Re: dynamic get from variantArray() data table

[Alex Parrill via Digitalmars-d-learn]

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);
}
}

Re: dynamic get from variantArray() data table

[Alex Parrill via Digitalmars-d-learn]

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);
}

Re: dynamic get from variantArray() data table

[data pulverizer via Digitalmars-d-learn]

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);
}
}