python list index - an easy question

BartC bc at freeuk.com
Mon Dec 19 10:48:05 EST 2016


On 19/12/2016 13:48, Ben Bacarisse wrote:
> BartC <bc at freeuk.com> writes:

>> You need to take your C hat off, I think.
>
> It's a computing hat.  Indexes are best seen as offsets (i.e. as a
> measured distances from some origin or base).

A 1-based or N-based index can still be seen as an offset from element 
0, if that is what you want, although it might be an imaginary element.

But if you needed a table of the frequencies of letters A to Z, indexed 
by the ASCII codes of those letters, then with 0-based you will either 
waste a lot of entries (table[0] to table[64]), or need to build extra 
offsets into the code (table[letter-ord('A')]), or have to use a 
clumsier, less efficient table (ordered dict or whatever).

An N-based array can simply have bounds of ord('A') to ord('Z') 
inclusive. And the array would have just 26 elements.

   It's a model that grew
> out of machine addressing and assembler address modes many, many decades
> ago -- long before C.  C, being a low-level language, obviously borrowed
> it, but pretty much all the well-thought out high-level languages have
> seen the value in it too, though I'd be interested in hearing about
> counter examples.

Older languages tend to use 1-based indexing. Newer ones zero, but it's 
possible C has been an influence if that is what designers and 
implementers have been exposed to.

Among the 1-based or N-based languages are Fortran, Algol68 and Ada:

https://en.wikipedia.org/wiki/Comparison_of_programming_languages_(array)

(I'd only known 1- or N-based ones so the languages I've created have 
all been N-based with a default of 1. With exceptions for certain types, 
eg: strings are 1-based only; bit-indexing within machine words is 0-based.)

However, if the base for arrays is fixed at 0 or 1, then 0-based is 
probably more flexible, as you can emulated 1-based indexing by adding 
an extra element and ignoring element 0; it's a little harder the other 
way around!

> The main issue -- of using a half open interval for a range -- is
> probably less widely agreed upon, though I think it should be.  EWD is
> correct about this (as about so many things).

With 0-based lists, then half-open intervals are more suited. Otherwise 
a list of N elements would be indexed by a range of [0:N-1]. While 
1-based is more suited to closed intervals: [1:N].

But then, suppose you also had an alternate syntax for a range, of 
[A^^N], which starts at A and has a length of N elements. Then ranges of 
[1^^N] and [0^^N] are both specified the same way. You don't need to 
worry about open or closed intervals.

(I've seen two arrows ^^ used on road signs to denote the length of an 
impending tunnel rather than the distance to it.)

-- 
Bartc




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