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?

On 10.05.20 12:02, Adnan wrote:
> 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?

You're going from this (simplified):

    delt = a == b ? 0 : 1
    result = x + delt

to this:

    result = x + a == b ? 0 : 1

But that new one isn't equivalent to the old one. The new one actually means:

    result = (x + a == b) ? 0 : 1

You need parentheses around the ternary expression:

    result = x + (a == b ? 0 : 1)

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)