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

behaviour.

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@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@tffenterprises.com> wrote:

> On 20 juil. 2016, at 14:21, Xiaodi Wu <xiaodi.wu@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.

Cheers,

Guillaume Lessard

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