[swift-evolution] Change Request: Make myString.hasPrefix("") and myString.hasSuffix("") return true
jaden.geller at gmail.com
Wed Jul 20 21:57:58 CDT 2016
This discussion is getting out of control. Both of these functions have mathematical precedent as well as consistent behaviors in other languages. Observe:
Python: `"hello world".startswith("")` => `True`
Java: `"hello world".startsWith("")` => `true`
Ruby: `"hello world".start_with? ""` => `true`
Rust: `"hello world".starts_with("")` => `true`
Go: `strings.HasPrefix("hello world", "")` => `true`
It's pretty hard to argue against this. Even if you think these other languages are wrong, Swift must regard the empty String as a prefix to be consistent with itself.
`str.hasPrefix(String(str.characters.prefix(0)))` => `false` ?!
I would expect every prefix of `str` to return true as an argument to `str.hasPrefix`…
> On Jul 20, 2016, at 6:49 PM, Ted F.A. van Gaalen via swift-evolution <swift-evolution at swift.org> wrote:
> Don’t Panic !
> At the risk of seeing things in a somewhat trivial perspective,
> combined with an almost complete absence of abstraction:
> Note that to relatively simple persons like me:
> String instances are just rows of characters (when not empty, of course)
> There are only two kinds of Strings:
> 1. empty Strings, which do not contain amy characters at all
> 2. Strings containing 1 or more characters.
> Ergo ad Infinitum :
> Empty Strings do not occur in Strings that contain characters.
> I’d say, please try to find possible empty strings
> that might perhaps be embedded e.g. in the string below:
> “Don’t Panic: Please read Hitchhiker’s Guide to the Galaxy 42”
> With all due respect:
> This might void the discussion below :o)
> I have nothing against Mathematics as long
> as it is applicable.
> Kind Regards
>> To the question of whether any given string has the empty string as prefix:
>> yes it does. This is a correct answer, and returning true is a correct
>> To the question of how many times the empty string occurs in a string: yes,
>> this can be infinite. "a" == "a" + "" == "a" + "" + "" == "a" + "" + "" +
>> "" == "a" + "" + "" + "" + "" == ... etc.. Concatenating an empty string,
>> like adding zero or multiplying by zero for a numerical value, can be done
>> infinitely many times without making a difference.
>> However, there's correctness and convenience. For example, every integer
>> can be expressed as a multiple of prime factors. 1 is technically a prime
>> number - it's divisible by 1 and itself - but for convenience we say it
>> isn't a prime number, because if it isn't, every integer can be expressed
>> uniquely as a multiple of prime factors, whereas if it is, there are an
>> infinite number of such expressions for each integer.
>> There may be many algorithms which rely on an empty prefix returning false.
>> If hasPrefix and hasSuffix are corrected, those algorithms should be
>> altered to recognise that correction. For example, if breaking up a string
>> using the empty string as a separator, it seems sensible that the sequence
>> of substrings would never contain consecutive empty strings.
>> On Wed, Jul 20, 2016 at 11:58 PM, Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> I'd run this by someone who actually knows math, but afaik there are
>>> finitely many empty strings in any given string.
>>> How many e's are in any given string? (Ignoring Unicode issues for now,)
>>> for each index in the string's indices, form a substring one character in
>>> length starting at that index and compare the value of that substring to e.
>>> How many empty strings are in any given string? For each index in the
>>> string's indices, form a substring zero characters in length starting at
>>> that index and compare the value of that substring to an empty string.
>>> On Wed, Jul 20, 2016 at 17:35 Guillaume Lessard <
>>> glessard at tffenterprises.com <mailto:glessard at tffenterprises.com>> wrote:
>>>>> On 20 juil. 2016, at 14:21, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>>>>> Doesn't your second argument undermine your first? If it's a trivial
>>>> solution and one rarely ever considers empty strings when invoking
>>>> `hasPrefix`, then returning the technically correct result must be a
>>>> trivial departure in behavior.
>>>> I specifically used an example where the trivial solution (y=0 instead of
>>>> y=exp(x)) is a pitfall.
>>>> How many empty strings are contained in any given string?
>>>> If the answer is infinitely many, it sounds like a pitfall to me.
>>>> Guillaume Lessard
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