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

[bauss via Digitalmars-d-learn]

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)