Re: How do I use CTFE to generate an immutable associative array at compile time?

[Chad Joan via Digitalmars-d-learn]

On Tuesday, 21 February 2017 at 23:30:52 UTC, Chad Joan wrote:
On Tuesday, 21 February 2017 at 22:43:15 UTC, H. S. Teoh wrote:

Parsing strings at program startup is ugly and slow.  What 
about parsing at compile-time with CTFE into an array literal 
and transforming that into an AA at startup, which should be a 
lot faster?

        // Warning: untested code
        pure string[2][] parseTwoColumnCsv(string inputCsv)
        {
                import std.csv;
                import std.typecons;

                string[2][] result;
		foreach (record; 
csvReader!(Tuple!(string,string))(inputCsv)) {
                        result ~= [record[0], record[1]];
                }
                return result;
        }

        immutable string[string] dataLookup;
        static this()
        {
		enum halfCookedData = 
parseTwoColumnCsv(import("some_data.csv"));
                foreach (p; halfCookedData) {
                        dataLookup[p[0]] = p[1];
                }
        }


T

Hi,

That makes a lot of sense and it had crossed my mind.

In my case the data sets are super small, so I'm probably going 
to be lazy/productive and leave it the way it is.

Anyone else from the internet copying these examples: try to 
just do it H.S. Teoh's way from the start ;)

Thanks for the suggestion.
- Chad

OK I couldn't help it:

---
pure private string[][] parseTwoColumnCsv(string inputCsv)
{
        import std.csv;
        import std.typecons;
        
        string[][] result;
        
        foreach ( record; csvReader!(Tuple!(string,string))(inputCsv) )
                result ~= [record[0],record[1]];
        
        return result;
}

pure private string[string] twoColumnArrayToAA(const string[][] 
arr)
{
        string[string] result;
        foreach ( pair; arr )
                result[pair[0]] = pair[1];
        return result;
}

pure private string[string] importTwoColumnCsv(string csvFile)()
{
        // Force the parse to happen at compile time.
	immutable string[][] tempArray = 
import(csvFile).parseTwoColumnCsv();
        
	// Convert the parsed array into a runtime Associative Array and 
return it.
        return tempArray.twoColumnArrayToAA();
}

immutable string[string] dataLookup;
immutable string[string] dataLookup1;
immutable string[string] dataLookup2;

static this()
{
        import std.range;
        dataLookup1 = importTwoColumnCsv!"some_data1.csv";
        dataLookup2 = importTwoColumnCsv!"some_data2.csv";
	foreach( pair; chain(dataLookup1.byKeyValue, 
dataLookup2.byKeyValue) )
                dataLookup[pair.key] = pair.value;
}

void main()
{
        import std.stdio;
        writefln("dataLookup = %s", dataLookup);
}
---

This example also shows joining associative arrays.