Custom separator in array format

[Simen Kjærås]

On Tuesday, 28 January 2020 at 07:36:25 UTC, Malte wrote:
> I want to format an array using the %(...%) syntax. How can I 
> change the separator? I tried to use ? and add it as additional 
> parameter, but that doesn't seem to work on arrays:
>
> import std;
> void main()
> {
>     writeln("This works:");
>     writefln("%,2?d", '_', 2000); // 20_00
>
>     auto vec = [1000, 2000, 3000];
>     writeln("This should fail (separator character expected) 
> but the ? is just ignored:");
>     writefln("%(%,2?d\t%)", vec); // 10,00	20,00	30,00
>     writeln("This throws:");
>     writefln("%(%,2?d\t%)", '_', vec); // 
> std.format.FormatException@/dlang/dmd/linux/bin64/../../src/phobos/std/format.d(2271): incompatible format character for integral argument: %(
> }

I think I see why it's not working. Essentially, for each element 
of vec, format is called with only that element as an argument. 
Essentially, rather than:

     foreach (e; vec)
         writef("%,2?d\t", '_', e);
     writeln();

You get:

     foreach (e; vec)
         writef("%,2?d\t", e);
     writeln();

For whatever reason, it doesn't throw when missing an argument 
for the separator - I'd say this is a bug 
(https://issues.dlang.org/show_bug.cgi?id=20541).

For now, you can work around the issue this way:

     import std.stdio : writefln;
     import std.format : format;
     import std.algorithm : map;

     auto vec = [1000, 2000, 3000];

     writefln("%-(%s\t%)", vec.map!(e => format!"%,2?d"('_', e)));

--
   Simen