Hi,

I am trying to use variantArray() as a data table object to hold columns each of which is an array of a specific type. I need to be able to get values from data table but I am having problems ...


import std.stdio; // i/o
import std.variant; // type variations

void main(){
  // Columns of the table
  string[] names = ["walter", "paul", "jeff", "andrie"];
  int[] age = [55, 62, 27, 52];
  string[] language = ["D", "Haskell", "Julia", "D"];
  Variant[] dt = variantArray(names, age, language);

  foreach(col; dt){
    foreach(el; col){
      // here I try a kind of dynamic cast operator
      auto x = el.get!(type(el)); // gives error
      write(x);
    }
    write("\n");
  }
}

data_table.d(37): Error: cannot infer type for el
data_table.d(38): Error: undefined identifier 'el'

Help

DP

On Tuesday, 13 October 2015 at 15:17:15 UTC, data pulverizer wrote:
> Hi,
>
> I am trying to use variantArray() as a data table object to hold columns each of which is an array of a specific type. I need to be able to get values from data table but I am having problems ...
>
>
> import std.stdio; // i/o
> import std.variant; // type variations
>
> void main(){
>   // Columns of the table
>   string[] names = ["walter", "paul", "jeff", "andrie"];
>   int[] age = [55, 62, 27, 52];
>   string[] language = ["D", "Haskell", "Julia", "D"];
>   Variant[] dt = variantArray(names, age, language);
>
>   foreach(col; dt){
>     foreach(el; col){
>       // here I try a kind of dynamic cast operator
>       auto x = el.get!(type(el)); // gives error
>       write(x);
>     }
>     write("\n");
>   }
> }
>
> data_table.d(37): Error: cannot infer type for el
> data_table.d(38): Error: undefined identifier 'el'
>
> Help
>
> DP

You're trying to iterate over a `Variant`, which isn't implemented.

You don't want to use a variant here anyway; you should use a struct or tuple for each entry in the table.



import std.typecons;

alias Entry = Tuple!(string, int, string);

void main() {
    auto table = [Entry("walter", 55, "D"), Entry("paul", 62, "Haskell"), ... ]; // complete array
    foreach(entry; table) {
        writeln(entry.expand);
    }
}

Thanks for the suggestion Alex, however I need the dynamic behaviour properties of variantArray(), writing a struct each time would be undesirable.

Perhaps I could boil down the question to something like, is there a way of writing

auto x = dt[0][0];
auto y = x.get!(x.type - or whatever); // to get the actual value of x rather than .VariantN! ... type

For some kind of auto cast back to basic type.

On Tuesday, 13 October 2015 at 15:51:40 UTC, Alex Parrill wrote:
> On Tuesday, 13 October 2015 at 15:17:15 UTC, data pulverizer wrote:
>> Hi,
>>
>> I am trying to use variantArray() as a data table object to hold columns each of which is an array of a specific type. I need to be able to get values from data table but I am having problems ...
>>
>>
>> import std.stdio; // i/o
>> import std.variant; // type variations
>>
>> void main(){
>>   // Columns of the table
>>   string[] names = ["walter", "paul", "jeff", "andrie"];
>>   int[] age = [55, 62, 27, 52];
>>   string[] language = ["D", "Haskell", "Julia", "D"];
>>   Variant[] dt = variantArray(names, age, language);
>>
>>   foreach(col; dt){
>>     foreach(el; col){
>>       // here I try a kind of dynamic cast operator
>>       auto x = el.get!(type(el)); // gives error
>>       write(x);
>>     }
>>     write("\n");
>>   }
>> }
>>
>> data_table.d(37): Error: cannot infer type for el
>> data_table.d(38): Error: undefined identifier 'el'
>>
>> Help
>>
>> DP
>
> You're trying to iterate over a `Variant`, which isn't implemented.
>
> You don't want to use a variant here anyway; you should use a struct or tuple for each entry in the table.
>
>
>
> import std.typecons;
>
> alias Entry = Tuple!(string, int, string);
>
> void main() {
>     auto table = [Entry("walter", 55, "D"), Entry("paul", 62, "Haskell"), ... ]; // complete array
>     foreach(entry; table) {
>         writeln(entry.expand);
>     }
> }

On Tuesday, 13 October 2015 at 16:22:36 UTC, data pulverizer wrote:
> Thanks for the suggestion Alex, however I need the dynamic behaviour properties of variantArray(), writing a struct each time would be undesirable.
>
> Perhaps I could boil down the question to something like, is there a way of writing
>
> auto x = dt[0][0];
> auto y = x.get!(x.type - or whatever); // to get the actual value of x rather than .VariantN! ... type
>
> For some kind of auto cast back to basic type.

The problem is that you can't do `x.get!(x.type)`, because the type of the expression must be known at compile time, but `x.type` is a runtime value. You'd have to create a branch for each type that `x.type` might be.

But again, unless you are dealing with a truly dynamic layout (and not a bunch of fixed layouts), there's better options. For example, you can use `std.range.zip` to iterate through each column array in lockstep, returning tuples for each element.


foreach(entry; zip(names, ages, languages)) {
    write(entry.expand);
}