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digitalmars.D.learn - Switch between two structs with csvreader

reply Selim Ozel <sozel wpi.edu> writes:
Hi There,

I am trying to switch between two structs as I am using the 
csvReader on a raw string. The pseudo-code below throws a "cannot 
implicitly convert" error due to difference between struct_type1 
and struct_type2. I must be doing something wrong or have a wrong 
understanding of how this function works. Could someone give a 
good suggestion on handling this?

Thanks!

Best,
Selim

auto records = rawtext.csvReader!struct_type1(';');
if(aControlCondition) {
records = rawtext.csvReader!struct_type2(';');
}

// Iterate through each data row.		
foreach (record; records) {
writeln(record);
}
Nov 05 2020
parent reply Anonymouse <zorael gmail.com> writes:
On Thursday, 5 November 2020 at 21:18:52 UTC, Selim Ozel wrote:

 auto records = rawtext.csvReader!struct_type1(';');


D is statically typed and `auto` means "deduce this type for me 
based on this one function's return value". It is not like 
JavaScript's `var` whose type may change.

If I'm not mistaken the `csvReader` function returns a range 
struct, and the full type is something long and unwieldy like 
`CsvReader!(struct_type1, cast(Malformed)1, string, dchar, 
string[])`. So just think of `records` as being that.

(You can tell what type it is at compilation with `pragma(msg, 
typeof(records).stringof)`.)


 if(aControlCondition) {
 records = rawtext.csvReader!struct_type2(';');


Here `csvReader!struct_type2(';')` returns a value of type 
`CsvReader!(struct_type2, ...)`, which is a different type from 
that of `records` (and they're not implicitly convertible). So 
the error message is right. If `auto` worked like `var` your code 
would work, but it doesn't.

You need two different variables and two different `foreach`es. 
For the same code to work on both types, the easy solution is 
templates. Perhaps make the `foreach` part after the reads a 
templated function that accepts any type passed to it?
Nov 05 2020
parent reply Selim Ozel <sozel wpi.edu> writes:
On Thursday, 5 November 2020 at 22:36:36 UTC, Anonymouse wrote:

 If I'm not mistaken the `csvReader` function returns a range 
 struct, and the full type is something long and unwieldy like 
 `CsvReader!(struct_type1, cast(Malformed)1, string, dchar, 
 string[])`. So just think of `records` as being that.


I actually first going this route but couldn't figure out the 
correct name for that data type. It is quite long.


 You need two different variables and two different `foreach`es. 
 For the same code to work on both types, the easy solution is 
 templates. Perhaps make the `foreach` part after the reads a 
 templated function that accepts any type passed to it?


Embedding the foreach loop inside a template function and 
deciding on the data type at the higher level function solved my 
issue. Thanks for the pointer!

Best,
Selim
Nov 06 2020
parent reply "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Fri, Nov 06, 2020 at 07:17:53PM +0000, Selim Ozel via Digitalmars-d-learn
wrote:

 On Thursday, 5 November 2020 at 22:36:36 UTC, Anonymouse wrote:

 If I'm not mistaken the `csvReader` function returns a range struct,
 and the full type is something long and unwieldy like
 `CsvReader!(struct_type1, cast(Malformed)1, string, dchar,
 string[])`.  So just think of `records` as being that.


 
 I actually first going this route but couldn't figure out the correct
 name for that data type. It is quite long.


[...]

You can use the typeof() operator to capture the type of a long,
unwieldy type in an alias. This is useful if you ever need to store such
a return type somewhere, e.g.:

	alias T = typeof(csvReader(...));

	struct MyStorage {
		T result;
	}

	MyStorage s;
	s.result = csvReader(...);

Let the compiler figure out the type for you. :-)


T

-- 
Guns don't kill people. Bullets do.
Nov 06 2020
parent Selim Ozel <sozel wpi.edu> writes:
On Friday, 6 November 2020 at 19:35:47 UTC, H. S. Teoh wrote:

 You can use the typeof() operator to capture the type of a 
 long, unwieldy type in an alias. This is useful if you ever 
 need to store such a return type somewhere, e.g.:

 	alias T = typeof(csvReader(...));

 	struct MyStorage {
 		T result;
 	}

 	MyStorage s;
 	s.result = csvReader(...);

 Let the compiler figure out the type for you. :-)


 T


That's great, thanks!

S
Nov 07 2020