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July 17, 2010
Equivalent of scanf
I'm having trouble figuring out how to do formatted console input, something
like C scanf() or C++ templated stream input. Unfortunately, console input
isn't covered in much detail in TDPL book. There doesn't appear to be much
discussion about the standard library at all, which was a bit disappointing.

But anyway, what different ways are there to properly do input from the
console? Fetching a string with readln() is easy enough, but how could I
fetch, say, an integer value? Conversely, what is the preferred method for
converting string into integer or floating point values?

Thanks,
Michael
July 17, 2010
Re: Equivalent of scanf
On 18.07.2010 00:41, Michael Koehmstedt wrote:
> I'm having trouble figuring out how to do formatted console input, something
> like C scanf() or C++ templated stream input. Unfortunately, console input
> isn't covered in much detail in TDPL book. There doesn't appear to be much
> discussion about the standard library at all, which was a bit disappointing.
>
> But anyway, what different ways are there to properly do input from the
> console? Fetching a string with readln() is easy enough, but how could I
> fetch, say, an integer value? Conversely, what is the preferred method for
> converting string into integer or floating point values?

The standard library (Phobos) is getting overhauled, which is probably 
why there's not a lot of detail about it in the book.  It's still quite 
limited and buggy.  Up until now, most real D work has probably been 
done using D1 and the Tango library.

There's a scanf implementation in the works, but it's not released yet. 
 You can probably get by with the to() and parse() functions in 
std.conv. C's isdigit() will probably come in handy too, it's defined in 
core.stdc.ctype.
July 17, 2010
Re: Equivalent of scanf
On Saturday 17 July 2010 15:41:14 Michael Koehmstedt wrote:
> I'm having trouble figuring out how to do formatted console input,
> something like C scanf() or C++ templated stream input. Unfortunately,
> console input isn't covered in much detail in TDPL book. There doesn't
> appear to be much discussion about the standard library at all, which was
> a bit disappointing.
> 
> But anyway, what different ways are there to properly do input from the
> console? Fetching a string with readln() is easy enough, but how could I
> fetch, say, an integer value? Conversely, what is the preferred method for
> converting string into integer or floating point values?
> 
> Thanks,
> Michael

Parsing strings like that isn't something that I've done much of lately, so I 
don't know what the best way to do it in D is. However, std.conv has essentially 
what you're looking for. For most conversions, just use to!T() - e.g. 
to!int(str) will convert the string str into an int and to!string(i) will 
convert the integer i to a string. For more complicated parsing, there's 
parse!T() which does essentially the same thing except that instead of 
converting the whole string, it only converts the portion which makes sense and 
leaves the rest. e.g.

string test = "123 \t  76.14";
auto a = parse!(uint)(test);
assert(a == 123);
assert(test == " \t  76.14");

So, with multiple calls to parse, you can parse the string. I'm not aware of a 
way to do it in manner like sscanf though, where you parse it all in one 
command. Worse comes to worse, if you really want that, you can always use C's 
sscanf and it'll work.

TDPL purposely didn't touch on Phobos much. One reason for this is because it 
was focusing on the language itself rather than whatever libraries there might 
be. The other main reason, as I understand it, is that Phobos has been in too 
much flux to have put in TDPL. A lot of work has been done on Phobos in the past 
few months, and TDPL was already turned in to the editor. And a lot of work 
continues to be done on Phobos. So, anything that was put in TPDL on Phobos 
would either have constrained Phobos so that it couldn't change when it needed 
to or would have made it so that TDPL didn't match Phobos - either that or we 
would have had to wait longer for TDPL.

- Jonathan M Davis
July 17, 2010
Re: Equivalent of scanf
Thanks for the informative reply.

I am getting a run-time error with to!int(str). Apparently to!() seems to only
support one-way converting other types to strings. That is part of the reason why
I was getting so confused in figuring out how to read in an integer value. But you
guys pointed me towards parse!() and that does work with string->int.

So there is no scanf equivalent, but there is also nothing similar to C++ cin with
the << operator?

And as for Phobos not being included much in TDPL, makes sense. I hope they
include a reference section on it in the 2nd edition after Phobos is stabilized.
That would make the book near perfect imo.
July 17, 2010
Re: Equivalent of scanf
== Quote from Michael Koehmstedt (mkoehmstedt@runner.csub.edu)'s article
> So there is no scanf equivalent, but there is also nothing similar to C++ cin with
> the << operator?

I meant the >> operator, of course. *slaps forehead*
July 17, 2010
Re: Equivalent of scanf
On 18.07.2010 01:21, Michael Koehmstedt wrote:
<snip>
> So there is no scanf equivalent, but there is also nothing similar to C++ cin with
> the<<  operator?
>

Equivalents of those are available in std.stream and std.cstream, but 
those modules will probably go away in a while.
July 18, 2010
Re: Equivalent of scanf
On Saturday 17 July 2010 16:35:11 torhu wrote:
> On 18.07.2010 01:21, Michael Koehmstedt wrote:
> <snip>
> 
> > So there is no scanf equivalent, but there is also nothing similar to C++
> > cin with the<<  operator?
> 
> Equivalents of those are available in std.stream and std.cstream, but
> those modules will probably go away in a while.

We'll be getting new streams which are range based which may or may not use the 
name std.stream, but the current implementation is certainly going away. Like a 
number in things in Phobos right now, it's being updated and replaced with 
something better. Phobos is evolving for the better, and it's great to see that 
it's finally starting to come together.

- Jonathan M Davis
July 18, 2010
Re: Equivalent of scanf
On Saturday 17 July 2010 16:21:04 Michael Koehmstedt wrote:
> Thanks for the informative reply.
> 
> I am getting a run-time error with to!int(str). Apparently to!() seems to
> only support one-way converting other types to strings. That is part of
> the reason why I was getting so confused in figuring out how to read in an
> integer value. But you guys pointed me towards parse!() and that does work
> with string->int.


Hmm... to!int(str) seems to be working for me. The issue is probably that when 
doing such a conversion, the _whole_ string must be convertable. So, no extra 
whitespace or whatnot. If you're looking to take multiple values out of a 
particular string or you might have whitespace in it, parse() is a better 
choice. But to() should work just fine as long as the string is exactly 
convertable to the type. As I understand it, to() should be able to convert 
pretty much anything to anything as long as the type has the appropriate 
conversion functions defined.

- Jonathan M Davis
July 18, 2010
Re: Equivalent of scanf
I am doing this and it is throwing an exception.

string d = readln( );
int i = to!int( d );

I tried

string d = "50";
int i = to!int( d );

and that worked ok.
July 18, 2010
Re: Equivalent of scanf
On Sunday 18 July 2010 02:58:59 Michael Koehmstedt wrote:
> I am doing this and it is throwing an exception.
> 
> string d = readln( );
> int i = to!int( d );
> 
> I tried
> 
> string d = "50";
> int i = to!int( d );
> 
> and that worked ok.

That would be because readln() was likely giving you more than just an int. All 
it would take would be a character of whitespace or a newline (there's a decent 
chance that readln() returns a string that ends in a newline) and to!int() would 
throw. So, if you're dealing with strings which may include stuff other than what 
you're actually trying to convert to, then parse!T() would likely be a better 
choice. You can also use std.string.strip() on a string to get rid of any 
whitespace on its ends if you're sure that it's only a number with whitespace.

If you really feel that we should have a version of scanf() for D, then you can 
always open up a bug report with enhancement request as the severity. If one 
isn't in the works, it might encourage it, and if one is already in the works, 
it might make it happen faster (since there would be an obvious desire for it 
plus there would be a bug report as a reminder).

- Jonathan M Davis
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