:n] L[1:n]
last n characters of L L[-n:] L[len(L)-n+1:]
The identity slice L == L[0:len(L)] == L[:] L[1:len(L)]
The empty slice L[a:a] is empty for any a. perhaps L[a:a-1]?
A slice of length n, from point a L[a:a+n] L[a:a+n-1]
Split L[a:b] at index c L[a:b] == L[a:c]+L[c:b] L[a:c-1]+L[c:b]

Another important property is an empty sequence can be expressed by L[a:a], while there is no natural way to express an empty sequence with the inclusive notation. But do we really need to care about a special case? Absolutely! In fact failure to account for empty input is one of the most common error. Just like zero is a fundamental concept in mathematics, always think how you program can handle null input. An inferior approach is to represent empty sequence by None or null pointer. This creates a special case so that a variable need to be tested before dereferencing. Failure to do so contributes to unexpected exceptions. It is an elegant design that L[a:b] can also represent sequences with 0 length.

C++'s STL also choose this notation to represent a range. According to the literature this is crucial because "algorithms that operate on n things frequently require n+1 positions. Linear search, for example (find) must be able to return some value to indicate that the search was unsuccessful." I have seen so many people flunked link list or data structure exercises because they have trouble dealing with the end of a list. Often a good solution is shift the focus beyond the n concrete objects to the n+1 positions around them. I hope this help to make sense of the half open notation.

2005.06.16 [, ] - comments

 

 

blog comments powered by Disqus

past articles »

 

BBC News

 

Deadly attacks hit Egypt's Sinai (30 Jan 2015)

 

EU extends sanctions against Russia (29 Jan 2015)

 

Balloonists surpass distance record (30 Jan 2015)

 

Philippine mourning for slain police (30 Jan 2015)

 

China warns universities on values (30 Jan 2015)

 

Saudi king reshuffles cabinet (30 Jan 2015)

 

Congress approves Keystone pipeline (29 Jan 2015)

 

Russian planes 'disrupted UK aviation' (30 Jan 2015)

 

Man with fake gun disrupts Dutch TV (30 Jan 2015)

 

German ranchers killed in Paraguay (30 Jan 2015)

more »

 

Slashdot News for nerds, stuff that matters

 

Microsoft To Invest In Rogue Android Startup Cyanogen (2015-01-30T00:55:00Z)

 

LibreOffice Gets a Streamlined Makeover With 4.4 Release (2015-01-30T00:11:00Z)

 

D-Link Routers Vulnerable To DNS Hijacking (2015-01-29T23:27:00Z)

 

Spire Plans To Use Tiny Satellites For More Accurate Weather Forecasts (2015-01-29T23:06:00Z)

 

FSF-Endorsed Libreboot X200 Laptop Comes With Intel's AMT Removed (2015-01-29T22:43:00Z)

 

'Anonymized' Credit Card Data Not So Anonymous, MIT Study Shows (2015-01-29T22:12:00Z)

 

VP Anthony Moschella Shows Off Makerbot's Latest Printers and Materials (Video) (2015-01-29T21:28:00Z)

 

US Air Force Selects Boeing 747-8 To Replace Air Force One (2015-01-29T20:46:00Z)

more »

 

TechPsychic Tech Rumors and Invented News

more »

 

SF Gate

 

Bay Area News (7 Jan 2012)

 

City Insider (11 Feb 2012)

 

Crime Scene (13 Feb 2012)

 

C.W Newius Column (10 Jan 2012)

 

C.W. Nevius Blog (11 Feb 2012)

 

Education News (10 Jan 2012)

 

KALW (11 Feb 2012)

 

Matier and Ross Blog (11 Feb 2012)

 

Twitch hits milestone: 100 million monthly viewers (29 Jan 2015)

 

Judge tosses challenge to SF 'Airbnb law’ but future suits likely (29 Jan 2015)

 

Twitter can predict heart disease, study says (29 Jan 2015)

 

How Yahoo plans to save billion in tax on Alibaba stock (29 Jan 2015)

 

Your genes may be to blame for your motion sickness, 23andMe says (29 Jan 2015)

 

Fed will stay 'patient’ on interest rate increase (29 Jan 2015)

more »

 

Asia Times Online

 

SPENGLER How US policy blunders hurt Israel (Wed 28 Jan 2015 11:00:00 GMT)

 

West's agri-giants snap up Ukraine (Wed 28 Jan 2015 11:00:00 GMT)

 

India v China: Border games (Wed 28 Jan 2015 11:00:00 GMT)

 

'Turk Stream' far from a done deal (Wed 28 Jan 2015 11:00:00 GMT)

 

The collapse of Europe? (Wed 28 Jan 2015 11:00:00 GMT)

 

Citic sale faced government doubts (Wed 28 Jan 2015 11:00:00 GMT)

more »

 


Site feed Updated: 2015-Jan-30 00:00