Why does indexing a string inside of a recursive call yield a different result?

[bauss]

On Sunday, 10 May 2020 at 10:02:18 UTC, Adnan wrote:
>
>
> In my naive implementation of edit-distance finder, I have to 
> check whether the last characters of two strings match:
>
> ulong editDistance(const string a, const string b) {
>     if (a.length == 0)
>         return b.length;
>     if (b.length == 0)
>         return a.length;
>
>     const auto delt = a[$ - 1] == b[$ - 1] ? 0 : 1;
>
>     import std.algorithm : min;
>
>     return min(
>         editDistance(a[0 .. $ - 1], b[0 .. $ - 1]) + delt,
>         editDistance(a, b[0 .. $ - 1]) + 1,
>         editDistance(a[0 .. $ - 1], b) + 1
>     );
> }
>
> This yields the expected results but if I replace delt with its 
> definition it always returns 1 on non-empty strings:
>
> ulong editDistance(const string a, const string b) {
>     if (a.length == 0)
>         return b.length;
>     if (b.length == 0)
>         return a.length;
>
>     //const auto delt = a[$ - 1] == b[$ - 1] ? 0 : 1;
>
>     import std.algorithm : min;
>
>     return min(
>         editDistance(a[0 .. $ - 1], b[0 .. $ - 1]) + a[$ - 1] 
> == b[$ - 1] ? 0 : 1, //delt,
>         editDistance(a, b[0 .. $ - 1]) + 1,
>         editDistance(a[0 .. $ - 1], b) + 1
>     );
> }
>
> Why does this result change?

Wrap the ternary condition into parenthesis.

editDistance(a[0 .. $ - 1], b[0 .. $ - 1]) + (a[$ - 1]
> == b[$ - 1] ? 0 : 1)