Find in assoc array then iterate (page 2)

October 22, 2022

Re: Find in assoc array then iterate

Posted by Paul Backus
in reply to Kevin Bailey

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Paul Backus

Posted in reply to Kevin Bailey

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On Saturday, 22 October 2022 at 15:21:07 UTC, Kevin Bailey wrote:

OTOH, a forward iterator (i.e. copyable but does not need to
go backwards) solves the problem elegantly and efficiently.

The builtin function byKeyValue returns a forward range (D's version of a forward iterator) over the contents of an associative array. To save the current iteration state, use the .save method.

Here's a simple example:

import std.algorithm, std.range, std.stdio;

void main()
{
    auto aa = ["x": 123, "y": 456, "z": 789, "w": 10];

    // name of range type is private, so use auto + typeof
    auto it = aa.byKeyValue;
    typeof(it) saved;

    // iterate once and save our position partway through
    for (; !it.empty; it.popFront)
    {
        auto e = it.front;
        if (e.key == "y")
            saved = it.save;
        writefln("%s: %s", e.key, e.value);
    }

    writeln("--------");

    // iterate again from saved position
    foreach (e; saved)
        writefln("%s: %s", e.key, e.value);
}

On 10/22/22 08:21, Kevin Bailey wrote: > his claim that there was no value. I think he was questioning the need for iterating from a point forward inside an unordered container. When the container is unordered, the elements that are accessed after a found element could be anything. I think that's the puzzling part in your question. > package main Here is my interpretation of Paul Backus's solution: module main; import std.stdio; void main() { int[string] m; m["key1"] = 7; m["key2"] = 8; auto iter = m.byKeyValue(); // Advance iterator iter.popFront(); // Preserve position auto iter2 = iter.save(); // Exhaust first iterator foreach (kv; iter) { writeln("iter: ", kv.key, " = ", kv.value); } // What does second iterator see? foreach (kv; iter2) { writeln("iter2: ", kv.key, " = ", kv.value); } } May print (because its unordered): iter: key2 = 8 iter2: key2 = 8 Ali

ah, I knew that I could iterate over byKey, but I didn't know that it was a tangible thing, that you could hold in your hand. This should be fine, and I'll use your template trick for passing it to a function. Thanks Paul and Ali!

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